SPIN-CROSSOVER COMPLEXES IN SOLUTION, .2. SOLVENT EFFECTS ON THE HIGHSPIN-EQUILIBRIUM LOW SPIN-EQUILIBRIUM OF [FE(BZIMPY)2](CLO4)2

The influence of the solvent on the spin-equilibrium of [Fe(bzimpy)2]( ClO4)2(bzimpy = 2,6-bis(benzimidazol-2'-yl)pyridine) has been investig ated in several nonaqueous solvents by means of UV-Vis spectroscopy an d magnetic susceptibility measurements. In methanol, a bis-tris ligati on equilibrium is found. Both, the FeL2(2+) and FeL(3)2+ Species show spin-crossover behaviour in solution. Photometric and magnetic propert ies of the complex in various solvents are related. The spin-equilibri um of [Fe(bziMPY)2](ClO4)2 shows the strongest sensitivity towards cha nges of the solvent that has ever been observed for spin-crossover com pounds. Increase in solvent donor number (DN) from DN = 14.1 (Acetonit ril) to DN = 19.1 (Methanol) causes a shift from mu(exp) = 4.21 (78% H igh-Spin (HS)) to mu(exp) = 3.0 (56% HS). In solvents with DN greater- than-or-equal-to 30 deprotonation of the complex occurs yielding a low -spin species. The solvent effects are discussed in terms of the donor -acceptor concept.