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Abstract

The complexes {(mu(4)-TCNX)[Fe(CO)(2)(C5H5)](4)}(BF4)(4) were prepared as light-sensitive materials from [Fe(CO)(2)(C5H5) (THF)](BF4) and the corresponding TCNX ligands (TCNE = tetracyanoethene, TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNB = 1,2,4,5-tetracyanobenzene). Whereas the TCNE and TCNQ complexes are extremely easily reduced species with reduction potentials >+0.3 V vs ferrocenium/ferrocene, the tetranuclear complex of TCNB exhibits a significantly more negative reduction potential at about -1.0 V. Even for the complexes with strongly pi-accepting TCNE and TCNQ, the very positive reduction potentials, the unusually high nitrile stretching frequencies > 2235 cm(-1), and the high-energy charge-transfer transitions indicate negligible metal-to-ligand electron transfer in the ground state, corresponding to a largely unperturbed (TCNX degrees)(Fe-II)(4) formulation of oxidation states as caused by orthogonality between the metal-centered HOMO and the pi* LUMO of TCNX. Mossbauer spectroscopy confirms the low-spin iron(II) state, and DFT calculations suggest coplanar TCNE and TCNQ bridging ligands in the complex tetracations. One-electron reduction to the 3+ forms of the TCNE and TCNQ complexes produces EPR spectra which confirm the predominant ligand character of the then singly occupied MO through isotropic g values slightly below 2, in addition to a negligible g anisotropy of frozen solutions at frequencies up to 285 GHz and also through an unusually well-resolved solution X band EPR spectrum of {(mu(4)-TCNE)[Fe(CO)(2)(C5H5)](4)}(3+) which shows the presence of four equivalent [Fe(CO)(2)(C5H5)](+) moieties through Fe-57 and C-13(CO) hyperfine coupling in nonenriched material. DFT calculations reproduce the experimental EPR data. A survey of discrete TCNE and TCNQ complexes [(mu(4)-TCNX)(MLn)(4)] exhibits a dichotomy between the systems {(mu(4)-TCNX)[Fe(CO)(2)(C5H5)](4)}(4+) and {(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)}(4+) with their negligible metal-to-ligand electron transfer and several other compounds of TCNE or TCNQ with Mn, Ru, Os, or Cu complex fragments which display evidence for a strong such interaction, i.e., an appreciable value delta in the formulation {(mu(4)-TCNX delta-)[Mx+delta/4Ln](4)}. Irreversibility of the first reduction of {(mu(4)-TCNB)[Fe(CO)(2)(C5H5)](4)}(BF4)(4) precluded spectroelectrochemical studies; however, the high-energy CN stretching frequencies and charge transfer absorptions of that TCNB analogue also confirm the exceptional position of the complexes {(mu(4)-TCNX)[Fe(CO)(2)(C5H5)](4)}(BF4)(4).