PREPARATION AND THE PH DEPENDENT REVERSIBLE COLOR-CHANGE OF COMPLEXES OF COBALT(III) AND CHROMIUM(III) WITH A HEXAFLUOROACETYLACETONATO LIGAND

Thirteen complexes of Co(III) and Cr(III) with a hexafluoroacetylacetonato (hfac) ligand were newly prepared as perchlorate salts. These are CoL(hfac)2+ with L = (tn)2, tren, beta-2,3,2-tet, beta-3,2,3-tet, (EtNH2)4, (n-PrNH2)4, (n-BuNH2)4, (bpy)2, (phen)2; and CrL(hfac)2+ with L = (NH3)4, (tn)2, alpha-2,3,2-tet, 3,2,3-tet. In these complexes the ligand L occupies four coordination sites and the hfac ligand acts as a bidentate. Aqueous or methanolic solutions of these complexes show a pH dependent rapid and reversible color change; orange in an acid and violet in a base. The origin of this chromism may be a covalent hydration equilibrium, where a lyate anion forms a sigma-bond with one of the carbonyl carbon atoms of the coordinating hfac ligand. The equilibrium constant in water is 2 approximately 12 x 10(6) M-1 for Co(III) complexes and 1 approximately 6 x 10(10) M-1 for Cr(III) complexes (M = mol dm-3).