SPIN-TRANSITION BEHAVIOR OF TRANSITION-METAL COMPLEXES WITH 2,6-BIS-(BENZIMIDAZO1-2'-YL)PYRIDINE INDUCED BY DEPROTONATION OF THE COMPLEX

2,6-bis-(BenzimidazoI-2'-yl)-pyridine (bzimpy=H(2)L) acts as a bidenta te ligand when combining with transition metal ions. The complexes [M( bzimpy)(2)](ClO4)(2) (M = Fe2+, Mn2+, Zn2+, Co2+, and Ni2+) were obtai ned as solids. The protonation constants (log K) for the ligand and th e complexes were evaluated in 30:70 (v/v) H2O:EtOH at 293 K and at con stant ionic strength of 0.12 M KCl. Coordination of the ligand to the metal ions leads to an increase of acidity of the imino-hydrogen of th e benzimidazole group of the ligand as a function of the complex stabi lity. Deprotonation leads to a spin-state transition (intermediate spi n-state --> low-spin) of the iron(II)-complex, followed by a shift of the metal-to-ligand charge transfer band (MLCT) to lower energies (lam bda(max) = 563 to 580 nm). The d-d absorption bands are found to shift to higher energies and the low-spin isomer is favoured at room temper ature. An opposite shift of the MLCT band (lambda(max) = 563 to 557 nm ) is observed when HClO4 is added to the complex solution, rendering t he high-spin state of the complex more favourable.