koordinacija 5 - anorganska
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Transi t ion Metal Pentacoordinat ion
Inorganic Chemistry, Vol. 14, No. 2 , 1975 365
(28) J . Bjerrum, G. Schwarzenbach, nd L. G. Sillen,
Chem.
Soc.,
Spec.
Publ.,
No.
7 ,
8 (1958) .
(29) R. Giovanoli,
W.
Stadelmann and H . Gamsjager,
Chimia, 27.
170 (1973).
(30) G . Bazsa and H. Diebler,
Proc. Int.
Conf.
Coord. Chem.,
15,442 (1973).
(31) We note
also
that W.
E . Jones
and
T.
W .
Swaddle,
Can. J . Che m. ,
45,
2647 (1967),
have
not succeeded
in
repeating the preparation
of
[Co(N H3)5C 104]2 + eported by Duval, nor has it been possible to detect
the brom ato complex by allowing excess TiaBrO3 to react w ith [C o-
(NH3)5Hz0]3+:
J .
F.
Ojo,
unpublished work.
(32)
N .
1. Lobanov,
Izv . Se k t . P lar iny Drugikh Blagorod. M e t a l . , Insr.
Obshc h. Ne org . Kh i m ., A k ad . Nauk S SS R , 28,
277 (1954).
(33) H . Siebert, personal communication.
(34) L. Monsted and
0 .
Monsted,
Acta Chem. Scand. ,
27,
2121 (1973).
(35) G. Guastalla and
T. W .
Swaddle,
Inorg. Chem., 13,
61 (1974).
(36) See, for example,
C.
Hwang and A . Haim,
Inorg. Chem ., 9,
500 (1970).
and references therein.
(37) J. H. Espenson,
Inorg. Chem., 8, 1554
(1969) .
(38)
A .
Haim,
Inorg. Chem., 9,
426 (1970).
(39)
H .
M
omley and W.
C.
E. Higginson,
J . Ch em Soc . , Dalton Trans . ,
2522 (1972).
(40) R. Holwerda,
E.
Deutsch, and H . Taube,
Inorg. Chem., 11,
1965 (1972).
(41)
M.
T. Beck, J .
Inorg.
Nuc.1.
Chem.. 15, 250
(1960).
(42)
M . V.
Olson,
Inorg.
Chem.,
12,
1416 (1973).
(43) E. B. Fleischer and R . Frost,
J.
Amer. Chenz.
Soc., 8 7 , 3998 (1965).
(44) J.
Halpern.
R.
A .
Palmer,
and
L.
M
lakley, J .
Amer. Chem. Soc.,
88,
2877 (1966).
(45)
J .
E. Byrd and
W . K.
Wilmarth,
Inorg. Chim cta Rev.. 5,
7 (1971).
(46) R.
S.
Murray, D. R. Stranks, and J.
K.
Yandell,
Chem. Commun. ,
604
N .
Lobanov,
Russ. J . Inorg. Chem., 4, 151
(1959) .
M. Mori ,
Inorg.
Syn . ,
5
132 (1957).
D.
W .
Hoppenjans and J. B. Hunt,
Inorg. Chem.,
8,
505,
(1969) .
W. W. Fee, C. S. Garner, and J.N. MacB. Harrowfield,
Inorg. Chem.,
6,
87 (1967).
R .
K.
Wharton, R. S. aylor, and A.
G.
Sykes,
Inorg. Chern., 14,
33
(1975).
R. K.
Wharton, Ph.D. Thesis, University
of
Leeds, 1973.
R.
Duval, C. Duval, and
J .
Leconite,
Bull.
Soc.
Chim. Fr.,
1048 (1947).
N. I . Lobanov, Z h .
Neorg.
Khim.,
2,
1035 (1957).
J. Fujita, A.
E.
Martell, and
K .
Nakamoto, J .
Chem. Phys . , 36,
324
(1962) .
K.
Nakamoto, “Infrared Spectra of Inorganic and Coordination
Compounds,”
2nd
ed, Wiley, New York,
N.
Y., 1970, p 238.
Reference 14, p 173.
For a discussion of the infrared spectra
of
iodates see
J . R .
Durig,
0.
D. Bonner, and W. H. Breazeale, J .
Phys . Chem., 69,
3886 (1965).
W .
E.
Dasent and
T.
C. Waddington, J .
Chem. Soc . ,
2429 (1960).
D.
A .
House,
Acta Chem. Scand. , 26,
2847 (1972).
R .
Davies and R. B. Jordan,
Inorg. Chem., 10,
2432 (1971).
0.
Nor and A. G. Sykes,
J. Chenz.
Sor.,
Dalton Trans.,
1232 (1973).
C .
Andrade and
H.
Taube,
Inorg. Chem., 5
1087 (196 6).
Calculated from data in
ref
20.
H. H . Cady and
R.
E.
Connick, J .
Amer. C hem. Soc . ,
80,2646 (1958).
T. C. Matts and P. Moore, J .
Che w. Soc. A ,
1632 (1971).
N . V .
Duffy and
J .
E. Earley, J.
A me r . Che m. Soc., 89,
272 (1967).
J. E.
Earley and W. A lexander, J .
Amer. Chem. Soc., 92,
2294 (1970);
M. A. Levine, T.
P.
Jones, W . E. Harris , and
W . J .
Wallace,
ibid., 83,
2453 (1961). (1969).
W .
K.
Wilmarth, H . Graff, and
S.
T.
Gustin, J .
Amer. Chem. Soc. 78,
2683 (1956). (48) P. J. M.
O’Neill,
unpublished work.
(47)
H .
Siebert and G. Wittke,
Z . Anorg. Allg . Chem., 399,
52 (1973).
Con t r ibu t ion f r om th e Depar tm en t s o f Chemis t r y , Un iver s i t y o f Connec t i cu t ,
S t o r r s , C o n n e c t i c u t 06268, a n d C o r n e l l U n i v er s it y , I t h a c a , N e w Y o r k 14850
Transition Metal Pentacoordination
A K G E L O R .
ROSS1
a n d R O A L D H O F F M A N N *
Received June
21 ,
1974
AIC40399E
A un i f ied molecu l a r o r b i t a l t r ea tmen t o f pen t acoor d ina t e t r ans it i on me ta l complexes f o r t he
D3h
t r i gona l - b ipy ramida l and
t h e
C4”
squar e - py r am ida l geomet r i e s i s p r esen t ed . Sym met r y a r g ume n t s and ca lcu l a t ions
on
model compounds f o r wh ich
a-bon ding ef fects are excluded yield basic u-bonding t rends. Thu s the axial bond should be weaker
in
t he t r i gona l b ipy r amid
f o r do -d4 an d d lo and s t r onger f o r d8 l ow- sp in complexes ; t he ap i ca l bond o f a s quar e py r amid wi th 6
<
165’
i s s t r onger
f o r do- ds and d lo and weaker f o r dg.
A
s imi l a r pa t t e r n i s ob t a ined f o r u - subs t i t uen t e f f ec t s : t he s t r onger
CT
d o n o r p r e f e r s
the equa tor ial posi t ion of a t r igonal bip yramid for dO-d4, dlo and t he axial si te for d*;in t h e s q u a r e p y r a m i d t h e p r e fe r re d
posi t ion for
a
s t r onger
u
donor i s ap i ca l f o r do -ds and d lo and basa l f o r d8. T h e subs t i t u t i ona l p re f e r ences o f subs t i t uen t s
bear ing cy l ind r i ca l and s ing l e- f aced a - donor an d - accep to r o r b i t a l s a r e exp lo r ed . T h e
a
i n t e r ac t i on i s g r ea t es t when the
subs t i t uen t
is
equa to r i a l in a t r i gona l b ipy r amid , w i th i t s donor o r acce p to r o r b i t a l i n t he equa to r i a l p l ane . A s ing l e- f aced
a
accep to r wi l l o r i en t i t sel f e q l f o r d*- d lo ;
a
single- faced
a
dono r , eql
.
