LAUSR

Abstract The structures and energetics of the experimentally known CpFeRe(CO)7 system as well as the related unsaturated CpFeRe(CO)n (n = 6, 5) systems have been studied by density functional theory. The experimental unbridged Cp(CO)2Fe–Re(CO)5 structure is the lowest energy CpFeRe(CO)7 structure by the substantial margin of 24 kcal/mol. However, it is a metastable structure since its disproportionation into the homometallic species Cp2Fe2(µ-CO)2(CO)2 and Re2(CO)10 is exothermic by ∼10 kcal/mol. Obviously this disproportionation process must have a high activation energy. The isomeric doubly bridged CpFeRe(µ-CO)2(CO)5 structure is not a genuine minimum in contrast to the CpFeMn(CO)7 system. The lowest energy structures for the unsaturated CpFeRe(CO)n (n = 6, 5) can be derived from the experimental and lowest energy unbridged CpFeRe(CO)7 structure by removal of CO groups from the rhenium atom. Higher energy CpFeRe(CO)n structures contain bridging CO groups, which may be either two-electron donor bridges involving only M–C bonds or four-electron donor η2-µ-CO bridges forming an M–O bond as well as M–C bonds. These higher energy structures include a triplet CpFe(µ-CO)3Re(CO)3 structure closely related to the experimentally known stable organometallic triplet CpFe(µ-CO)3FeCp with a σ + 2/2π metal–metal bond containing the two unpaired electrons.