A comparison of anionic transition metal hydrides and anionic transition metal carbonyls : establishing the factors which influence relative reactivity

Abstract

The anionic transition metal hydride, HM(CO)₄L⁻ (M = Cr, W; L = CO, P(OMe)₃, P(C₆H₅)₃, P(Me)₃) reacts with transition metal carbonyl dimers (Cp₂W₂(CO)₆, Cp₂Mo₂(CO)₆, Mn₂(CO)₈{P(C₆H₅)₃}₂, Mn₂(CO)₁₀, Co₂⁻ (CO)₈) to give μ-HM₂(CO)₁₀⁻ and the corresponding anionic carbonyl. Through the reaction of the hydride with different dimers, the relative reducing ability of the hydride and the resulting anion has been evaluated. This relative reducing ability order is compared with both King's nucleophilicity order of transition metal anions and the reactivity order of the group VI anionic hydrides. The reaction of CpMo(CO)₃⁻ with allyl chloride showed an "inverse" counterion effect; reactions are faster in the presence of interacting cations. The nucleophile (CpMo(CO)₃⁻) is found to attack at the α-carbon of allyl halide rather than at the γ-carbon. The ligating ability of the anionic metal hydrides, metal carbonylates, and traditional ligands to M(CO)₅° (M = Cr, W) has been compared. With respect to the electron donor-acceptor metal-metal bond, ν(CO)IR band resolutions for HFeW(CO)₉⁻ and CrMn(CO)₁₀⁻ have been attempted. The relative electron-donating ability of each ligand attached to M(CO)₅ (M = Cr, W) was assessed, based on Cotton-Kraihanzel force constants. Kinetic studies of CrMn(CO)₁₀⁻ dimer disruption by PR₃, a reverse process to ligation of ligand to M(CO)₅° (M = Cr, W) were carried out under pseudo-first order conditions of PR₃. The rate determining step is believed to be Cr-Mn bond-breaking. The leaving group ability of CpMo(CO)₃⁻ vs Br⁻ was compared both in the reaction of MH⁻ (MH⁻ = HW (CO)₄P(OMe)₃⁻, HW(CO)₅⁻, HCr(CO)₅⁻ HFe(CO)₄⁻ with M'-R (M'-R = CpMo(CO)₃(CH₃), CpMo(CO)₃(ΛΔ) and in the analogous reaction with BrΛΔ in THF at ambient temp. Due to the several sites available for hydride attack, several mechanistic pathways seem to be involved in the reaction of the anionic metal hydride with CpMo(CO)₃(ΛΔ). Several results suggest that CpMo(CO)₃⁻ is a better leaving group than Br⁻.