The rich and varied chemistry of group 6 cyclopentadienyl nitrosyl complexes

The syntheses and characteristic physical and chemical properties of representative members of important types of cyclopentadienyl nitrosyl complexes of the Group 6 elements are reviewed. These organometallic compounds contain either 14-valence-electron (14e) Cp′M(NO) or 17e Cp′M(NO)2 core fragments [Cp′ = an η5-cyclopentadienyl ligand, most often η5-C5H5 (Cp) or η5-C5Me5 (Cp*); M = Cr, Mo, or W]. All the complexes originate from Cp′M(NO)(CO)2 which can be readily converted into the precursor complexes Cp′M(NO)2X, Cp′M(NO)X2 (M = Mo, W; X = halide) and [Cp′Cr(NO)I]2. The latter compounds are convenient starting materials for the synthesis of [CpCr(NO)2]2 dimer, [CpM(NO)2]+ cations, and neutral compounds such as 17e Cp′Cr(NO)(L)(X) (L = Lewis base; X = halide or alkyl), 18e Cp′M(NO)L2 (L = Lewis base), and 16e or 18e Cp′M(NO)(R)(R′) (M = Mo, W; R, R′ = hydrocarbyl or H). Consideration of the distinctive chemical properties of these compounds has been restricted to transformations in which the Cp′M(NO) and Cp′M(NO)2 scaffolds are retained. The synthetic utility of these types of cyclopentadienyl nitrosyl complexes of the Group 6 elements for mediating specific organic transformations is also highlighted throughout this review.