Molecular magnetism via resonating valence bonds for conjugated radicals and selected transition metal complexes

Currently there is considerable interest in the nature of exchange interactions leading to unpaired electrons in molecular and cluster magnets. Here, the focus is largely at a qualitative level, via a novel "mean-field" resonance-theoretic view, to deal with exchange couplings, so as to allow unpaired electrons in either (or both of) the pi- and sigma -parts of a (largely organic) bipartite (or alternate) molecular network. The (quantitative) number and (qualitative) location of unpaired spins are dealt with by this simple approach, which also offers some (qualitative) information on the occurrence of low-lying higher-spin states. To illustrate the approach it is applied to a variety of systems where the spin sources are conjugated pi -network molecules and polymers, carbenes, variously defected graphites, and a few species involving transition metals, especially Cu. The discussion deals not only with traditional conjugated organics compounds but also with selected inorganic species. (C) 2001 John Wiley & Sons, Inc.