A cyl indr ical
a
accepto r wil l favor the equato r ial
site for dg-dlo; a
a
donor , t he ax i a l s i t e . I n t he i n t e r es t i ng dg case t he e f f ec t
of
a
a
accep to r
on
t he r e l a t i ve bond s t r eng ths
c o u n t e r ac t s t h e u ef f ec t, wh i l e a donor r e in fo r ces i t. I n a squa r e py r amid t he ex t en t o f7r i n t e rac t i on va r i es wi th t he deg r ee
o f py r amida l i t y . For
a
near ly f l a t squa r e py r amid cy l ind r i ca l
a
in teract ion
is
gr ea t es t i n t he basa l s i t e bu t chan ges t o t he
apical posi t ion
as
t he py r amida l i t y i nc r eases . In a basa l s i t e t he r e is a lways mor e i n t e r ac t i on in t he ba o r i en t a t i on .
Pentacoordinate transition metal complexes occupy a unique
position in inorganic chem istry. As unstable reactive inter-
mediates such species are commonly implicated in associative
primary reactions of tetracoordinate molecules and dissociative
reactio ns of hexa coord inate compounds.l-3 Wh en penta-
coordinate molecules are stable enough to be isolated, they
confront us with a fascinating geometrical problem-the choice
between trigonal bipyramid, squa re pyramid, and even other
extrem e conformations, and a generally soft potential energy
surfac e connecting these minima.4-7
In this paper w e build on our previous analysis of the bo nding
in pen tacoo rdinate phosphorus8 to derive a general theory of
substitu ent effects and g eom etrical preferences in penta-
coord inate t rans i t ion metal complexes. Th e geometr ies
considered in detail are th e
D3h
trigonal bipyramid, 1 and the
C 4 v
square pyramid, 2. Th e electronic effects were modeled
on ML5, where M is a metal atom of the third transition series,
L a pseudoligand carrying either s orbitals alone, when the
.
* To
whom correspondence should be addressed at Cornell University.
l
i
1
2
isolated effects of CT bonding were to be studied,
or
a full
complement of s and p orbitals. Th e qualita tive discussion
of bonding effects should be valid for metal atoms in any
transition series. Details of the extended Huck el calculations
are given in the App endix.
A caut ionary note must be inserted here . Th e arguments
to be presented in this paper are primari ly symmetry and
overlap based, with detailed calculations playing only a
supportive role. Even so, the conclusions should be viewed by
the reader critically, not as the last word of theory but as th e
working out of the consequences of one particu lar model. It
is legitimate to question som e of the foundations of the m odel,
for instance t he cru cial role we will assign to hyb ridization with
( n + l ) p orbitals. And th e geometrical features of transition
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366 Inorganic Chemistry, Vol. 14,
No. 2, 1975
Angelo
R.
Rossi and Roald Wofi'mann
r - - - - - 1
Figure 1.
I n t e r ac t i on d i agr am
for
a
D,, MI2,
c o m p l e x .
The lig-
ands bea r
u
orbi tals alon e. Mixing with meta l
s
a n d
p
i s not
exp l i c it l y show n .
me tal com plexes will be influenced no t only by the electronic
factors we explore but also by steric and electrostatic effects.
Trigonal
Bipyramid
Figure
I
shows the expected interaction diagram for an ML5
system with D3j symmetry, wi th 7r bonding from the l igands
assumed ab sent. Th e d-level splitt ing schem e, confirmed by
ou r calcula tions, is not nove1,5,9 bu t, since it ca rrie s within it
all the information required for an understanding of substituent
effects, we will anal yze it in some detail .
Lowest in energ y is the e" set
3
and
4 .
In th e absence of
I
3 4
l igand orbital s of
T
symmetry the e" set is pure metal
d ,
xz
a n d y z . A t som ewha t higher energy lies the e ' set shown in
5
and 6 . These orbi ta ls are primari ly m etal d, x y and x 2 -
5
6
y2,
with a smal l ant ibonding admixture of equatoria l l igand
o orbitals . Th e reason for th e relatively small metal-ligand
interaction has been discussed previously by us8-though the
precise D3j symmetries of metal
xy
and x2 -
y
and
symmetry-adapted ligand combination match, their pseudo-
symmetries are di fferent . Th e l igand e ' combinations have
one nodal surface eac h, while the m etal se t has
two.
But the mos t impo r t an t fea ture
of
the e ' orbi ta ls , the one
which will be determinative in the subsequent
T
bonding, is
only fa int ly appa rent in 5 and 6. This is a hybridization of
the metal component away from the equatoria l l igands . W e
may tr ace down the hybridization as follows. Th e primary
mixing is of metal nd with ligand v orbitals, yielding the usual
bonding an d antibon ding combinations shown in the middle
of Figure 2. Note that the bonding combination, mainly
on
the ligands, is at very low energy, and it is the antibonding
combination, mainly metal nd, which is of primary concern
to us . Th e metal ( n + l) p orbital now is mixed in. Its mode
of interaction is such as to stabilize all orbitals that it interacts
with. Thu s it mixes into the nd-ligand bonding combination
so
as to increase that bonding, thereby producing a d--p hybrid
pointing towa rd th e ligands. And it mixes into the nd--ligand
F i g w e 2.
Schemat i c o r ig in of hybr id i za t i on i n on e e ' o r b i t a l .
Metal d mixing with l igand orbi tals i s shown
a t
the lef t ; fol lowcd in
t h e
center
by mix ing i n
of t he
meta l
p
o r b i t a l .
4-11
0
Figure 3 . Calculated e nergy levels of
ML;
as a
func t ion of
t h e
Lbasal-hf-Lbaal
angle
0 .
T he l abe ls i den t i f y t he p r imar y char ac tc r
of
t h e 210, ven thoug h these o r b i t a l s a r e
to various
degrees
dc io-
cal ized. The ver t ical cnergy scale is
in
e l ec t r on vo l t s .
ant ibonding orbi ta l
so
to decrease that antibonding, t h u s
producing a hybrid that points away from the ligands.10
In
6
the descriptor "pointing away from th e ligands" i s not
sufficient, since the ligand function has density on all the
equatorial sites. Th e mixing in of metal p~ is here determined
by the largest antibon ding interaction, tha t with the ligand
atom located
on
t he
y
axis.
The re rem ains the highest lying orbital
of
the d set, of
a i
symmetry. This is 7, strongly metal-axial ligand antibonding
and weakly metal-equatorial l igand antibo ndin g.
Within the
C4,
constraint there remains a single degree of
freedom, defined by an Lbasai -M-kbaYai angle 0 in 2. Tt is
instructive to show the variation of the individual energy levels
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Transi t ion Metal Pentacoordinat ion
Inorganic Chemistry, Vol.
14,
No. 2,
I975
367
6
=
180°).15 For the d6 case one might expect an equilibrium
between a low-spin squ are pyramid a nd a high-spin trigonal
bipyramid. Th e situation has a direct analogy to the low-spin
square-planar an d high-spin tetrah edral equilibrium for Ni(I1)
and other d8 systems, as well as corresponding spin-state and
geometry questions for cyclobutadienes and methylenes.
An interesting case in which the square-pyramidal and
trigonal-bipyramidal conformers of one molecule both exist
as distinct structure s, at least in th e solid state, is that of the
low-spin d7 Co(I 1) complexes Co(dpe)2Cl+.16 Th e red
modification of this complex is squ are pyram idal,
C1
apical ,
while the green form is a trigonal bipyramid, with C1 equa-
torial. In solution th e two geometries interconvert rapidly.
u-Bond Strengths in Pentacoordinate Complexes
Th e natu re of the orbitals which we have derived allows an
obvious inference of the effect of the num ber of d electrons
on
the u-bond strength. First we confirmed th at in our model
D3h ML5 system, for do, the axial bonds a re weaker, just as
in
PR5.
Orbi ta ls
3
and 4 in the D3h geometry ar e pure d an d
thus should have no effect on the bond s t rength. Orbi ta ls
5
and 6 , filled next in a low-spin complex, are M-equatorial L
antibonding. Thu s the initially stronger equatorial bonds ar e
weakened, to the extent tha t in our calculation the dg complex
has stronger axial bonds. As one fills in the last two d electrons
into 22,
7,
the axial bonding is again weakened,
so
that by dlo
the do order, axial weaker than equatorial, is restored.
