Electron transfer reactions of (C5R5)2(CO)2Ti (R = H or Me) with TCNE or TCNQ -: Spectroelectrochemical assignment of metal and ligand oxidation states in [(C5Me5)2(CO)Ti(TCNX)]2-/-/o/+

The TCNX ligands TCNE (tetracyanoethene) and TCNQ (7,7,8,8-tetracyano-p-quinodimethane) react instantaneously with (C5R5)(2)(CO)(2)Ti, R = H or Me, to yield highly air-sensitive mononuclear complexes (C5R5)(2)(CO)Ti(TCNX) of which the soluble species (R = Me) were characterized also in the oxidized and reduced forms through cyclic voltammetry, EPR, IR and UV-vis spectroelectrochemistry. While oxidation at rather low potentials yields labile carbonyltitanium(IV) species of the TCNX.- ligands, the reduction occurs stepwise at unusually negative potentials, first on the ligand (to yield coordinated TCNX2-) and then on the metal (to form Ti-II). For the neutral complexes (C5R5)(2)(CO)Ti2+q (TCNX-q) the results support a rather large amount of charge transfer 1 < q < 2 from the metal to the acceptors TCNX. Evidence for the previously formulated {(mu-TCNE2-) [(C5H5)(2)Ti-IV(CO)](2)} (TCNE2-) could not be found. The complexes (C5R5)(2)(CO)Ti(TCNE) are compared with related compounds (C5R5)(2)BrV(TCNE), (C6R6)(CO)(2)Cr(TCNE) and (C5R5)(CO)(2)Mn(TCNE). (C) 2003 Elsevier B.V. All rights reserved.