Ligand Substitution on Hru₃(μ-Cx)(co)₁₀(X = OMe or NMe₂). Syntheses and Characterizations of Three Isomeric Forms Hru₃(μ-Cx)(co)₉L and Kinetics of Substitution. Implications for the Mechanism of Cluster Hydrogenation

Ligand substitution by L = AsPh₃ or PPh₃ on Hru₃(μ-Cx)(co)₁₀, X = OMe or NMe₂, sequentially forms HRu₃(μ-CX)(CO)_10nLn, n = 1, 2, and 3, in which only one L is coordinated to each Ru atom. The “lightly stabilized” complexes Hru3(μ-Cx)(co)₉L', X = NMe₂, NMe(CH₂Ph), or N(CH₂Ph)₂ and L' = NCMe or py, can be prepared by the addition of a stoichiometric quantity of trimethylamine N-oxide to Hru₃(μ-cx)(co)₁₀in the presence of an excess of L'. The py ligand of HRu₃(μ-CNMe₂)(CO)₉(py) is coordinated on a bridged Ru atom and in the axial position trans to the CNMe₂ ligand (b-a isomer). Upon mixing L = SbPh₃, AsPh₃, PPh₃, P(c-C₆H₁₁)₃, or PBu₃ with HRu₃(μ-CX)(CO)L' immediate ligand exchange generates HRu₃(μ-CX)(CO)₉L in which the kinetic product contains an equatorially coordinated ligand on a nonbridged Ru atom (n-e isomer). This product then rearranges by an intramolecular process to give an equilibrium mixture, the same as that obtained by thermal substitution on Hru₃(μ-cx)(co)₁₀, containing a second isomer in which L is coordinated to a bridged Ru atom in an equatorial position (b-e isomer). The equilibrium constant depends upon the identity of both X and L. For HRu₃(μ-CX)(CO)₉(PPh₃) the n-e/b-e ratio decreases in the order: X = N(CH₂Ph)₂ (1:0) > NMe₂ (0.12:0.87) > OMe (0:1). For HRu₃(μ-CNMe₂)(CO)₉L the n-e/b-e ratio decreases in the order: L = SbPh₃ (2.7) > AsPh₃ (0.4) > PPh₃ (0.14) and PBu₃(1.5) > PPh₃P(c-C₆H₁₁)₃(0.14). These products have been characterized by spectroscopic methods and in the cases of HRu₃(CNMe₂)(CO)₉(py) and HRu₃(CN(CH₂Ph)₂)(CO)₉(PPh₃) by X-ray crystallography. The kinetics of thermal substitution by AsPh₃ on HRu₃(μ-CNMe₂)(CO)₁₀and by AsPh₃, PPh₃, and P(OMe)₃ on HRu₃(μ-COMe)(CO)₁₀were determined. The rate laws and activation parameters are consistent with a mechanism which is primarily CO dissociative. The rate of AsPh₃ substitution on HRu₃(μ-CNMe₂)(CO)₁₀is slower than for substitution on the OMe analogue (X = OMe, ΔH‡ = 26.6 kcal, ΔS‡ = 8 eu; X = NMe₂, ΔH‡ = 26.9 kcal, ΔS‡ = 3 eu). Activation parameters for replacement of AsPh₃ from Hru₃(μ-CNMe₂)(CO)₉(AsPh₃) by CO were also determined (ΔH‡ = 25.4 kcal, ΔS‡ = 2 eu). A mechanism is proposed for ligand substitution on Hru₃(μ-cx)(co)₁₀in which the CX ligand changes from a μ-η¹three-electron donor ligand to a M3 V five-electron donor in the transition state. These data in conjunction with results of a previous study suggest that the rate-determining step for dihydrogen elimination from H₃Ru₃-(COMe)(CO)₉to form HRu₃(COMe)(CO)₁₀under CO involves intramolecular rearrangement of the hydride ligands from bridging to terminal coordination sites.