Reactions of coordinated .beta.-polyketonate ligands. 1. Synthesis and structure of bis[1,7-diphenyl-1,3,4,5,7-heptanepentaonato(2-)]tetrakis(pyridine)dicobalt(II) resulting from the oxidation of bis[1,7-diphenyl-1,3,5,7-heptanetetraonato(2-)]tetrakis(pyridine)dicobalt(II)