T h e C4v do case was not analyzed in great detail in our
previous work on PRj, though it was noted that for a geometry
with 0
= 160"
the apical bond was stronger.8 This was
consistent with electron-rich multicenter bonding for th e basal
bonds, normal apical bonding. Th e introduction of d orbitals,
as in our model ML5, results in a perturbation of the system.
At 8 = 180" the basal bonds a re s l ightly s t ronger than th e
apical one, but a t smal ler
8
the trend reverses, with the
crossover coming at 8
= 165".
Filling the xy orbi ta l has
no
effect on the do trend. Th e next four e lect rons go into the e
orbital, whose metal-ligand antibo ndin g cha racte r depends
on
the pyramidality of the structure. For a flat square pyramid
there is little effect, but for smaller
0
there could be a significant
weakening of the basal bonds. For
8 =
160°, a typical value,
d6 has the basal bonds definitely weaker than the apical one.
Th e next orb ital, ai , is strongly M-apical L ant ibonding.
Thus a t d8 the bond s t rengths are reversed from the do case.
With two more e lect rons in the s t rongly M-basal L ant i -
bonding x2
- y2
orbital the original order is restored.17-22
W e summ arize the ant ic ipated trends for the s t rength of
the metal-ligand u bonding, noting that a comparison with
experiment must be postponed until we complete our analysis
of r-bonding effects.
W I S I I s
I w
C4
D3h C2
YQ
Figure
4. Wave
f unc t ion and en er gy changes a long a Berry pseudo-
r o t a t i on coor d ina t e .
with 8, which is done in Figure
3.
Th e charac ter of the levels
is most clearly exhibited a t 6 = 180°, an octahedron minus
one l igand. Jus t as in an octahed ral complex there is a t r iad
of
levels, xz, z , and xy, ot engaged in u bonding. The re is
the high-lying x2 - y2 orbita l, strongly metal-basal l igand
antibonding. At intermediate energy lies the z2 orbital, m ainly
metal-apical l igand antibonding.11
Th e degeneracy of the e set
xz , y z )
and the b2 x y ) orbital
holds only at 8 = 180". At smal l er 0 t he xy orbi ta l remains
pur e d, but t he e set begins to mix in antibond ing way with
basal l igand orbitals.lzb>c
Not
only is the e orbital set thus
destabilized, but with significant consequences for P bonding
the e orbitals become hybridized away fro m the basal l igands
and toward the ap ical si te . Th e effect is shown in an exag-
gerated manner in
8.
The explanation of the hybridization
8
i s the same as for the e' orbital of the trigonal bipyramid.
T h e z2 orbital decreases in energy with decreasing 8 for two
reasons. First, the antibonding between z2and th e basal ligand
orbi ta ls is decreased, as the basal l igands move toward the
nodal surface of the z2 orbita l. Seco nd, the basal-apical
ligand-ligand interactions, which ar e antibonding, are reduced.
Th e equilibrium conformation of an M L5 molecule, if i t is
a squ are pyram id, will clearly depend
on
the number of d
elect rons. From Figure
3
it can be seen that d6 systems will
favor a "flat" squa re pyram id, with 0 =
180"
but tha t t he
addition of more electrons will tend to dis tort the molecule
so that 0 <
180 .
In our model dg M L j the optimum
0
is 164 .
It is also simple to construct the correlation diag ram for a
Ber ry pseudoro ta t ion~3~14hich relates the D3h and
C4v
ex-
tremes by a pathway maintaining C2 symmetry. This is shown
in Figure
4.
If the pseudorotation process is continued to
another trigonal bipy ramid, as was don e by Eaton ,l4 it becomes
clear tha t t he symmetry-allowed interconversions a re for do-dz
and d6-d10 low-spin species. As fa r as the preferences for
C ~ L
or D3h ar e concerned, we note that, superimposed on the slight
do framewo rk preference for a trigonal bipyramid,g th e level
t rends in Figure 4 imply tha t d3-d4 would favor D3h by still
more, and d5 4 6 a square pyramid (and from Figure
3
one with
d e0-d6,d1Oeo-d4,d l0
w
=
weaker
s =
s t r onge r
Th e cases not listed above a re interm ediate in their preferences.
Th e computed overlap populations mak e the crossover in axial
v s . equatoria l bond s t rengths in the t r igonal bipyramid at d j
and d9, with a stronger axial bond for d6 and d7. In th e square
pyram id d7 and d9 show a reduced trend for a stronger basal
bond.
a-Substituent Effects
Th e detai led form of th e molecular orbi ta ls of the two
coordination geometries yields in an obvious manner the
preferred substitution sites for dono rs and acceptors. T he
arg um ent is simple-it
is
assumed th at more electronegative
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368 Inorgunic Chemislry, Vol. 14, No. 2 ,
1975
l igands will preferentially enter those sites which carry an
excess of electrons in the unperturbed
M L j
sys tem. The
further identification of a CT acceptor with the more elec-
tronegative and (r dono r with the m ore electropositive (or less
electronegative) ligand is obvious.
Th us in do, D?h, the axial atom s are more negative, which
fol lows from Rundle 's s imple picture of e lect ron-r ich
three-center bonding.
Th e consequent preference for elec-
tronegative su bstituen ts in the axial sites has been discussed
previously.* T he e" orbital
(3,
q d ,
which is filled f rom do to
d4, has
no
electron density
on
the ligands and
so
will not change
the do trend. Th e e' orbital,
5
and 6 , which is next filled, puts
electron density on the eq uatorial atom s. At d6? and especially
at d*, they become more negative than the axial atoms. Thus
fo r dX D3h complexes we would clearly expect a reversal of the
do Muetterties rule, with more electronegative atoms preferring
the equatorial positions.
T h e
z2
orbi ta l ,
7,
has most of its
electron density on the axial l igands.
I t reverses the trend
again,
so
tha t dlo is like do. Similar argu ments for the square
pyramid lead again to a reversal of the do and dlo trend at de,
The predictions of favored sites for
a
donors, D, and u
acceptors , A, can be summarized as
Angelo R. Wossi and Roald Hoffmarin
For a n acceptor the s i te wi th m aximum interaction will bc
stabilizing; for a donor th e interaction may be stabilizing or
destabilizing. The important axial and equa ~or ialnteractions
which are allowed by symm etry ar e shown
in
11-14. Bn thc
A
D
D
I
a
I
A
A
D
d0-d4,d1' de d'-d6,d1' d8
Extend ed Hiickel calculations confirm our analysis. W e defer
a discussion of the e xperim ental evidence for the pa ttern of
substitution until we consider the preferences arising from
T
bonding.
n-§ubstituent
Effects in the Trigonal
In
this section the geometrical preference of n-electron
donors and acceptors will be examined, independent of
u
effects. The restrictions
on
interaction as a result of symmetry
will be considered, coupled with calculations utilizing model
donors and acceptors. A n donor is defined as a substituen t
with one or two high-lying occupied molecular orbitals of local
7r symm etry while a n- cceptor is defined as a substituent with
one or two low-lying unoccupied m olecular orbitals of a
T
type.
Th e simplifying theoretical a rgum ent will be applied to th e
D3h case first. Axial substitu tion is shown in
9
while equatorial
subs t i tut ion is given by 10.
The molecular symmetry is
9
IO
reduced from D3h to
C3y
on axial substitution and to C'zVon
equatorial substitution. The substituent orbitals transform as
e in C3) and as bi, b2 in CZL.
The conventional axis choice
in
CzV,
amely,
z
axis along
C2.
is inconsistent with o ur original
coordinate system, given next to
I .
which in turn was the
natural choice for an unsubstituted trigonal bipyramid. Th ere
is no good way out of this notational problem.
W e h av e
retained the original d orbitals but have gone to a conventional
C2
axis system by redefining w hat was
y
in
1
as
z ,
and
z
as
x.
