Defining Reactivity Differences in Sterically Crowded (η⁵-C₅Me₅)₃M Complexes Based on Metal Size and Lanthanide vs Actinide Effects

Reactions between (η⁵-C₅Me₅)₃M and (η⁵-C₅Me₅)₂M′(μ-Ph)₂BPh₂ (M, M′ = La, Ce, Pr, Nd, Sm, and U; M radius < M′ radius) were examined to evaluate the relative steric crowding in the (C₅Me₅)₃M series as a function of metal size and 4f vs 5f electron configuration. The sterically more crowded (C₅Me₅)₃M complexes transfer (C₅Me₅)⁻ to (C₅Me₅)₂M′(μ-Ph)₂BPh₂ to generate less crowded (C₅Me₅)₃M′ products and (C₅Me₅)₂M(μ-Ph)₂BPh₂ in mixtures with equilibrium constants in the range that allows all four components to be observed by NMR spectroscopy. The (C₅Me₅)₃M to (C₅Me₅)₃M′ ratios depend on the difference in size of the two metals. Hence, (C₅Me₅)₃Sm and (C₅Me₅)₂La(μ-Ph)₂BPh₂ form a mixture with 99% (C₅Me₅)₃La and 1% (C₅Me₅)₃Sm, but the analogous reaction with (C₅Me₅)₂Nd(μ-Ph)₂BPh₂ contains 90% (C₅Me₅)₃Nd and 10% (C₅Me₅)₃Sm. In analogous reactions with (C₅Me₅)₂U(μ-Ph)₂BPh₂ and (C₅Me₅)₃Ln lanthanide complexes, a size dependence is also observed, but the (C₅Me₅)₃Ln complexes are favored over (C₅Me₅)₃U to a greater extent than expected based on size differences.