The coordination chemistry of TCNE, TCNQ and related polynitrileπ acceptors

Conjugated polynitriles such as TCNE or TCNQ can act as non-chelating polydentate ligands towards metal centres. Among the outstanding features of these ligands are (a) their propensity to undergo π/π stacking, (b) their non-innocent nature, i.e. their facile interconversion between three oxidation states, including a spin-bearing radical form, (c) their σ/π coordination ambivalence towards metals, and (d) their ability to bridge several metal centres, thus giving rise to oligonuclear complexes and coordination polymers. This article contains a survey of the structural and electronic features of metal complexes with TCNE, TCNQ and some related molecules. Guidelines are given to elucidate the type of coordination using vibrational data and to assign reasonable oxidation states to the redox-active polynitrile ligand; close-lying ligand π* MOS and d orbitals of coordinated transition metals can cause considerable orbital mixing. Attempts are made to relate coordination modes with electronic structures. Electron transfer dominates the chemical reactivity of the polynitriles and of their complexes, various consequences of initial electron transfer such as isomerization, polynucleation or substitutional rate enhancement are discussed. Interesting physical properties resulting from the extended π conjugation in metal complexes of the polynitrile ligands are briefly referred to by example of electrical conductivity, magnetic coupling and long-wavelength optical absorption.