isolobal analogy

Chapter 15 Parallels Between Main Group and Organometallic Chemistry

15-4 Cluster compounds

15-3 Metal-metal bonds

15-2 The isolobal analogy

15-1 Main group parallels with binary carbonyl complexes


Consider several parallels between main group and organometallic compounds.

7 electrons 17 electronsElectronically equivalent


Tetrahedral tetramers


Ronal Hoffmann in his 1981 Nobel lecture;

Hoffmann defined molecular fragments to be isolobal

if the number, symmetry properties, approximate energy and shape of the frontier orbitals and the number of electrons in them are similar-not identical, but similar.


Orbitals of octahedral and tetrahedral fragments


Cyclic trimers


The isolobal species Ir(CO)3, Co(CO)3, CR, and P

Structures resulting from combinations of Co(CO)3 and CR

15-2-1 Extensions of the analogy

1. The isolobal definition may be extended to isoelectronic fragments having the same coordination number.

2. Gain or loss of electrons from two isolobal fragments yields isolobal fragments.

Mn(CO)5 CH3, Re(CO)5 CH3[Fe(CO)5]+[Cr(CO)5]-

Mn(CO)5 CH3, Cr(CO)5 CH3+

Mo(CO)5W(CO)5

Fe(CO)5 CH3-

Ru(CO)5Os(CO)5


3. Other 2-electron donors are treated similarly to CO

4. Ligands η5-C5H5 and η6-C6H6 are considered to occupy three coordination sites and to be 6-electron donors

Mn(CO)5 Mn(PR3)5 [MnCl5]5- Mn(NCR)5 CH3

Mn(CO)5 CH3,

(η5-C5H5)Fe(CO)2

(η6-C6H6)Mn(CO)2

[Fe(CO)5]+ CH3,


5. Octahedral fragments of formula MLn (where M has a dx

configuration) are isolobal with square-planar fragments of formula MLn-2 (where M has a dx+2 configuration and L is a 2-electron donor).

Cr(CO)5d6

Fe(CO)4d8

[PtCl3]-d8

Pt(PR3)2d10

MLn MLn-2

Octahedral Fragments Square-planar Fragments

15-2-1 Extensions of the analogy5. Octahedral fragments of formula MLn (where M has a dx

configuration) are isolobal with square-planar fragments of formula MLn-2 (where M has a dx+2 configuration and L is a 2-electron donor).

Comparison of square-planar fragments with octahedral fragments


6 5 4 3 2

5 4 3

6 5 4 3

n-1

n-2n-3

15-2-2 Examples of applications of the analogy

The 5-electron fragment CH is isolobal with P and other Group 15 atoms.A potential application of this relationship is to seek phosphorous-containing analogues to organometallic complexes containing cyclic ligands such as C5H5 and C6H6

15-3 Metal-Metal Bonds

Single, double, triple, and quadruple bonds

We need to consider how metal atoms can bond to each other

15-3-1 Multiple Metal-Metal Bonds

Main group –highest possible bond order –--- 3 How about transition metal?

Bonding interactions between metal d orbitals


Relative energies of orbitals



Re(III) – 4 d electrons

Occupied by ligand electrons



Structures ?

staggered

eclipsed

Relative energies of orbitals


Bond order and electron count


The effect of population of δ and δ* orbitals on bond distances can sometimes be surprisingly small.


Quintuple bonds

Formal shortness ration = 0.774- Trans-bent geometry and apparent interactions

Influence of bridging ligandsNonlinearityMore complex interactions

Science, 2005 JACS, 2007 Angew. Chem. Int. Ed., 2008

Chromium(I) complexes with extremely short metal-metal bonds

15-4 Cluster compounds

Some of boron compounds exhibit similarities in their bonding and structures to transition metal clusters.

15-4-1 Boranes

13 bonding orbitals (= 2n + 1)

7 bonding orbitals (= n + 1) 6 bonding orbitals (= n) – boron-hydrogen

bonding11 nonbonding or antibonding orbitals

Each boron has four valence orbitals---- total 24 boron orbitals

6 of the hybrids

7 among remaining 6 of the hybrids and 12 of the unhybrides

s and 3p

15-4-1 Boranes

15-4-1 Boranes

Closo, nido, and arachno borane structure

Structures of closo, nido, and arachno boranes having 6 to 12 borons

15-4-1 Boranes

15-4-2 Heteroboranes

Isoelectronic species such as the carboranes (carbaboranes)CH+ unit is isoelectronic with BH.

15-4-2 Heteroboranes

Formally each C should be converted to BH

C2B8H10 B10H12

B10H12 - 2H+ = B10H102-

Therefore closo Many derivatives of boranes containing other main group atoms

15-4-3 Metallaboranes and metallacarboranes

Fe(CO)3 unit is isolobal with BH.

C should be converted to BH

Organometallic derivatives of B5H9

Orbitals of isolobal fragments BH and Fe(CO)3


As ligand

Carborane analogs of ferrocene.

Comparison of C2B9H112- with C5H5

-.

The approach used to describe bonding in boranes may also be applicable to bonding in carbonly clusters and other clusters.

Total # of valence electrons in cluster

# of electrons involved in framework bonding

# of electrons involved in metal-ligand bonding= +

Related to the classification of the structure as closo, nido……

Closo-B6H62-

26 valence electrons= 14 + 12

86 valence electrons= 14 + 72

Nine valence orbitalsFour valence orbitals

Five extra orbitals→ 10 more electrons per framework atom

15-4-4 Carbonly clusters


n designates the number of framework atoms


Metal cores for clusters containing seven skeletal bond pairs.

Seven metal-metal framework bonding pairs are the most common


The predictions of structures of transition metal-carbonly complexes, using Wade’s rules are often, but not always, accurate.

Co4(CO)12 60 valence electrons

nido

A trigonal bipyramid (the parent structure) with one position vacant

However, tetrahedral metal cores


Ionic clusters of main group elements (Zintl ions).


An extension of Wade’s rules has been described for electron counting

mno rule – states that for a closed cluster structure to be stable, there must be m + n + o skeletal electron pairs, where

m = # of condensed (linked) polyhedran = total # of verticeso = # of single-atom bridges between two polyhedra

A fourth term

p = # of missing vertices (e.g. p =1 for nido, p = 2 for arachno)


B12H122-

m = # of condensed (linked) polyhedran = total # of verticeso = # of single-atom bridges between two polyhedrap = # of missing vertices (e.g. p =1 for nido, p = 2 for arachno)

m = 1n = 12o = 0p = 0

13 pairs

m = 2n = 23o = 1p = 0

26 pairs

15-4-5 Carbide clusters

Carbon exhibits unusual coordination numbers and geometries not found in classic organic structures

Carbide clusters

Encapsulated carbon contributes its 4 valence electrons.

86 electrons


The formation of four C-Ru bonding orbitals

The octahedral Ru6 core has framework bonding orbitals of the same symmetry as in B6H6

2-.

Bonding interactions between central carbon and octahedral Ru6.


Examples of large clusters.

Encapsulated iron

Homework

Exercise 15-1~15-9

Problem 1, 2, 5, 6, 11, 21, 23, 25.

isolobal analogy

Documents

binary carbonyl

main group

bonding interactions

electron donors

transition

planar fragments

octahedral

wades rules