trans-Influence of anionic, neutral, and bridging ligands on the nuclear magnetic resonance spectra of methyl- and fiuorobenzyl-bis(dimethylglyoximato)rhodium(III) complexes. Some observations on bridge formation

Abstract

The ¹⁹F and ¹H n.m.r. spectra of a series of 3- and 4-fluorobenzylbis(dimethylglyoximato)ligandrhodium(III) com-plexes have been recorded and compared with those of the corresponding cobalt(III) complexes. The chemical shifts of the fluorine substituents show that the strong electron-donating effects of the metallomethyl substituent is composed of a small hyperconjugative contribution which increases markedly in the order ligand = l^– < [SCN]^– < Br^–≤ Cl^–≤[N₃]^– < [NO₂]^– < [CN]^–, and a very strong inductive contribution which increases slightlyinthesame order. The chemical shifts of the 4-fluorine substituents are paralleled by the ¹H chemical shifts of the benzylic methylene groups and of the methyl groups of the corresponding methylbis(dimethylglyoximato) ligandrhodium(III) complexes, and by the ¹³C–¹H coupling of the methyl groups in the latter complexes. The ¹³C chemical shifts show no correlation with any of the usual trans-effect parameters for a wide range of neutral and anionic vans ligands. Similar considerations have been applied to a number of bridged complexes in which fluoro-benzyl- and/or other alkyl-bis(dimethylglyoximato)-rhodium(III) and -cobalt(III) complexes are bonded to the two ends of cyanide, azide, or thiocyanate ligands.