Heterometallic bonding between a first row transition metal and a third row transition metal: The cyclopentadienyliron rhenium carbonyls CpFeRe(CO)n (n = 7, 6, 5)

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) 2 Fe–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 Cp 2 Fe 2 (µ-CO) 2 (CO) 2 and Re 2 (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 η 2 -µ-CO bridges forming an M–O bond as well as M–C bonds. These higher energy structures include a triplet CpFe(µ-CO) 3 Re(CO) 3 structure closely related to the experimentally known stable organometallic triplet CpFe(µ-CO) 3 FeCp with a σ +  2 / 2 π metal–metal bond containing the two unpaired electrons.