Comparison of Isoelectronic Heterometallic and Homometallic Binuclear Cyclopentadienylmetal Carbonyls: The Iron–Nickel vs. the Dicobalt Systems

The heterometallic binuclear cyclopentadienylironnickel carbonyl compounds Cp 2 FeNi(CO) n ( n = 3, 2, 1; Cp = η 5 ‐C 5 H 5 ) have been studied by density functional theory (BP86) for comparison with the isoelectronic homometallic dicobalt derivatives Cp 2 Co 2 (CO) n . The FeNi tricarbonyl is shown to be the doubly bridged isomer Cp 2 Fe(CO)Ni(μ‐CO) 2 with an Fe–Ni distance of 2.455 Å (BP86), in accord with experiment and in contrast to Cp 2 Co 2 (CO) 3 where singly and triply bridged but not doubly bridged isomers are found. The dicarbonyl compounds Cp 2 FeNi(μ‐CO) 2 and Cp 2 Co 2 (μ‐CO) 2 both have analogous doubly bridged structures with M=M distances around 2.35 Å, suggesting formal M=M double bonds. The monocarbonyl compounds have analogous singly bridged axial structures Cp 2 FeNi(μ‐CO) and Cp 2 Co 2 (μ‐CO) with metal–metal distances in the range 2.05 Å (M 2 = Co 2 ) to 2.12 Å (M 2 = FeNi) consistent with the formal M≡M triple bonds required for the favored 18‐electron configuration. Open‐shell states of Cp 2 FeNi(μ‐CO) are found to have even lower energies than the closed‐shell structure, which indicates that the ground state of Cp 2 FeNi(μ‐CO) might be a high spin structure. However, the global minimum for the monocarbonyl is found to be a singlet “hot dog” perpendicular Cp 2 NiFe(CO) structure with a terminal CO group bonded to the iron atom. Other higher energy perpendicular structures are also found for Cp 2 FeNi(CO) n ( n = 3, 2, 1) with terminal CO groups and bridging Cp rings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)