114
H.A. KUSKA,M. T. ROGERS, AND R. E. DRULLINGER
the spin density is ?r delocalized through a configuration interaction mechanism similar to that proposed by Kivelson and Lee20 and by Fortman and Hayes.21 Esr Theory. Although the revised equations (15’) and (16’), which take into account the copper 4s contribution do give the apparent correct ordering of covalency for this series, the agreement cannot be extended to other copper complexes. For example, Figure 2 is a plot of the value of the nitrogen hyperfine splitting values, A N us. the a2 values calculated from eq 15’ and 16‘. If the sp2 hybridization of the nitrogens in different copper chelates remains constant, then the A N values are expected to be good indications of the covalency; however, no apparent correlation is found in Figure 2. Also, in Table IV there are listed a number of compounds for which eq 15 predicts a more covalent bond than predicted by eq 16. This discrepancy cannot be explained by a 4s contribution. It could be attributed to admixture of z2 and/or 4p orbital into the xy ground state. The possibility of z2 admixture could be checked by solvent effect esr data on a square-planar copper chelate which has only copper-nitrogen bonds to see if the change in covalency as predicted from eq 15”, 8’, lo’, and 11’ correlated with the change in covalency as predicted from the nitrogen splittings with
the use of O’Brien’s equationss for the dependence of the ligand splittings on 6.
a**
a2 f
Ref
Bisdithiocarbamate Bis-%hydroxyquinoline Tris-l,l0-phenanthroline Bismaleonitriledithiolate
0.582 0.818
0.474 0.763 0.697 0.489
Q
b C
d
’ R. Pettermon and T. Vannghrd, Arkiv Kmi, 17,249 (1960). G. F. Kokoszka, H. C. Allen, Jr., and G. Gordon, J . Chem. Phys., 42, 3730 (1965). e H. C. Allen, Jr., G. F. Kokoszka, and R. G. Inskeep, J . Am. Chem. SOC.,86, 1023 (1964). A. H. Maki, N. Edelstein, A. Davison, and R. H. Holm, ibid., 86, 4580 (1964). Calculated from the isotropic data with eq 16. Calculated from the anisotropic data with eq 15.
’
The J o u r d
of
Physical Chemktry
B. R. MCGARVEY (Polytechnic Institute of Brooklyn, Brooklyn). I do not feel that a vibronic interaction could introduce enough 4s character to account for your results, but, if we were to accept your explanation, your results would indicate that the 4s character increases as the bond becomes more ionic. Do you have any comments? H. A. KUSKA. The admixture is directly proportional to the metal-ligand vibrational frequency and is inversely proportional to the xy to 211 energy separation. From compound 8 in Table I to compound 1 the metal-ligand vibrational frequency d e creases from 455 to 415 cm-1 and the zy to z* energy separation decreases from -14,500 to -12,000 cm-*. It would appear that the decrease in the energy separation is the dominant effect since the 4s contribution increases. In Table I1 the f* values indicate that for the highest symmetry complex (compound 8) only 0.003 4s character is required, and this amount would be reduced if either our approximations and choice of values for the constants were such that O L ~ is too low or as is too high. If the 4s character is due to a vibronic mechanism, we would expect an inverse dependence of A on temperature. An inverse dependence has been reported for copper acetylacetonate; see ref 10.
N. Y.). Could the 4s character, required in the ground state of the copper complexas to account for the observed hyperfine
Ligand
0.617
Discussion
K. KRIST(Fairchild Camera and Instrument Corp., Syosset,
Table IV: Esr Data for Copper Complexes Which Cannot Be Explained by Only a 4s Correction
0.804
Acknowledgment. We wish to thank Professor J. P. Fackler, Case Institute of Technology, and Professor R. H. Holm, University of Wisconsin, for helpful discussions.
splittings, arise because of zero-order orbital mixing between the Cu 4s and 3d,, orbitals? Since it has been proposed that d,, and d,, orbitals are split in the complex, it would seem that the symmetry is low enough for this to be the case.
H. A. KUSKA. In D2h zy contains the unpaired electron and transforms as Bt,; the z* orbital transforms as At,. In CsV (which would be the symmetry if one solvent molecule coordinates along z ) xy is A2 and 2%is AI. In Clh (0-CR‘-CHCR”-O with R’ # R”) both xy and z* transform as A, and would mix. (20) D. Kivelson and 9. K. Lee,J . C h m . Phys., 41, 1896 (1964). (21) J. J. Fortman and R. G . Hayes, &id., 43, 15 (1965).
