Canadian Journa canadien of Chemistry de chimie

Published by Publit? par THE NATIONAL RESEARCH COUNCIL OF CANADA LE CONSEIL NATIONAL DE RECHERCHES DU CANADA

Volume 52 Number 22 November 15, 1974 Volume 52 numCro 22 15 novembre 1974

Cobalt-59 Nuclear Quadpupole Resonance Studies on Five-coordinate Cobalt Complexes

LIAN SAI CHIA, WILLIAM R. CULLEN, AND MICHAEL C. L. GERRY Chemistry Department, University of British Columbia, Vancouver, British Colurnbiu V6T 1 W5

Received June 17, 1974

LIAN SAI CHIA, WILLIAM R. CULLEN, and MICHAEL C. L. GERRY. Can. J. Chem. 52,3695 (1974).

The n.q.r. spectra of a number of complexes of the type [Y,PCo(CO),], have been recorded. The results are compared with those from related studies.

LIAN SAI CHIA, WILLIAM R. CULLEN et MICHAEL C. L. GERRY. Can. J. Chem. 52,3695 (1974).

On a enregistre les spectres n.q.r. d9un certain nombre de complexes du type [Y3PCo(CO),12. Les rtsultats sont compares avec ceux rapportes dans des etudes apparentis.

[Traduit par le journal]

Recently there has been an interest in the cobalt-59 nuclear quadrupole resonance (n.q.r.) of axially symmetric five-coordinate cobalt com- pounds (1-5). These studies have mainly been concerned with complexes of the type Y,MCo- (CO), (Y = C1, CH,, c6H5 , etc., M = Ce, Sn) and their carbonyl substituted derivatives. The n.q.r. spectra of other axially symmetric cobalt complexes have received little attention. For example, for the large class of dimeric complexes of the form [Y,PGO(CO),]~, which contain a Co-Co o bond and the Y3P groups in the axial positions (6-9), only one spectrum has been reported (3) (for U = C6H5). Similarly for the ionic complexes [(Y,P),Co(CO),]+X- only one spectrum (3) (for Y = C6H,, X = Co(CO),) has previously been described.

To determine the effects of the substituents Y and the anions X- on the n.q.r. spectra of these complexes, both dimeric and ionic, we have obtained the spectra of several examples.

The following compounds were prepared by literature methods as follows : [Y,ECo(CO),], :

(E = P; Y = C6H,0 (10, ll), C 2 H 5 0 (lo), CH30 (lo), n-C,H9 (lo), C6H5 (10; Y, =

(C6H5)2(CH30) (lo); Y3E = (C6Hj)3As (lo), (C,H,),Sb (10)); (Y,P),Co(CO),'X-: (Y =

C6H, or n-C4H9, X = Co(CO), or B(C6H,), (12)); [(CH,O),PI,Co(CQ), +B(C6H5)4- (13).

Nuclear quadrupole resonance spectra were recorded using a Decca instrument as described previously (14). The recorded frequencies are listed in Table 1 . Note that some compounds did not give a detectable signal in the region 7-60 MHz. No signal attributable to the ion Co(CO),- was found.

Results and Discussion For 59Co, which has I = 712, both the quad-

rupole coupling constant e2Qq and the asym- metry parameter q can be obtained from the experimental frequencies using a series of ap- proximations for the transition frequencies (1 5). The resulting values can be confirmed with a frequency ratio plot (16). To carry out this pro- cedure a computer program was written to cal-

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3696 C A N . J . CHEM. VOL. 5 2 , 1974

TABLE 1. 5 9 C ~ n.q.r. spectra of [(Y3P),Co(CO),]+X- and [Y3PCo(C0)3]2 complexes

Compound Temperature ("C) Frequencya (MHz) rl e2Qq (MHz)

"The numbers in parentheses are signal to noise ratios. The first line gives v,, the second l ~ n e v,, and the third line v,. Experimental errors: t -0.050 MHz.

bReference 3.

culate the energy levels and transition frequencies for any values of the nuclear spin, e2Qq, and 11 (17).

Since Z = 712 for j9Co each nucleus should give three strong transitions, having frequencies v,, v,, v,, which are directly proportional to e2Qq, so that the ratios v2/v,, v3/v2, and v,/v, are independent of e2 Qq, though dependent on q. Accordingly the program was used to calculate the values for these ratios for a series of values of q differing by 0.0001. The observed ratios were then compared with the calculated ones to obtain q. In a similar fashion, because the transition frequencies are directly proportional to e2Qq, the calculated frequencies were ob- tained with e2Qq = 1. With q obtained, the calculated frequencies were divided into the

observed values to evaluate e2Qq for the complex. The averaged values of both e2Qq and q are given in Table 1 .

Table 1 also contains the results of Ogino and Brown (3). It is noteworthy that our data for [(C,H,),P)2Co(CO),] +Co(CO),- are in excel- lent agreement with theirs, with our frequencies slightly lower at the higher temperature. We were unable to repeat their measurements for [(C,H5),PCo(CO),I2; this was probably a func- tion of the nature of our sample.

The crystal structures of a number of deriva- tives of the type [Y,PCo(CO),], have been de- termined, and have the general structure 1 (6-9) where each cobalt atom is five-coordinate with the group V base in the axial positions. The low values of q recorded for the compounds in

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CHIA ET AL.: COBALT-59 NUCLEAR QUADRUPOLE RESONANCE 3697

TABLE 2. Colnparison of the 59Co n.q.r. constants" and asymmetry parameters" of substituted cobalt carbonyl dimers and related compounds

Compound L = CO L = P(OCH3)~ L = P(OChH5)3 L = P(C6H5)3 L = P(n-C4H&

"Coupling constants are in MHz, except for Fe(CO)?L, which are in mm s-'. Asymmetry parameters ( in parentheses) are unitless. hPresent work. =From ref. 3. dFrom ref. 4. eFrom ref. 2. ,From ref. 21. 9Two d~fferent resonances from the same complex hResonances from trio different complexes. 'From ref. 5: T = 77 "K.

parameter to intermolecular interactions in the solid state (18).

