Systematic CO dissociation kinetics of (μ2-H)(H)Os3(CO)10L clusters (L=P-donor ligands)

Article abstract of DOI:10.1016/s0020-1693(97)05466-2

The kinetics of decarbonylation of (μ₂-H)(H)Os₃(CO)₁₀L (L=P(OPh)₃, P(OCH₂)₃CEt, PPh₃, P-o-tol₃, PEt₃, PCy₃, P-t-Bu₃) to form (μ₂-H)₂Os₃(CO)₉L in paraffin hydrocarbons have been studied over a range of temperatures. Rates are ～10⁵ times faster than CO dissociation rates of analogous Os₃(CO)₁₁L clusters but no appreciable kinetic isotope effect is observed at 60°C when L=PPh₃. Rates decrease with increasing net electron donor ability of the ligands L according to the equation log k=α_L+β_Lδ(¹³CO) (β_L=0.09±0.02 and 0.11±0.03 ppm^-1 at 50 and 100°C, respectively), only the largest substituent P-t-Bu₃ (Tolman cone angle 182°) showing an enhanced rate due to steric effects. The absence of any detectable steric effects when the cone angles of the substituents are ≤ 170° is attributed to a close balance between the opposing effects of Os-CO bond breaking and the contraction of the hydride-bridged Os-Os bond. This picture is supported by generally small or negative values of ΔS≠. These electronic and steric effects are quite small, the largest difference in rates corresponding to only ～4% of the average value of ΔG≠. However, this is due to a change of ～4 kcal mol^-1 in ΔH≠ that is almost balanced by a change in TΔS≠. It is the energy provided by strengthening the bonding in the hydride-bridged Os-Os region of the cluster that largely compensates for the energy needed to break the Os-CO bond and that leads to the greatly enhanced rates for these hydrido clusters. Reactions of (μ₂-H)₂Os₃(CO)₉L with other P-donor ligands L′ appear likely to lead to adducts (μ₂-H)HOs₃(CO)₉LL′ in which both P-donor ligands are attached to the same Os atom in a way that allows for isomerism depending on whether L′ is added to (μ₂-H)₂Os₃(CO)₉L or L to (μ₂-H)₂Os₃(CO)₉L′.