T h u s
ah D3h)
- U~ JJZ) CL).
he important molecular
orbi ta ls of
M L j
transform as indicated below
c,,
< D q l - c2
a 1 a1
a
e
c
b2 f a ,
e
e
b , t
a2
The prime consideration is whether a given donor or acceptor
interact s with molecular orb itals of the ML5 skeleton more
or less at a given position, rather t han the extent
of
interaction.
- ax
12
1 3
14
axial case we have made the approximation that the sub-
stituents effectively do not interact with th e fram ewor k e orbital
derived from e' , but only wixh the orbital derived from e".
The axial interactions 11 and 12 are identical by symmetry.
Th e equatoria l interactions are labeled as e q l (13) and eq'l
(14) according to the orientation of the substituent donor or
acceptor orbital p erpendicular or parallel to the threefold axis.
The eq,l interaction is identical with either of the axial ones.
Th us the distinction between axial and equato rial substituents
bearing a cylindrically symmetrical acceptor set (CY , CQ,
PR3-.)
depends on the magni tude of the e q l interact ion 13.
It was noted above that the
D3h
e' orbitals, one m ember
of
which is engaged in the interaction 13,are hybridized
away
from the eq uatoria l l igands . A direct consequence of this is
tha t the 3d-p r overlap in e q l , 13, is significantly greater than
the corresponding interactions 11, 12, and
Equatorial r bonding is thus stronger than axial. This has
been noted by others.23 but it is important
to
be careful not
to
draw the inference therefrom that .ir-interacting substituents,
whether donors or acceptors, will always prefer the equatorial
site. Donor interaction can be destabilizing, and one must
carefully examine the number of d electrons and thc level
scheme before reaching a
particu lar conclusion.
Let us first consider the orientational preferences
of
single-faced 7r donors or acceptors. Wh en such substituents
appear in the axial site, there is no effective discrimination
amon g possible orientations. In the eq uatoria l site there is .
Figure
5
shows an interaction diagram comparing eq ~ nd e q l
orientations for single-faced donors
or
acceptors.
In the
construction of this diagram it is assumed th at the encrgy g ap
denominator in a perturbation theory expression for the in-
teraction is not dominant but that instead the magnitude of
the interaction is controlled by the overlap.
For d*-dlo th e acceptor will clearly prefer an e q l orientation
15. In th e donor case one is filling both the donor orbital a nd
the m etal levels interacting with it . Th e interactions become
four-electron destabilizing ones,*4 and conformational pref-
erences ar e set by seeking out the site of least interaction. For
ds-dlo tha t is the eqll orientation,
I 6
eq, eqt1
15
16
'The analysis of cases with less than eight d electrons requires
some care. On e has to distinguish between instances of weak
and strong 7r interaction. Th e situation illustrated in Figur e
5 is th at of weak interaction-namely. the pertu rbation caused
by the substituent is smaller than t he energy separation between
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Transi t ion Metal Pentacoordinat ion
Inorganic Chem istry,
Vol. 14, No. 2, 1975 369
/?
Figure 5.
In t e ra c t i o n d i a gra m
fo r
a we a k e q u a t o r i a l a c c e p t o r
(top)
or d o n o r (b o t t o m ) . Th e e q o r i e n t a t i o n o f t h e s i n gl e - fa c e d
7r
d o n o r
or a c c e p t o r i s a t t h e l e f t ; t h e e q l o r i e n t a t i o n a t t h e r i g h t .
e’ and e” set by the bonding. With s trong
x
acceptors or
dono rs the ratio of eqli and e qJ- interactions may rem ain the
same, but the level ordering might be l ike that in Figure 6.
The f igure is based on calculat ions wi th model donors or
acceptors. No te that in the weak interaction scheme, Figure
5,
one would conclude that a low-spin d2 system with an
equato rial acceptor ligand would prefer
qli
but from the strong
interact ion diagram, Figure
6,
o n e w o ul d ch oo se e q i . W e
think it is better to leave the analysis of individual cases to
the reader-the diag ram s are complex, but the principles
required for their construction ar e now understood.
An especially interesting case of such an interaction is for
a s ingle e thylene l igand. Th e ethylene x orbi ta l makes for
roughly equivalent bonding in an y position, but the ethylene
x* acceptor orbi ta l carr ies the sam e symmetry propert ies as
the single p orbital in
15
and 16. It follows that a coordinated
olefin will prefer a n orienta tion such as
17
rather than
18.
This
17
18
i s indeed the conformat ion found in a number of such
structures.25-32 In one complex, Os( CO )(N O)( C2 H4 )-
(PPh3)2+, for which a structur e like 17 is indicated, a barrier
to olefin rotation of 9.5 kcal/mol has been deduced.33 For
a series of complexes Fe(C0)4(olefin) the rotational barrier
would a pp ea r to be higher sti11.34 W e would pred ict an increa se
in that barrier with increasing x-acceptor capability of the
olefin.35
To turn from ethylene to other s ingle-faced donors or ac-
ceptors an interesting possible case of a don or orientation al
preference is available in the structure of dichlorotris(
1,2-
dimethylimidazole)copper(11), a d9 complex.36 Th e structu re,
shown schematically below, is a trigonal bipyramid with two
I
U
A C C
E PTO R
Figure 6 . In t e ra c t i o n d i a gra m fo r a s t ro n g e q u a t o r i a l a c c e p t o r :
e ql l a t l e f t ; e q l a t r i g h t .
I
Q
19
dimethylimidazole units axial and one equatorial. Th e or-
ientation of the equato rial imidazole ligand is of inte rest. If
we view it as a x donor, the observed orientation indeed is eqll.
Of course it ma y be th at steric and n ot electronic effects set
the observed solid-state preference.
Another ins tance of an equatoria l subs ti tuent wi th an o r-
ientational degree of freedom is that of
a
ni t rosyl . The
fascinating story of the pentaco ordina te nitrosyls is told
el~ewhere.37~38n most “good” trigonal bipyramids the M N O
angle is close to
180’.
However, many of the nitrosyl struc tures
ident i f ied as square pyramids are in fact intermediate
in
geom etry between the two extremes. An ex ample is IrC12-
(NO)(PPh3)2, wi th a PIrP angle of
170°,
while ClIrCl is
1 5 7 O . 3 9 Th e bent nitrosyl eclipses the PIr P axis, which better
P
P
20
approximates the axial locus of the trigonal bipyramid. No te
that th e observed bending puts the
NO
acceptor o rb it a l eq i ,
the dono r orbital eqll, as expected. A similar situation occurs
in IrI(C H3) (NO ) (PPh3)2,40 but not in IrX ( CO )(NO ) (PPh3)
2+
W e proceed to a n analysis of the site preferences of cyl-
indrically symmetrical x donors and acceptors. Th e requisite
information is already a t hand, namely, the estimate that the
ordering of interactions is e q i
>
eqll
=
ax. Figure
7
shows
a comparison of axial and equato rial substitution by donors
and acceptors, in the weak-interaction limit. For d*-dlo the
conclusion follows that a x acceptor will favor an eq uatorial
site; a x donor, an axial one. For complexes with less d
electrons an evaluation of the strength of the x bonding must
x = I, c1 41
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370 Inorganic
Chtvnistry,
Yo] .14, iz‘o.
2,
1975
ACCE PTOR
Angelo R . Kossi and Roald Hoffmann
DONOR
Figuse
7.
I n i e r ac l i on d i ag r am
fo r a
cyl indr ical ly sym metr ical ac-
cep to r ( t op ) and
donor (bottom).
T he l e f t
side of
t h e f igure
has
t he subs t i t uen t i n t he ax i a l s i t e ; t he r i gh t s i de , i n
the
equa to r i a l
po-
si t ion.
be m ade f i rs t, to see whether F igure
7
or another diagram,
appropria te to s t rong 7i interac tion, is suitable.
