Edge-bridging and face-bridging hydrogen atoms in trinuclear rhenium carbonyl hydrides

The structures of the trinuclear rhenium carbonyl hydrides H ₃Re₃(CO)_n (n = 12, 11, 10, 9) have been investigated by density functional theory (DFT). The only structure found for H₃Re₃(CO)₁₂ is the experimentally known structure with each approximately 3.3 Å edge of a singly bonded Re ₃ triangle bridged by a hydrogen atom and all terminal carbonyl groups. The two lowest energy H₃Re₃(CO)₁₁ structures lie within 0.5 kcal/mol in energy and are derived from this H ₃Re₃(CO)₁₂ structure by loss of a carbonyl group with retention of the central Re₃(μ-H)₃ core. A higher energy H₃Re₃(CO)₁₁ structure, closely related to the experimental structure for the isoelectronic dianion H ₃Re₃(CO)₁₀^2-, is also found in which one of the Re=Re edges of double bond length (2.79 Å) is bridged by two hydrogen atoms. Loss of a carbonyl group from this H₃Re ₃(CO)₁₁ structure with concurrent conversion of one of the edge-bridging hydrogen atoms to a face-bridging hydrogen atom gives the lowest energy H₃Re₃(CO)₁₀ structure.