The 13C nuclear magnetic resonance spectra of carbene and isonitrile complexes of chromium and tungsten and a reinvestigation of the 1H nuclear magnetic resonance spectra of phenyl carbene complexes

Abstract

The effect of changes in the electronic environment of the sp2 hybridised carbene carbon atom bound to the metal in a series of 18 complexes of the type (CO)5MCXY, where M = Cr or W; X = OR or NR1R2, and Y is an organic group, are discussed with the aid of 13C n.m.r. spectroscopy. The chemical shift of the carbene carbon is more strongly influenced by X than by either M or Y. Carbene complexes of the transition metals are better considered as metal-stabilised carbonium ions than as carbonyl (ester or amide) analogues. The resonances of the carbonyl ligands in carbene complexes are compared with those in isonitrile complexes of the type RNCCr(CO)5. Evidence is presented from both 13C and 1H n.m.r. spectra which shows that overall electron release rather than π→p interaction is important in the stabilisation of the carbene carbon by the group Y.