Organometallic Donor-Acceptor Complexes with Nonplanar Donors: The Zigzag Linear Chain Complex [(C₆Me₆)₂M²⁺][iso-C₄(CN)₆^2-] (M = Fe, Ru)

Donor-acceptor solids [(C₆Me₆)₂Me²⁺][iso-C₄(CN)₆^2-].MeNO₂ (M = Fe, la; M = Ru, 2a) and [(C₆Me₃H3)2M²⁺][iso-C4(CN)6²“] (M = Fe, 1b; M = Ru, 2b) prepared from (arene)₂M²⁺ and iso-C₄(CN)₆^2- are described. Single-crystal X-ray studies show that la crystallizes in the space group P2₁/c with a = 19.875 (6) Å, b = 10.512 (2) Å, c = 15.981 (4) Å, β = 96.65 (3)°, V = 3316 (3) ų, p = 1.36 g cm^-3, Z = 4, R_u = 0.061, and R_w= 0.080. The ruthenium analogue 2a also crystallizes in the space group P2₁/c with a = 19.973 (2) Å, b = 10.553 (2) Å, c = 16.144 (1) Å, β = 96.80 (7)°, V = 3378 (3) ų, p = 1.36 g cm^-3, Z = 4, R_u= 0.070, and R_w= 0.068. Both complexes exhibit “zigzag” linear chains of closely spaced alternating cations and anions. Donor-acceptor interactions arise from close intermolecular contacts between the C₆Me₆ ligands of the dications and nitrogen atoms of the nonplanar dianion with dianions “bridging” the dications via three of its cyano groups. The complexes exhibit strong charge-transfer bands and are best described as donor-acceptor (DA) complexes with nominally doubly charged (D^2-A²⁺) ground states and (D^-A⁺) excited states. The difference between the charge-transfer absorption energies of 1 and 2 is equivalent to the difference in the solution reduction potentials of the isostructural (arene)₂M²⁺ cations. The structure of the 19e (C₆Me₆)₂Fe⁺ monocation, reported here for the first time, is essentially identical with that of the 18e dication, suggesting that the charge-transfer transitions of these DA complexes can occur without the formation of energetically unfavorable molecular conformations.