Of course the choice between substitution sites is a resultant.
of preferences set by both the
i-
and 7r-donating capabilities
of
the ligand in question. Our conclusions often are parallel
for
t he iand
R
system (for d* both good a donors and good
R
donors will prefer the axial site). Given that m any ino rganic
ligands understandably show opposite a and K character , i.e.,
a re good i donors and good T
acceptors,
we are often led to
a hazardous evaluation of the relativc strength of i and
T
bonding. If we nevertheless persist we find that o ur conclusions
receive some support, though they a re not unambiguous, fro m
exist ing s t ructurai an d dynamic s tudies .
Osbo rn,Q Churchill,43 Raymond,*3 and their coworkers have
generalized that strong x-acceptor ligands will tend to occupy
equato rial sites in a trigonal bipyram id. This conclusion is
based on cry s ta l s t ructures such as those of
2 1 > 4 422,45
23 3
24,46 X , ? 3 and 26,47 as wel l as s ia t ic and dynamic nmr
studies.42%4* t is also in terestin g
i o
iioke the co ntrast between
the phosphine positions in 22 and 23. The methyl group, an
excellent idonor, appe ars in the axial site, in accord with ou r
theoretical conclusion.43 (py)F e(C0 )4 and (pyr) Fe(C0 )4 have
recently been characte rized? In both
of
these d* compounds
the ni t rogen base, a poorer acceptor than
CO
occupies the
axial site.
I n
Tr(CO)3(PMezPh)z+ the better acceptors, the
carbonyls, occupy the equatorial sites.50 Molecular orbi ta l
calculations on several substituted F e(C 0) 5 derivatives have
been carried out.72
Th e next topic to be discussed is the relative strength of axial
and equ atorial bonds in trigonal-bipyramidal complexes. W e
have alre ady mentioned in a previous section the trends to be
expected from
a
bonding. Th e analysis of
T
bonding is a
corollary of the energetic scheme of Figure 7 and th e attached
0
I
P
1
21 22 2 3
0
PPhpMe
C
I l -
24
2s 26
discussion. For d8-d10 com plexe s a cylindrically symn icirical
K
acceptor will strengthen the metal-ligand bond, but
i t
will
do so more when the acceptor is equatorial th an when
i t
is axial.
For the cor responding d W l 0 donor case , t he donor,’s f o w
electron destabilizing interaction will weaken both axial a r i d
equatorial bonds, but there will be differentially more
weakening
of
the metal-ligand bond when
t.hc
donor ligand
is equatorial.
It will be recalled tha t th e cr-bonding effect
i s
that i n
d 8
the
axial bond
is
stronger and in d10 it is weaker. T h u s in thc
interesting and common case
of
d* complexes we
h v c
t he
conclusion that if the ligands are K donors both a- and
T
bonding effects cooperate to make for
a
strong or short axial
bond and a weak or long equatorial bond. Wh en the ligands
are good K acceptors, unfortunately the a and P effects oppose
each other .
The results of (mainly) X-ray crystallographic stildics of
ML5 systems are presented in Tab le
I.
Since there is a hcasy
atom in the system, the precision of the M L distances
is
inherently limited. Nor have we tried to analyze critically the
reliability of the structu re determ ination s. Th e trigonal-
bipy ramid al d8 cases all have R-acceptor ligands. Th e
equatorial bonds are generally longer, which would be con-.
sistent with the
i
effect dominating. It is hazardous to ma ke
anything of the small axial-equatorial bond length diffe-Lnce
in the opposite direction in Mn(C 0)5 -, but a carbonyl ligand
on a negatively charged metal atom creates a situation of greal
relative acceptor streng th. Perha ps the K effect is coming to
the fore here . For dl0 cases an ou tstandi ng exception to
our
expectations, noted as such by others as
we11,6.7.51
is CdC1+.
T he axial bonds should be longer than the equatorial ones, but
in the avai lable crys ta l s t ructure they appear to b e shorter .
There are numerous s t ructures in which not all the ligands
are identical, but nevertheless
a
similar ligand occurs in bcth
equa torial and axial or apical and basal sites. ’These also
provide a probe of our theoretical conclusions. By way
of
examp e two such stru ctures are shown in
27
and
28.
n O C
25”
T h e d *
ML4L’
structures, of which ther e ar e many,6,’350352
show no consistent trend . Whe ther this is due to a trans effect
of the lone axial l igand rema ins to be analyzed . Less sym-
metric al structure s with polydentate ligands. of which
2’7
is
but one exam ple among many,20~-5? ften do possess structures
showing the theoretically anticipated trend for a domin ant
a
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Transi t ion Metal Pentacoordinat ion
T a b l e I ML Bond L eng ths in S o m e M L , S t r u c tu r e s
Inorganic Chemis t ry, Vol
14,
No.
2,
1975
371
M-L, a
Geom- Con- E qua to r i a l
Molecule et rya f ig& Axial
or
ap ica l
or
basal Ref
Nb( NMe, ) ,
C , ,
d n 1 . 9 8 2 . 04 C
MnC1, '-
C4
d 4
(hs)
2 . 58 ( 2 . 46 ) 2 . 30 ( 2 . 27 ) d
Ni(CN),
3
I n t d 8 1 . 8 4 1 . 9 9 , 1 . 9 1
g
N i p s 2 +
D,h
d 8 2 . 1 4 2 . 1 9
h
F e ( C O ) ,
D h d 8 1 . 8 1
1 . 8 3
i
Pt ( SnC13) j3 -
D , h
d 8 2 . 54 2 . 54
k
Mn( CO) , -
D h d 8 1 . 8 2 1 . 8 0
m
CUC1,
3
D,h dq
2 . 30
2 . 39 n
CuBr
3-
D 3 h
d9
2.45
2 . 52
0
F e ( N , ) -
D h
d s ( h s ) 2 . 0 4 2 . 0 0
e
C O ( C , H , N O ) , ~ +
D3h
d 7 ( h s )
2 . 10 1 . 98 f
c
d 8 2 . 1 7 1 . 8 6
C o ( C N C H , ) , I
D , f 1 d 8 1 . 8 4
1 . 8 8 i
Pt ( GeC1 , ) ,3 - I n t d8 2 . 40 2 . 43
1
CdCl
3-
D,h d" 2 . 53 2 . 56
P
InC1,'-
C , ,
d " 2 . 4 2
2 . 4 6 4
Sb(C, H
j 5
C , ,
d l ' 2 . 1 2
2 . 2 2 S
AsF
D , h
d" 1 . 71 1 . 66 r
a Int impl ies a
C , ,
s t r u c tu r e i n t e r m e d i a t e b e t w e e n
D 3 h
a n d
C 4 , .
See re f 4 f o r a de t a i l ed d i scussion o f such s t r uc tu r es . b hs
=
high
spin.
( 1971) . S imil a r d i s t ances wer e ob t a ined f o r
a
pen tak i s ( p ipe r id ina -
t o ) n i o b i u m c o m p l ex .
Chem.
Cor nmun . , 803 ( 1971) . T he d i s t ances i n pa ren theses a r e
fo r
ano t he r sa l t , w i th a phena n th r o l in iu m ca t i on : M. Mat su i ,
S.
K o d a ,
S.
O o i ,
H.
K u r o y a , a n d I . Ber na l ,
:hem.
L e t t . , 5 1 ( 1 9 7 2 ) . e J.
S.
Wood and
J .
D r u m m o n d ,
Chem. Comrnun.,
1 3 7 3 ( 1 9 6 9 ) ;
W.
Beck,
W.
P . Feh lhamm er , P . Po llmann ,
E.
Schu ie r e r , and K . Fe ld l ,
Chem.
Ber., 100,
2335 ( 196 7) . T he d i s tances l i s ted i n t he t ab l e a r e f r om
r ef 6 . T he g iven equa to r i a l Fe - N d i s t ance i s somewh at l onger t han
in t h e o r ig inal c r ys t a l l og r aph ic r epor t . f B. A . Coy le and J A. Ibers,
I no r g .
Chem. ,
9 , 7 6 7 ( 1 9 7 0 ) .
Terzis, K.
N.
R a y m o n d , a n d
T. G .
Spi r o , Inorg.
Chem. ,
9 , 2 4 1 5
( 1 9 7 0 ) a n d t h e r e c e n t p a p e r b y F . A . J u r n a k a n d K . N .
