Structural trends in first-row transition-metal bis(benzimidazole) complexes

Dimethyl-substituted bis(benzimidazole) (Me(2)BBZ) is a novel macrocyclic l igand that possesses an intrinsic nonplanarity. To examine how metal-ion bi nding affects the magnitude of this nonplanarity, we have determined the st ructures of a periodic series of Me(2)BBZ complexes bound to Mn(II), Fe(II) , Co(II), Ni(II), and Cu(II). These studies demonstrate that the extent of ligand ruffling and metal doming is indeed influenced by the nature of the metal. Concomitant with the periodic decrease of the ionic radii of the enc apsulated divalent metal ion, a decrease in the magnitude of both the ligan d nonplanarity and the metal out-of-the-plane distance is observed. For the metal out-of-the-plane distance, this correlation persists until the metal finally moves into the mean ligand plane. For the nonplanar distortion, th e extent of the nonplanarity decreases to a limiting value that is intrinsi c to the Me(2)BBZ ligand due to steric factors. These observations indicate that the relative sizes of the metal ion and the Me(2)BBZ ligand cavity ha ve profound effects on the structural features of the metal-ligand complex.