THE COORDINATION CHEMISTRY OF TCNE, TCNQ AND RELATED POLYNITRILE PI-ACCEPTORS

Conjugated polynitriles such as TCNE or TCNQ can act as non-chelating polydentate ligands towards metal centres. Among the outstanding featu res of these ligands are (a) their propensity to undergo pi/pi stackin g, (b) their non-innocent nature, i.e. their facile interconversion be tween three oxidation states, including a spin-bearing radical form, ( c) their sigma/pi coordination ambivalence towards metals, and (d) the ir ability to bridge several metal centres, thus giving rise to oligon uclear complexes and coordination polymers. This article contains a su rvey of the structural and electronic features of metal complexes with TCNE, TCNQ and some related molecules. Guidelines are given to elucid ate the type of coordination using vibrational data and to assign reas onable oxidation states to the redox-active polynitrile ligand; close- lying ligand pi MOS and d orbitals of coordinated transition metals c an cause considerable orbital mixing, Attempts are made to relate coor dination modes with electronic structures. Electron transfer dominates the chemical reactivity of the polynitriles and of their complexes, v arious consequences of initial electron transfer such as isomerization , polynucleation or substitutional rate enhancement are discussed. Int eresting physical properties resulting from the extended pi conjugatio n in metal complexes of the polynitrile ligands are briefly referred t o by example of electrical conductivity, magnetic coupling and long-wa velength optical absorption.