Applications of ⁹⁵Mo N.M.R. Spectroscopy. XVIII. Relaxation Times, Quadrupole Coupling Constants and Partial Field Gradients in [Mo(CO)₅L] Complexes

Abstract

A study of ,⁹⁵Mo spin-lattice relaxation times (T₁) and linewidths of [Mo(CO)₅L] complexes, where L = PPh₃, AsPh₃, SbPh₃, pyridine and Cl^-, has shown that the relaxation times are due entirely to the quadrupolar mechanism, with no scalar coupling contribution to linewidth where molybdenum is bonded to a quadrupolar nucleus. Based on the literature value of the quadrupole coupling constant obtained by n.q.r . for the PPh₃ complex (1.972 MHz), the quadrupole coupling constants of the arsine and stibine complexes are determined to be 3.36 and 3.75 MHz respectively. These values, and that of the pyridine complex (2.80 MHz), are found to correlate with ligand partial field gradient parameters obtained from Mossbauer spectra of Fe^II complexes, and are rationalized in terms of metal- ligand bonding interactions. For Et₄N [Mo(CO)₅Cl], the correlation is very poor; this result is attributed to the effects of ion in solution.