Isotope and temperature dependence of transition-metal shielding in complexes of the type M(XY)6

Abstract

The temperature coefficients of the chemical shift of /sup 51/V in carbonyl complexes in solution are reported here. For transition metal nuclei the effects of rovibrational averaging can be obtained from these temperature coefficients without gas-phase studies in the zero-pressure limit. They present a theory for the large temperature coefficients and large isotope shifts observed in transition metal complexes. They use the vibrational analysis of V(CO)/sub 6//sup -/ and Co(CN)/sub 6//sup 3 -/ to provide dynamic averages of the displacements in the V-C, C-O, Co-C, and C-N bonds as typical examples. The temperature and mass dependencies of these dynamic averages successfully account for the signs and magnitudes of the observed temperature coefficients and isotope shifts. They report observations of a correlation between the temperature coefficients and chemical shifts. This correlation is qualitatively consistent with a simple electrostatic model for shielding. The limitations of the electrostatic model for shielding are discussed.