Nuclear magnetic resonance study of arene metal tricarbonyl complexes

Abstract

The n.m.r. spectra of all the methyl-substituted benzene complexes of chromium tricarbonyl, molybdenum tricarbonyl, and tungsten tricarbonyl have been obtained. The unusual features include a large upfield shift of alkene proton chemical shifts on complex formation, but within the metals the order of increasing shift is Cr > W > Mo, a non-periodic trend. It has also been observed that the individual alkene proton chemical shifts in an unsymmetrical methyl benzene become markedly different on complex formation, in contrast to the behavior found in the free arene. Symmetrical methyl-substituted benzene metal complexes, on the other hand, show the same symmetry characteristics as the free arenes, and hence, on an n.m.r. time scale, there is no permanent distortion of the ring. A theory is presented, invoking partial three-point bonding of the M(CO)3 moiety to the arene ring. This bonding is postulated to occur most strongly at arene carbon atoms bearing methyl groups and, as a consequence, results in an abnormally strong electron withdrawal from carbon positions meta to this carbon, even though this meta position may be unsubstituted.