REACTIONS OF 16-ELECTRON CP'M(NO)R2 COMPOUNDS [M = MO, W R = ALKYL, ARYL] WITH CARBON-MONOXIDE

This paper reports the reactions of 16-electron Cp'M(NO)R2 complexes [Cp' = CP (eta5-C5H5), Cp* (eta5-C5Me5); M = Mo, W; R = alkyl, aryl] with carbon monoxide. The reactions proceed in a stepwise fashion, and their outcomes are dependent on both the natures of Cp' and R and the experimental conditions employed. Thus, treatment of solutions of Cp'W(NO)R2 with CO under ambient conditions affords the corresponding 18-electron monoacyl species Cp'W(NO)-(eta2-C{O}R) (R) (Cp' = Cp*, R = CH2CMe2Ph, p-tolyl; Cp' = Cp, R = CH2CMe2Ph, CH2CMe3, p-tolyl). An initially formed carbonyl adduct is isolable for Cp*W(NO)(p-tolyl)2 at-38-degrees-C, but at higher temperatures it converts to Cp*W(NO)(eta2-C{O}-p-tolyl)(p-tolyl). The atom connectivity in CpW(NO)(eta2-C{O}CH2CMe2Ph) (CH2CMe2Ph) has been established by a single-crystal X-ray crystallographic analysis which was hampered by severe disorder. The cases having R = CH2Ar (Ar = C6H5, C6H4-4-Me) are unique in that exposure of the Cp'M(NO)(CH2Ar)2 complexes to CO does not produce isolable acyl alkyl complexes but rather leads to the reductive elimination of (ArCH2)2CO and formation of the well-known Cp'M(NO)(CO)2 compounds. Upon exposure to CO at higher pressures, only the (perhydrocyclopentadienyl)tungsten monoacyl alkyl complexes undergo insertion of a second equivalent of CO, thereby affording the novel bis(acyl) complexes CpW(NO)(C{O}alkyl)2, again probably via an initially formed carbonyl adduct. Such an adduct is detectable by IR spectroscopy for Cp*W(NO)(eta2-C{O}-p-tolyl)(p-tolyl), but it only persists in an atmosphere of CO. The spectroscopic properties of the CpW(NO)(C{O}alkyl)2 complexes are consistent with their being stereochemically nonrigid molecules having the instantaneous 18-electron molecular structures CpW(NO)(eta2-C{O}alkyl)(eta1-C{O}alkyl). A unified rationale of how steric and electronic factors influence the eventual outcomes of the carbonylation reactions is presented.