R a y m o n d , ih id . , 1 3 , 2 3 8 7 ( 1 9 7 4 ) .
Ni(CN), conta ins two crystal lographical ly dist inct anions. The
ligand is 2,8,9-trioxa-l-phosphaadamantane:
R. A . Jacobs on , I no r g .
Chim.
A c t a , 4 , 4 0 7 ( 1 9 7 0 ) . I B. Beagley
a n d D .
G .
Schmid l ing , J .
Mol .
S t r u c t . , 2 2 , 4 6 6 ( 1 9 7 4 ) ;
B.
Beagley,
D.
W.
J .
Cr u ickshank , P .
M.
Pinder , A . G . Rob ie t t e , and
G. M.
She ld r i ck , Ac ta Crystallogr. Sect.
B ,
2 5 , 737 ( 1969) . See
a l so A . Almenn ingen , A . Haa l and , and
K.
Wah1 , Acta
Chern.
Scand . ,
2 3 , 2 2 4 5 ( 1 9 6 9 ) ; M. I. Davis a n d H. P . Hanson , J.
Phys . Chem. , 69,
3 4 0 5 ( 1 9 6 5 ) ; 7 1 , 7 7 5 ( 1 9 6 7 ) ;
J .
D o n o h u e a n d A . C a r o n , ihid., 7 0 ,
6 0 3 ( 1 9 6 6 ) ; 7 1 , 7 7 7 ( 1 9 6 7 ) ; A c t a C r y s t al l og r ., 1 7 , 6 6 3 ( 1 9 6 4 ) .
I
F. A . C o t t o n , T . G . D u n n e , a n d J S. Wood, I no r g .
Chem. ,
4 , 3 1 8
( 1 9 6 5 ) .
S to lbe r g , J.
Arner. Chern.
S o c . , 8 7 , 6 5 8 ( 1 9 6 5 ) . E .
D.
E stes and
D. J .
H o d g s o n , I n o r g .
Chem. ,
1 2 , 2 9 3 2 ( 1 9 7 3 ) . T h e st r uc t u re
is
c loser t o t he t r i gona l - b ipy r amida l lim i t , bv t t he ang les i n t he equa-
t o r i a l p l a n e a r e 1 1 1 , 1 0 7 , a n d 1 4 1 " .
The
d i s t ances g iven i n t h e
t ab l e a r e aver ages f o r t he t r i gona l -b ipy r amida l l im i t. T he ac tua l
s t r uc tu r e has a l ong bon d ( 2 . 48
A )
oppos i t e t he l a r gest equa to r i a l
ang le, wh ich i s cons i s t en t w i th our pr ed i c t i ons f o r t he squar e - py r a -
midal l imi t . B.
A .
F r e n z a n d
J. A .
I be r s , I no r g .
Chern.,
1 1 , 1 1 0 9
( 1 9 7 2 ) .
a n d
T.
Br ennan , "P r ogress i n Coor d ina t i on Chemis t r y , " M. Ca is , E d . ,
E l sevi e r , Ams te r dam , 196 8 , p 518 .
R a y m o n d , Z n o r g .
Chem. ,
1 0 , 2 6 0 4 ( 1 9 7 1 ) . p
E. F.
E pste in and I .
Bernal , J .
Chem. SOC.
, 3 6 2 8 ( 1 9 7 1 ) .
E ins t e in , and D. G. T u c k , I n o r g .
Chem. ,
8 , 1 4 ( 1 9 6 9 ) . . B.
Cl ippar d ,
Jr . ,
a n d L.
S.
Bartel l ,
ibid.,
9 ,
805
( 1970) . A . L.
Beaucham p, M.
J
B e n n e t t , a n d
F. A .
C o t t o n ,
J.
A m e r . Chem. SOC. ,
90, 6 6 7 5 ( 1 9 6 8 ) ; P . J. Wheat l ey , J .
Chem.
S o c . , 3 7 1 8 ( 1 9 6 4 ) .
effect.
A
particularly instructive series of square-p yram idal
Ni(I 1) complexes ha s been discussed by Orioli.*Oc Th e
high-spin dS complexes have approximately equal basal and
apical distances, with the basal distances perhaps slightly
longer. Th e low-spin complexes have much elongated apical
bonds , up to
0.6 A
longer than the corresponding basal
separat ions .
Bond length comparisons for dl-d6 complexes ar e rare.
In
C.
Heath and M. B . Hur s thouse ,
Chem. Cornmun.,
1 4 3
I Ber na l ,
N.
Ell iot , and R. Lalancet te ,
g Ref er ence 21b .
See also A.
T he s t r uc tu r e of Cr ( en ) -
E .
R.
Riede l and
R.
D.
Cr amer , R . V. L indsey , J r . , C . T . P r ewi t t an d
U. G.
Ref er ence 21a ; I. Ber na l, N . E l l i o t t , R . A . L a l ance t t e ,
S.
A .
Goldf i e ld and K . N .
D. S.
Br own, F . W. B .
one of the two isomers
of Ru(CO)(PP~~)~((CF~)~C~S~
e
have one triphenylphosphine group which is apical and one
basal, in square-p yram idal coordination.54 In this d6 complex
we would anticipate weaker basal bonding, and in agreement
Ru-Pba = 2.35
A
and Ru-Pap =
2.21
A.
a-Subst i tuent Effects in the Squ are Pyramid
The important donor and acceptor interactions in the C4v
geometry will now be analyzed. A pair
of
a-donor or -acceptor
orbitals in th e apical and b asal positions of a square pyram id
ar e shown in 29 and
30,
respectively. Th e molecular symmetry
2 9 3 0
remains C ~ Vpon apical substitution but is lowered to CS by
a single basal substituent. Th at this symm etry is so low will
prove a complication in our analysis. Th e d-orbital correlations
C4v
- S
a r e
c8
a '
a '
a' + a
a' '
The interacted pair of donor- or acceptor-framework
molecular orbitals of e symm etry in the apical position are given
in 31 and designated ap. For the case of basal
7r
bonding there
O P e,)
a p ( e y z )
3 1
ar e two framework orbi ta ls
of
a ' symmet ry and one wi th a"
symmetry which are capable of cons iderable interact ion.
These interactions for basal substitution are shown in 32. The
9
h I
bo , ,
yz bo , , ( z Z )
bo,(
x y
)
3 2
basal interactions which ar e parallel
to
the pseudo-C4" axis
ar e labeled a s ball while the one which is perpendicular is
d es ig n at ed a s b a i .
Is the interaction of the framework molecular o rbi ta ls of
a square pyram id with a substituent carrying
x
orbitals greater
in the apical or the basal position? In order to answer this
question, the impo rtant framew ork orbital-H orbital inter-
actions will be considered in detail . Over lap considerations
alone may not suffice , because the square-pyramidal energy
levels vary drastically with pyramidality angle, as was il-
lustrated in Figure 3. In th at figure note especially the slope
of the e (xz, y z ) levels, which a re strong ly involved in
T
bonding.
Just as the energy levels vary with the angle 8 (see
2
for its
definition),
so
do the imp ortant coupling overlaps between the
donor
or
acceptor orbi ta l and the framework orbi ta l . Tha t
variation is exhibited in Figu re 8. Th e ba l-x y and bal l-yz
overlaps decrease from their maximum at
8
=
180°,
the "flat"
squ are pyramid. Th e ap-xz,yz and ball-zz overlaps increase
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372
Inorganic Chemistry,
Vol. 14, No. 2,
1975
Angelo R. Ross i and Roald Hoffmann
V
05
I
I L O 170 160
150
140 130 120
I I I 1
Ang le of bend ing
8
-
Figure
8. Computed va r i a t i on of over l ap be tween donor
or
ac-
cep to r o r b i t a l s and t he f r ame wor k molecu l a r o r b i t a l s o f a squar e
pyr amid as a f unc t ion o f t he bend ing ang le 0 .
as 8 decrease s. Th e bal1-22 effect is easily understood by a
motion of the perturbing ligand toward the nodal plane of the
2 2 orbital.
