TCNE and TCNQ ligands as efficient bridges in mixed-valence complexes containing iron-cyclopentadienyl and other organometallic systems

Reaction between the pi-accepting tetracyanoeethene (TCNE) or tetracyano-p- quinodimethane (TCNQ) ligands and the rr-electron-rich organometallic speci es Cp(dppe)Fe+ results in the formation of the complexes [{Cp(dppe)Fe}(n){e ta(n)-TCNX}](PF6)(n) (n = 1, 2 and 4). Infrared spectroscopy, magnetic mome nt measurements, electron-spin resonance (ESR) and cyclic voltammetry data indicate a net transfer of one rr electron to the TCNX acceptor ligand. The polynuclear complexes have an intense intervalence electron transfer absor ption band in the near-infrared region. The values for the intervalence par ameters alpha = 0.02-0.06 and V-ab = 200-600 cm(-1) indicate that these com plexes can be classified as class II according to Robin-Day. Estimation of the rate of electron transfer affords values in the order of 10(8) s(-1), w hich are compared and discussed with values estimated for other polynuclear systems containing TCNE and TCNQ ligands as bridges. The solvent effect on the intervalence transition follows Hush's prediction for highly polar sol vents, thereby permitting evaluation of the reorganizational energy. For th e related series [{Cp(dppe)Fe}(n)-(mu-X)] (PF6)(n), the reorganization ener gy appears to depend mainly on the inner reorganization energy through chan ges to vibrational modes of the bridge. Reorganizational energy is not as i mportant in determining the electron transfer rate as the activation energy , E-a. A linear relationship is found between the rate constant ln(k(et)) a nd the activation energy. These results can be discussed within the Marcus theory of electron transfer. The fast and efficient electron transfer betwe en the metal fragments is discussed using molecular orbital arguments. (C) 2000 Elsevier Science Ltd All rights reserved.