Low-Temperature Neutron Diffraction Study of HMn₂Re(CO)₁₄ and Studies of a Metal-Metal Exchange Equilibrium That Converts HMn₂Re(CO)₁₄ into HMnRe₂(CO)₁₄

The crystal and molecular structure of (CO)₅Re(μ-H)Mn(CO)₄Mn(CO)₅, prepared from reaction of Mn₂- (CO)₉(η¹-tolualdehyde) with HRe(CO)₅, has been determined from neutron diffraction measurements at 15 K: unit-cell constants, a = 9.145 (1) Å, b = 15.557 (3) Å, c = 14.040 (3) Å, β = 106.60 (2)°, monoclinic, space group P2₁/n, Z = 4, V = 1914.2 (6) ų. R(F²) = 0.110 for 4859 reflections with F₀² ≥ 3σ(F₀²) and (sin θ/λ)_max = 1.054 Å^-1. The Re-H distance (1.827 (4) Å) is longer than the Mn-H distance (1.719 (5) Å). Spectroscopic and crystallographic data indicate that a small amount (~9%) of (CO)₅Re(μ-H)Mn(CO)₄Re(CO)₅ has cocrystallized with the major component. Further evidence for the identity of (CO)₅Re(μ-H)Mn(CO)₄Re(CO)₅ comes from an independent synthesis by a known route. A mechanism is proposed that accounts for the formation of (CO)₅Re(μ-H)Mn(CO)₄Re(CO)₅ from the reaction of (CO)₅Re(μ-H)Mn(CO)₄Mn(CO)₅ with HRe(CO)₅. The equilibrium constant for the metal-metal exchange equilibrium, (CO)₅Re(μ-H)Mn(CO)₄Mn(CO)₅ + HRe(CO)₅ ⇌ (CO)₅Re(μ-H)Mn(CO)₄Re(CO)₅ + HMn(CO)₅, has been determined; K_eq= 1.00 ± 0.05 at 22 °C in C₆D₆.