T
bonding is maximized when the ligand is in that
nodal plane. Th e increase in the ap-xz or -yz overlap is
a
result
of
th e increasing hybridization, m ixing in of PX into dsz
and of py into dyz, alluded to in t he f irst section.
Calculations with model donors an d acceptors show tha t the
net ball interactio n, mad e up of z2 and y z components , i s
approximately cons tant as a function of 8. On e way of un-
derstanding this is to note th at one component rises as the other
one falls, as 8decreases from
180".
Another explanation comes
to mind if instead of z2 and yz one considers the linear
combinations ciz2 C ~ J J Z . n the y z plane such combinations
possess nodal lines rotated by some angle from those of their
components. This is shown below.
,
z
t Y
-
*
33
34
If the ligand b earing th e p orbital is located a t the dot in
33
o r
34,
then the two rotated d orbi ta ls a re such that one,
34,
has negligible T overlap with t he substituent, while the other,
33,
has m aximal d-p
T
overlap. T o put i t in another way yz
and
22
combine to
form
one combination which "follows" the
ligand position.
Th e net magn itude of th e ball interaction is, however, mo re
than that of a simple d orbital on the metal center interacting
wit h a ligand p. T he z2 comp onent of ball is significantly
hybridized away fro m the apical l igand. The re are important
consequences of this hybridization, which will be discussed
elsewhere.55 For the present case the consequence is that the
ball interac tion, by virtue of the p adm ixtu re in z2, acquires
partial p-p n charac te r . A t 8
=
180" the ball interaction is
greater than al l others .
Schem atically , the net interactions behave as in Figure
9.
Sever al corollaries follow.
1.
A single-faced H acceptor or donor in a basal site will
alway s have a greater interaction when it is in the bal or-
3
160 140 I 2 0
e
Figure 9.
Schemat i c r ep r esen t a t i on
of
t he deg r ee
of
i n t e r ac t i on
b e t w e e n d o n o r or accep to r o r b i t a l s and the f r amewor k molecu l a r
o r b i ta l s o f a squar e py r am id , a s a f unc t ion o f
t he
bending angle 0 .
Figure 10. I n t e r ac t i on d i ag r am f o r a cy l i nd ri ca l ly symmet r i c d onor
subs t i t uen t
in
t he ap i ca l and basa l pos i t i ons o f a squar e py r amid .
ientat ion. Whe ther an acceptor
or
donor assumes that or-
ientation will depend on the numb er of d electrons. Th e specific
preferences can be obtained by constructing interaction di-
agram s analogous to Figure
7 .
For a low-spin dg complex an
acceptor substituent should prefer the ball orientation, while
a donor should as sume b a i .
2. A cylindrical n donor
or
acceptor finds greate r interaction
in the basal s i te a t
8
= 180
and in the apical site at lower
8. In our mo del calculations the crossover comes at 8
=
175",
nearly a f la t squa re pyramid for a donor subs t i tuent , and at
smaller 8,- 60", for an acceptor. Most squa re pyramids have
0 =
150",
that i s , an Lap-M-Lba angle of -105".
Th e variety of interaction dia gram s that could result from
these conclusions is staggering, so that it is best to consider
some individual cases.
Th e vanadyl ion is a ubiquitous structural type in vanadium
chemistry.56
A
typical structure is that of VO(acac)z,57
exhibiting square-pyramidal coordination, an apical oxygen,
and
a
very short
VO
bond. Oft en a sixth ligand is weakly
coordinated trans to the oxygen58 One trigonal-bipyramidal
structu re, VOC12(NMe3)2, is known.59 These ar e formally
V(1V). The oxide substituent. formally
0 2 - ,
has two lone pairs
ready for interaction. W e have a dl case with a cylindrical
T
donor. Figure
10
compares basal with apical oxygen
substitution. Th e requirement in this case is to achieve
maximum stabilization for the donor (oxygen) lone pairs while
minimizing th e destabilization
of
th e lone "d" electron. Clearly
this is better accom plished when the oxygen is apical. No thin g
is changed if the system is viewed as V(I1) with a neutral
atomic oxygen ligand. Th e placement of the un paired electron
in an xy orbital is in agreement with other molecular orbital
calculations.60-63.56
C l e a r- c u t ex a m p le s of b a i
vs
ba , p re fe rences
for
-
8/16/2019 Koordinacija 5 - anorganska
9/10
Transi t ion Metal Pentacoordinat ion
single-faced ~onors or acceptors are difficult to find. In the
pentaamide complexes of Nb(V),G4
35,
the basal amide ligands
Inorganic Chemistry, Vol. 14,
No.
2,
1975
373
35
36 37
ar e twisted around the Nb-N bond, so that they assume a
geometry intermediate between parallel and perpendicular
orientations
of
the donor nitrogen lone pair and t he apical bond.
If electronic effects were dominant, we would expect in this
do, donor case the bail orientation. W e would guess tha t steric
effects would favor the b a i orientation and view the observed
intermed iate twist angle as an indication of the operation of
an electronic effect consistent with our analysis.
In the crys ta l s t ructure of bis phenoxyacetat0)triaqua-
copper(II),65 36, the d9 copper ato m is coordinated in basal
sites by two monod entate phenoxyacetate ligands. Of interest
to us is the orientation of the
OCO
planes of these ligands
relative to the copper coordination polyhedron. Th e aceta te
oxygen ligand c arries two donor functions. On e is the sp* lone
pair; the othe r, an oxygen p typ e lone pair which is part of a
delocalized four-electron
i~
system. It is likely that the donor
effect of the latter d ominates. For a d9 system a donor should
prefer a b a i geom etry, which is consistent with this structure.
Another case in which an oxygen atom serves as
a
basal ligand
is the high-spin dS system, of pentakis(trimethy1arsine ox-
ide)nickel(II)2+,66 37. Th e oxygen p type lone pairs assume
an approximate ball or ientation. W e would expect a b a i
preference, but that may be sterically impossible in this case.
If th e observed geometry is set by the steric constraints, then
it is easily seen from figure 10 why
a
high-spin complex is
likely.
It is interesting t o note a t this point the man y complexes
of F e( C 0) 3 with a co njugated diene,G7 exemplified by the
paren t compound C4H6Fe(C0)3 ,@
38.
Th e relative orien-
7
/Fe---
38
tation of the butadiene and th e Fe (C 0)3 moiety is that depicted
in
38
and is apparently common to all compounds whose
structures have been determined. Th e structure is to be viewed
as a sq uare pyramid, with th e diene occupying two basal sites.
If the ethylene components of the diene were to be viewed
individually, then their acceptor, a*, rbitals are in the favored
ball orientation.
A general problem for many of th e complexes under
considerat ion is tha t i t is difficult to determ ine what pa rt of
an observed conformational preference is du e to the electronic
effects analyzed by
us
and w hat par t is set by the often large
steric requirem ents of the coo rdinated ligands.
Acknowledgment.
W e are gra t e fu l to J. M. Howell, E. L.
Muet ter t ies , M . M . L. Chen, and M . Elian for valuable
discuss ions and comments , W e also wish to thank a referee
for some comments and corrections. Ou r research was
supported by the National Science Foundation through Gr ant
G P 28137.
Appendix
In o rder to perform calculations on model pentacoordinate
transition m etal complexes it is necessary to choose a suitable
transition metal along with a set of reasonable parameters.
