CONTROL OF THE REACTIVITY OF TRANS-[MO-2(CP)(2)(CO)(Y-Z)(MU-SR)(2)] (CP=ETA-C5H5, Y-Z=CO OR CN-) BY THE SULFUR SUBSTITUENTS (R=ME, PR-I, BU-T, PH OR CF3) - CRYSTAL-STRUCTURE OF TRANS-[MO-2(CP)(2)(CO)(CNME)(MU-SCF3)(2)]

Reaction of trans-[Mo-2(cp)(2)(CO)(2)(mu-SR)(2)] (cp = eta-C5H5; R = M e, Pr-i, Bu-t, Ph or CF3; syn and anti isomers) with cyanide ion gave the corresponding cyanide complexes trans[Mo-2(cp)(2)(CO)(CN)(mu-SR)(2 )](-), except with R = Bu-t where no reaction was observed. For R = CF 3, two isomers having a syn orientation of the sulfur substituents wer e obtained. The nature of the R groups is shown to have a crucial infl uence on the site of the reaction of the cyanide complexes with Me3O+. Complexes where R = Me, Pr-i (syn and anti isomers) or Ph (anti isome r) were S-methylated, whereas N-methylation was observed for R = Ph (s yn isomer) or CF3 (anti and both syn isomers). This is ascribed to ele ctronic effects of the R groups which control the site of methylation by switching the reaction from orbital control (S-methylation) to char ge control (N-methylation). For R = CF,, the R groups also affect the reaction of the dicarbonyl precursor with a Y=Z substrate since the pr eferred site of attack is different for Y=Z=CN- and Y=Z=RNC.