Metal Centers Connected by Conjugated Bridges. A Theoretical Evaluation of Delocalization Effects

A series of diiron complexes, in which the Fe atoms are linked by a conjugated bridge, have been investigated through Fenske-Hall molecular orbital calculations. The complexes studied were [CpFeL₂]₂[μ-(X)_m]ⁿ⁺, where Cp = η⁵-C₅H₅, L₂ = (CO)₂ or η²-H₂PCH₂PH₂, X = -CHCHCHCH-, -CCCC-, p-C₆H₄, or -NCHCHN-, m = 1 or 2, and n = 0, 1, or 2 (in selected combinations). For L = CO, the neutral complexes exhibit only weak delocalization, but the radical cationic and dicationic species show relatively strong π delocalization, as measured by population analysis and effective coupling parameter (V_ab) for the mixed-valence cations. All bridges studied participate in π delocalization, with the polyenediyl bridges promoting delocalization most effectively. The property of the bridges that most strongly affects the conjugating ability in the cationic and dicationic complexes is the energy of its highest π orbital relative to the metal π_d orbital, while the property that most strongly effects the conjugating ability in the neutrals is the energy of the lowest π^* orbital. For L₂ = H₂-PCH₂PH₂, delocalization in the neutrals is increased, but the extent of additional delocalization upon oxidation is decreased. In the mononuclear complexes [CpFe(CO)₂(CHCH)_mH]ⁿ⁺, where m = 1 or 2 and n = 0 or 1, π delocalization is less extensive than in the dinuclear complexes, but not dramatically so.