Toward this end it was decided to perform on PtC142- an
extended Huckel calculat ion modif ied to incorporate a
Table 11. Pa ra me t e r s Use d in Calcu la t ions of Model
Pt(I1) C o m p l e x e s
O r b i t a l e x p o n e n t
Orb i t a l
1
2
H, , , eV
6 p 2 . 5 5 4 - 7 01
6 s 2 .5 5 4 - 1 1 . 6 4
5 d 6 .0 1 3 (0 .6 3 3 4 0 ) ’ 2 6 9 6 (0 .5 5 1 2 5 ) ‘ - 1 2 .2 7
’ u m b e r s in paren theses ind ica te coeff ic ien t of t h e c o n t ra c t e d
me mb e r o f t h e 5 d ra d i a l fu n c t i o n .
self-consistent ch arge refinement of th e diagonal on e-electron
Hamil tonian matr ix e lements ,
H I , ,
or Pt. Th e basis set of
valence atom ic orbitals for Pt was 5d, 6s, and 6 p wave functions
expressed as Slater-type orbitals. Th e radial part of the
platinum 5d orbital was taken as a normalized linear (con-
tracted) combination of two Slater orbitals with principal
quantu m number 5, whereas the
6s
and 6p orbitals were chosen
as a single Slater-typ e orbital. Th e mixing ratios for th e
contracted 5d orbitals an d the 5d exponents as well as the 6s
and 6p exponents for pla t inum were obtained from wave
functions computed by B asch and Gray. @ Th e Slater orbitals
used for C1 had exponents of 2.033 (3s and 3p); the HIIwere
-30.00 eV (3s) and -15.00 eV (3p).70 T he Pt-Cl distance was
2.33 A Th e diagonal Hamilton ian matrix elements, Hli , for
platinum were approximated by valence-state ionization
potentials, VSIP’s, an d the neutral values were obtained fro m
Cotton and Harris.”
A
l inear cha rge dependence for the
on platinum was used and th e slope was chosen as 2.5 eV. Th e
Hll or C1 were not corrected for charge. Th e charge iteration
procedure was allowed to continue until the char ges on both
the plat inum and chlorine did not change by m ore than 1
O
X
10-5 charge uni t between the kth and k th + 1 cycles.
A
summar y of the platinum param eters along with the final
self-consistent HII or platinum is given in Table 11. All other
calculations mentioned in this paper were extended Hucke l
calculations without charge iteration unless otherwise specified.
The parameters for platinum listed in Table I1 were used
throughout .
Th e ML 5 calculations referred to in the body of the paper
were for
a
PtL53- system, with the Pt-L distance 2.33
8
or
both trigonal-bipyramidal an d square-pyramidal geom etries.
Th e pseudoligands L- car ried a single 3s orbital, Slater ex-
ponent 2.033,
HI,=
-18.00 eV.
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and
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This diagram
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Treichel. E. Pitcher, R . B. King, and F.
G . A .
Stone,
J .
Amer. Che m.
So c . ,
83.
2593 (1961).
Here there are three structures, differing in the composition
of
t h e
phosphine ligand:
\I,
R. Churchill and S. . A Bezman. Inorg .
Chern..
11. 2243 (1972);
12,
260. 531 (1973).
C .
P.
Brock.
J . P.
ollman,
G .
Dolcetti.
P. H .
Farnham,
J. A
Iber.;,
J .
E. Lester, and C . A . Reed,
Inorg.
C h r m . ,
12,
1304 ( 1 9 7 3 ) .
P.
Porta, H. M. Powell, R.
J .
Ma\+by,and
1 M.
enanzi,
J . C-heni. Sot..
A . 455 (1967).
J.
R. Shapley and
J .
A. Osborn.
J . Amer.
C h em.
Soc. , 92.
6976
(1970):
D.
P. Rice and
J . A .
Osborn,
J .
Organometal.
Chem., 30.
C84 (1971).
F.
A
Cotton and J .
M
roup,
J . A m e r .
Chern. S oc . , 96, 3438
(1974).
.A. J .
Deeming and B.
1
haw.
J . Chem. Sor. A ,
2705 (1970); G . Raper
and W.
S .
McDonald, Acrn
Crysiallogr..
Sec t .
B , 29,
2013 (1973).
J . E. Huh eey. “Inorganic Chemistr .” Har per and Row ,he + York, N
Y . , 1972,
p 378.
( 5 2 )
K. Emerson, P. R. Ireland, and W . T. Robinson,
I n o r g . Cheni. ,
9, 436
(1970).
(53)
1
acconi,
Coord. Chem.
Rev..
8 ,
351 (1972).
(54) I .
Bernal,
A.
Clearfield.
J .
S. Ricci,
Jr . , A .
Balch,
,I.
S.
Miller. J . C‘hem
Soc.. Chem.
C o m m u n . . 39 (1973).
(55) M
lian and R . Hoffman n, to be submitted for publication .
(56) See reviews by
J .
Selbin , Chenz. R e b 65. I53 (1965);
Coord.
C‘her i z .
R e v . . 1 . 293 11966).
( 5 7 )
( 71 )
( 7 2 )
~1
~
R .
P.
Dodge,
D.
H . Templeton. and A . Zalkin,
J . C h e m Phy.r. , 35. 5 5
(1961): P.-K. Hon, R.
L.
Belford, and C . E. Pfluger,
ibid. , 43,
1323,
3111 (1965).
A s
in the stru cture of VO(acac)>.dioxane [K. Dichmann ,G . ll;imcr,
S.
C . Nyburg, and
VI;.
F. Reynolds,
Chenz. Commuiz.,
1295
(1970) ] o r
VO(NCS)4*-- [A . C. HanAl.
Ar ia Crystnliogr.,
17.
1 1 5 0
( 1 9 6 4 ) \ ,
J . E.
Drake, J . Vekris, and
J .
S. Wood, J . Cherii.
S o t .
A. I 000 I 968) .
C . I. Ballhausen and H B. Gra y, Inorg. C h em. . 1 . I 1 (1962).
Thi i
is
a calculation
on
an octahedral VO( H:0 )9+. \+it11 hc Mater tnolcciulc
trans to the vanadyl oxygen weakly held Tha t sixth ligand
pushes
up
the
z*
orbital relatibe
to i ts
position in Figure
10.
L. G .
Vanquickenbourne and
S.
P. M?cGl>nn,Theor.
Chim. cru.
9, 390
(1968). This is also a calculation
on VO(l-IzO)i*+.
Vanadyl porphyrin: Ci. Zerner and M. P. Goutertnan,
Inorg
Cheni . .
5
1699 (1966).
See also the related calculations
on
\.IXLs systems do -d* by
f : . \1.
Shustorovich, Yu. A . Budaev. and
Y u. V.
Kokunov.
Z h .
Strukr .
Khini . .
13.
I l l
(1972).
C. Heath and
M .
B. Hursthouse,
C h e r n Commur7..
143 (1971).
C .
1’.
Goebel and R. J. Doedcns,
Chem.
Comnzun..
839 (1970):
Inorg .
Chern., 10, 260
(1971).
S.
H . Hunter, K. Emcrson, and
G . A .
Rodlcy.
Che m.
Conirnun
,
I398
(1969).
See the numerous examples cited by H .U uinn and J . H
Tsai. i l d v c i n
Inorg. Cheni. Radiochem.. 12,
217 (1969). The structural features and
bonding of these complexes have been analbzed bq M . R. Churchill ,ind
R. Mason,
Advan.
Organomelu l . C h em. . 3
(1967).
Sec
nlso
F. A
Cotton. V.
W ,
Day. B. A . Frenz, K. I. Hardcastle, and
J .
h4.
Troup.
J .
A i n u
Che m. Sot. , 95, 4522
(1973).
B. F. Hallam and P. L. auson ,
J . Cheni. Soc.. 642
(1958):
0 .
S . Mills
and
G .
Robinson.
Proc. C h em.
Soc.,
London, 421
(1960).
H . Basch and H . B. Gray ,
Theor
Chirri.
A c t a , 4,
367 (1966).
R. Hoffmann, G .
D .
Zeiss. and
G .
V, Van Dinc,
J .
Amer.
Chr m. Soi . .
90. 1485 (1968);
E.
Clementi and D. L. Raimondi.
J . Chem.
Phys . . 38.
2686 (1963).
F.
A .
Cotton and C
R.
Harris.
Inorg. Chriiz.,6 .
369
I
967 .
%’,
L. .Thornsberr).
J r . . Ph.D.
Thesis. Tulane Lnive rsit),
K c a
Orlcms.
La. . 1974. See also H . S. I ldr ich. .1.
T .
Clague, and L..
C .
C u s a c h , lni
J . Qiiantum C he m . ,Symp. .Vo.
7, 239 (1973), for some calculation\ on
five-coordinate rhodium complexes.