EPR EVIDENCE FOR LOW-LYING MLCT STATES IN TETRACYANOIRON AND TETRACYANORUTHENIUM COMPLEXES WITH STRONGLY PI-ACCEPTING CHELATE LIGANDS - COMPARISON WITH ISOELECTRONIC TETRACARBONYLMETAL SPECIES

The complex ions [(bpz)M(CN)(4)](2-) (M = Fe, Ru) and [(abpy)Fe(CN)(4) ](2-) with the strongly pi-accepting chelate ligands 2,2'-bipyrazine ( bpz) and 2,2'-azobis(pyridine) (abpy) were prepared as potassium or te trabutylammonium salts and were characterized by IR and UV/vis absorpt ion spectroscopy. Cyclic voltammetry in acetonitrile, DMF, or water re vealed reversible one-electron oxidation processes, while the reductio n waves showed reversibility only in aprotic solvents. EPR studies of the spectroelectrochemically generated (1-) and (3-) states confirm th e MLCT formulation for the lowest excited states of the (2-) precursor complexes by showing typical low-spin Fe(III) signals of the oxidized species [(abpy)Fe(CN)(4)](-) and partially hyperfine-structured spect ra for the reduced forms [(bpz)M(CN)(4)](3-). The latter are clearly t etracyanoiron(II) or -ruthenium(II) complexes of the corresponding ani on radical acceptor ligands and are thus isoelectronic with monoanioni c tetracarbonylchromium(0) or -molybdenum(0) radical complexes.