Nephelauxetic effect in paramagnetic shielding of transition metal nuclei in octahedral d6 complexes

Abstract

For transition metals, using reliable d electron radial wave functions, it was calculated that the d electron radial parameters, BRacsh and (rd-3), are equally influenced by change in the d-orbital occupancy. On this basis, introduction of the first power of the nephelauxetic ratio into the paramagnetic shielding term of octahedral d6-complex transition-metal nuclei is justified. Interpretations of rhodium NMR chemical shifts in rhodium(II1) complexes were analyzed. In the recent interpretation of metal N M R chemical shifts in octahedral d6 transition metal complexes,’ the paramagnetic shielding term (up) has been expressed by the ligand field parameters in the following way: (where ho is the vacuum permeability, p~ the Bohr magneton, ( rd-3)F the free-ion(atom) expectation value of d electron inverse cube distance, f13s the nephelauxetic ratio, and A E the energy of the ‘Alg fT i electronic transition). Some arguments for incorporation of the nephelauxetic ratio into the paramagnetic shielding term, based on molecular orbital analysis of the covalency effects, have been put forward.2 However, the general validity of the proposed relationship has been questioned by Bramley et since they would rather expect the third power of the nephelauxetic parameter to be incorporated in the paramagnetic shielding term. They suggested that rhodium chemical shifts in rhodium(II1) complexes are in accordance with that expectation. Therefore, I undertook further investigation of the theoretical foundation of eq 1, the results of which are presented here. Results Ramsey’s theory of nuclear paramagnetic shielding, applied to a metal with d6 configuration in a strong octahedral ligand field$J leads, in the molecular orbital to the expression: (1) (a) JuraniC, N. Inorg. Chem. 1980, 19, 1093. (b) JuraniE, N. Inorg. Chem. 1985, 24, 1599. (c) JuraniC, N. J . Magn. Reson. 1987, 71, 144. (2) JuraniE, N. Inorg. Chem. 1983, 22, 521. (3) Bramley, R.; Brorson, M.; Sargenson, A. M.; Schiffer, C. F. J . Am. Chem. SOC. 1985, 107, 2780. (4) Griffith, J. S.; Orgel, L. E. Trans. Faraday SOC. 1957, 53, 601. (5) Freeman, R.; Murray, G. R.; Richardson, R. E. Proc. R. Soc. London, Ser. A 1957, 242, 455. (6) Walstedt, R. E.; Wernick, J. M.; Jaccarino, V. Phys. Rev. 1967, 162, 301. (where 1, is orbital angular momentum operator). Molecular orbitals e,(t2g5eg:1T1g) and t2&t2 ‘?‘Alg), which in the ionic limit are reduced to the metal d,+$ and d, orbitals, respectively, contain information on the metal-ligand bond covalency. As a measure of the impact of covalency on the paramagnetic shielding term, the following ratio may be introd~ced:~?’ which allows eq 2 to be put into the form: