Cycloaddition and C–S Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with Alkynes and Phenyl Isothiocyanate

[Image: see text] Reactions of [MoReCp(μ-PMes*)(CO)(6)] with internal alkynes RC≡CR yielded the phosphapropenylidene-bridged complexes [MoReCp(μ-κ(2)(P,C):η(3)-PMes*CRCR)(CO)(5)] (Mes* = 2,4,6-C(6)H(2)(t)Bu(3); R = CO(2)Me, Ph). Terminal alkynes HC≡CR(1) gave mixtures of isomers [MoReCp(μ-κ(2)(P,C):η(3)-PMes*CHCR(1))(CO)(5)] and [MoReCp(μ-κ(2)(P,C):η(3)-PMes*CR(1)CH)(CO)(5)], with the first isomer being major (R(1) = CO(2)Me) or unique (R(1) = (t)Bu), indicating the relevance of steric repulsions during the [2 + 2] cycloaddition step between Mo=P and C≡C bonds in these reactions. Similar reactions were observed for [MoMnCp(μ-PMes*)(CO)(6)]. Addition of ligands to these complexes promoted rearrangement of the phosphapropenylidene ligand into the allyl-like μ-η(3):κ(1)(C) mode, as shown by the reaction of [MoReCp(μ-κ(2)(P,C):η(3)-PMes*CHC(CO(2)Me)}(CO)(5)] with CN(p-C(6)H(4)OMe) to give [MoReCp{μ-η(3):κ(1)(C)-PMes*CHC(CO(2)Me)}(CO)(5){CN(p-CH(4)OMe)}(2)]. The MoRe phosphinidene complex reacted with S=C=NPh to give as major products the phosphametallacyclic complex [MoReCp{μ-κ(2)(P,S):κ(2)(P,S)-PMes*C(NPh)S}(CO)(5)] and its thiophosphinidene-bridged isomer [MoReCp(μ-η(2):κ(1)(S)-SPMes*)(CO)(5)(CNPh)]. The first product follows from a [2 + 2] cycloaddition between Mo=P and C=S bonds, with specific formation of P—C bonds, whereas the second one would arise from the alternative cycloaddition involving the formation of P—S bonds, more favored on steric grounds. The prevalence of the μ-η(2):κ(1)(S) coordination mode of the SPMes* ligand over the μ-η(2):κ(1)(p) mode was investigated theoretically to conclude that steric congestion favors the first mode, while the kinetic barrier for interconversion between isomers is low in any case.