LIGAND-EXCHANGE REACTIONS OF CHROMIUM(V) - CHARACTERIZATION OF THE LIGAND-EXCHANGE EQUILIBRIA OF BIS(2-ETHYL-2-HYDROXYBUTANOATO(2-))OXOCHROMATE(V) IN AQUEOUS 1,2-ETHANEDIOL AND THE SOLUTION STRUCTURE OF BIS(1,2-ETHANEDIOLATO(2-))OXOCHROMATE(V)

The complex bis(2-ethyl-2-hydroxybutanoato(2-))oxochromate(V), [Cr(ehba)2O]- (I), undergoes a ligand-exchange reaction in aqueous solutions of 1,2-ethanediol to produce an equilibrium mixture of complex I and the complexes (1,2-ethanediolato(2))(2-ethyl-2-hydroxybutanoato(2-))oxochromate(V), [Cr(ed)(ehba)O]-(II), and bis(1,2-ethanediolato(2-))oxochromate(V), [Cr(ed)2O]-(III). The concentration ratios of these three complexes in solution were determined from the relative intensities of their EPR signals at g(iso) = 1.9783 (2) (I), g(iso) = 1.9791 (2) (II), and g(iso) = 1.9803 (2) (III), respectively. The g values and the nine-line H-1 superhyperfine coupling (A(iso) = 0.63 (3) G) observed for III and the five-line H-1 super hyperfine coupling observed for II (A(iso) = 0.69 (6) G) identify the products of the ligand-exchange reaction but also caused serious overlap of the signals. In order to deconvolute these peaks for the determination of equilibrium constants, experiments were performed in aqueous 1,2-ethanediol-d6. The equilibrium constant for the conversion of I to II, (K(c))1, increases from 1.5 (5) x 10(-5) in 2.0% v/v aqueous 1,2-ethanediol-d6 to 2 x 10(-4) in 20% v/v aqueous 1,2-ethanediol-d6 but remains constant at approximately 2 x 10(-4) over the range 20-95% v/v aqueous 1,2-ethanediol-d6 solutions at 23-degrees-C. The equilibrium constant for the conversion of II to III, (K(c))2, increases slightly from 5 x 10(-6) (20% v/v aqueous 1,2-ethanediol-d6) to 1.5 x 10(-5) (95% v/v aqueous 1,2-ethanediol-d6) at 23-degrees-C. If the reaction of I with neat 1,2-ethanediol is performed over 3-angstrom molecular sieves, the equilibrium is driven almost exclusively to the complex III, due to the absorption of the released 2-ethyl-2-hydroxybutanoate ligand on the surface of the zeolite. This method enables the facile preparation of essentially pure (99%) solutions of III or III-d8, which are important as dynamically polarizable proton and dueteron targets, respectively. Complexes II and III undergo fluxional behavior that makes the protons of the 1,2-ethanediolato(2-) ligand chemically equivalent on the EPR time scale. Despite numerous EPR studies on III, its dynamic properties have not been recognized previously. On the balance of evidence, this equilibration is expected to occur via an intramolecular Berry twist, since it is too fast for a ring opening or an intermolecular process.