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.