VITAMIN-B12 COMPLEXES WITH PHOSPHITE LIGANDS .1. UV-VISIBLE SPECTRA AND TEMPERATURE-DEPENDENT EQUILIBRIA

The cobalamins with P(OCH3)3, P(OCH2CH3)3, P(OCH2CH3)2O-, and P[OCH(CH3)2]2O- as ligands have been prepared and characterized. It is proposed that charge-transfer bands are present in the near UV region of the UV-visible spectra of vitamin B-12 complexes with phosphite ligands and that they obscure the shifts of the gamma-bands of the corrin ligand with changes in the sigma-donor ability of the ligand; the shifts of the alpha-beta-bands correlate with the sigma-donor ability of the ligand. Cobalamins with anionic phosphite ligands show temperature-dependent equilibria between the 6-coordinate H2O-Co-X and 5-coordinate Co-X forms and the equilibrium was increasingly displaced in favor of the Co-X form as X is varied in the order: P(OCH3)2O- < P(OCH2CH3)2O- < P[OCH(CH3)2]2O-.