Binuclear manganese and rhenium carbonyls M2(CO)n (n = 10, 9, 8, 7): comparison of first row and third row transition metal carbonyl structures

Abstract

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic binuclear rhenium carbonyls Re₂(CO)_n (n = 10, 9, 8, 7) were determined using the MPW1PW91 and BP86 methods from density functional theory (DFT) with the effective core potential basis sets LANL2DZ and SDD. In all cases triplet structures for Re₂(CO)_n were found to be unfavorable energetically relative to singlet structures, in contrast to corresponding Mn₂(CO)_n derivatives, apparently owing to the larger ligand field splitting of rhenium. For M₂(CO)₁₀ (M = Mn, Re) the unbridged structures (OC)₅M–M(CO)₅ are preferred energetically over structures with bridging CO groups. For M₂(CO)₉ (M = Mn, Re) the two low energy structures are (OC)₄M(μ-CO)M(CO)₄ with an M–M single bond and a four-electron donor bridging CO group and (OC)₄MM(CO)₅ with no bridging CO groups and an MM distance suggesting a double bond. The lowest energy structures for Re₂(CO)₈ have ReRe distances in the range 2.6–2.7 Å suggesting the triple bonds required to give the Re atoms the favored 18-electron configuration. Low energy structures for Re₂(CO)₇ are either of the type (OC)₄MM(CO)₃ with short metal–metal distances suggesting triple bonds or have a single four-electron donor bridging CO group and longer M–M distances consistent with single or double bonds. The 18-electron rule thus appears to be violated in these highly unsaturated Re₂(CO)₇ structures.