Y,P-co -Co ---PY, Table 2 contains a summary of the coupling

I / \ constants obtained in the present work together

CO CO co with data for related complexes. The coupling constants can be discussed in terms of the follow-

1 ing simple equation (1):

Table 1 are certainly in keeping with this struc- ture. It is interesting that the triphenylphosphite e2Qqm0, = e2Qq3,~[N,z2 + f(NdXz + Ndyz) derivative is normal in this respect since Boyd - (Nd,y + Nd,2-y2)1 and Brown (4) found that q for (C,H,),SnCo- (CO),P(OC,H,), was significantly greater than for other similar phosphine and phosphite derivatives. This increase was ascribed to con- formational effects of the triphenylphosphite. However, these same authors report that q for CI,SnCo(CO),P(OC,H,), is normal.

Theinfrared spectra (1 2, 13) of the((C,H,),P),- Co(CO),+X- compounds (X = Co(CO),, BPh,) show only one CO band attributable to the cation; this indicates a D,, structure 2. The small q values (0.1-0.2) of these compounds are also consistent ~ i t h the proposed structure. The variation in the values of q observed indi- cates once again the great sensitivity of that

where N, is the population of the appropriate Co 3d orbital; e2Qq,,, is the coupling constant of a single 3dZ, electron. It has been shown (1) that e2Qq,,, is negative, whereas e2Qq,,, is most likely positive, so that the last, negative term in the parentheses is greater than the sum of the first two.

Boyd and Brown (4) have suggested that the variations in 59Co resonance frequencies for XCo(CO),L complexes depend not only on the nature of X and L individually, but aiso on the interactions between them. Thus, for example, the trends observed on variation of L are ex- pected to be different when X is a strong n- acceptor from when X is a weak n-acceptor. In the case of the complexes studied in this work L is PY, and X is Y,PCo(CO),. Since the Co- Co bond is symmetrical a net o- or n-electron drift from one cobalt atom to the other is not expected. The complexes thus form the basis for a description when X is a very weak o-donor and n-acceptor. Indeed, one can go further and

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3698 CAN. .I. CHEM. VOL. 52, 1974

say that in the Co-Co bonded complexes, un- like all the other compounds in Table 2, the var~ations in e2Qq are functions of PY, alone. It is clear from our data that these variations are slight (about 64,). According to eq. 1 jlidZ2 gives a measure of o-donor ability, and (Ndx, + Ndyz) a measure of the n-acceptor ability of PY,. The small variation of coupling constants suggests that these orbital populations, and consequently the bonding properties of PY,, are virtually independent of Y. The same coaclusions can be reached for (C,H,),Sn, (CH,),Sn, Cl,Sn, in- deed for ebery other X.

Comparison of the coupling constants of the Co-Co bonded complexes with those of (C,H,),Sn - and ( C H , ) , S n derivatives illus- trates two noteworthy points: (i) The trends on varying Y in PY, are essentially the same and (ii) the coupling constants of the Sn derivatives are consistently 40-50 MHz below those of the dimers. Now ( C , H , ) , S n and (CH,),Sn- are relatively weak n-acceptors ( 1 , 4), and since Sn has a slightly hlgher electronegativity than Co they are probably slight o-acceptors as weli. Thus there is little surprise in the trends with Y in PY,. The 40-50 MHz general decrease of e2Qq in the Sn compounds cannot be accounted for in terms of the o- or rc-acceptor ab~lities of Sn, since both these should increase e2Qq. The decrease must thus result from a lowering of ( N d X y + Ndx2-y,), presumably as a secondary effect from the reduction of Ndi2, Nd,,, and

Nd,/ When the ligands X are strong n-acceptors

(e.g. C1,Sn-, C1,Ce-) the j9Co coupling con- stants increase again, presumably because of a decrease of (NdxL + ~ y i ) . This applies also to the ioil @o(CO),[P(C,H ,) , I ,+ , The effect of decreased o-donor ability when C1 replaces C,H, and CI-f, also helps increase e2Qq by decreasing Ndz2.

At the time of submitting this paper, we became aware that others (19, 20) have studied

some of the compounds described here. Their data are in good agreement with ours.

The authors are grateful to the National Research Council of Canada for financial support of this work in the form of operating grants (to W.R.C. and M.C.L.G.) and funds to purchase the spectrometer. One of us (L.S.C.) wishes to thank the International Nickel Com- pany of Canada, Limited, for the award of a scholarship. They would also like to thank Dr. T. L. Brown for comments and information concerning related work.

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1316 (1968). 10. A. SACCO. Ann. Chim. Rome, 43, 495 (1953). 11. G. H. WHITFIELD and H . W. B. REED. J. Chem. Soc.

1931, 1940 (1954). 12. (a) W. HIEBER and W. FREYER. Chem. Ber. 91, 1230

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13. S. ATTALI and R. POILBLANC. Inorg. Chim. Acta, 6, 475 (1972).

14. L. S. CHIA, W. R. CULLEN, M. C. L. GERRY, and S. Y. POH. Can. J . Chem. In press.

15. G. K. SEMIN and E. I. FEDIN. Eh. Strukt. Khim. 1, 252 (1960).

16. H. 6. ROBIS~OS. Phys. Rev. 100, 1731 (1955). 17. L. S. CHIA. Ph.D. Thesis. University of British

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