244 Inorganic Chemistry, Vol. 17, No. 2, 1978
Bramley, Creaser, Mackey, and Sargeson
The p2 ofbital on chlorine is considered to form the Cl-C u bond. The low value of the 3sClresonance frequency means that either the C1-C bond is relatively ionic in the complex, which causes a large Npzvalue for chlorine, or that C1 C r bonding is occurring, which causes Npx and Npvto be less than two electrons apiece, or both. If a-bond ionicity were responsible, then the CCo3(C0j9group would have to be a rather strong u donor. The 35Clresonance frequency is in the range of that found for (CH3)2CHC1 and (CH3)3CC1.34 Evidence is available which indicates that the CCo3(COj9 group is electron withdrawing, however. Therefore, a considerable amount of C1-C a bonding must be occurring. This x bonding tendency between the R group and the apical carbon is consistent with the fact that the C-R bond is often found to be shorter than a single bond2 and the fact that v(C-C1) in ClCCo3(C0)9 occurs a t a rather high frequency in the infrared spectra (906 ~ m - l ) The . ~ ~N Q R data when coupled with the other data available are consistent with a significant amount of a character in the C-Cl bond. A highly ionic u bond is less consistent with the data available. The triethylsilyl derivative behaves as more of an electron-withdrawing agent than might be anticipated from its substituent parameter.28 It was not included as a data point in Figures 2 and 3. More work is needed with substituents having low-lying unfilled energy states before detailed comments can be made about their electronic participation with the cobalt atoms. In summary, NQR data at both ends of the R C C O ~ ( C O ) ~ complex (59C0 and 35Cl) indicate a significant amount of delocalization of electron density in the region of the apical carbon atom. This delocalization is responsible for extensive a transmission of electron density between the Co atoms and the substituent group, R.
B. R. Penfold and B. H. Robinson, Ace. Chem. Res., 6 , 73 (1973). G . Palyi,
