J. Am. Chem. SOC.1986, 108, 5408-5412
5408
to -2J is antiferromagnetic, and its value ranges from 2-3 cm-I (as in 1) to 590-770 cm-l (as in 3) when one acetate ligand is considered. Conclusions The results of these nonempirical VB calculations support the conclusion obtained from earlier MO-CI and VB studiesI5 that 0-0overlap within the carboxylate ligands is primarily responsible for the antiferromagnetism that is observed for Cu( 11) carboxylate dimers. The resulting stabilization of the S = 0 state relative to the S = 1 state arises primarily from covalent-ionic resonance of the Pauling "3-electron bond" type, Le., it leads to the development of this type of bond (0.0)between the oxygen atoms
of the carboxylate ligands. The "cis 0-0overlap" contributions N203,41 and related to the rotation barriers for N204,24c-h*41 molecules are also associated with the development of in-plane Pauling "3-electron bonds" between pairs of cis oxygen atoms in the planar conformers. Acknowledgment. We thank the Australian Research Grants Scheme for financial support for this project. One of us (F.L.S.) acknowledges the award of a University of Melbourne Postgraduate Scholarship. (41) Harcourt,
R. D.; Skrezenek, F. L.; Volkoff, C., in preparation.
Electrochemical Oxidation of the Tetrakis( p-pyrophosphito-P,P')diplatinum( 11) Complex Pt2(p-P205H2)44Both in the Presence and the Absence of Halide Ions, and Reduction of the Axially Substituted Halodiplatinurn( 111) Complexes Pt2(p-Pz05H2)4X24Samuel A. Bryan, Russell H. Schmehl, and D. Max Roundhill* Contribution from the Department of Chemistry, Tulane University, New Orleans, Louisiana 70118. Received April 7, 1986
Abstract: In the presence of added halide ions X- (X = CI, Br, I), aqueous solutions of the complex Pt2(p-P205H2)2-can be electrochemically oxidized to Pt2(pP205H2)4Xz.eLowering the potential causes the reaction to reverse. The OTTLE cell data for all three reactions show isosbestic points and no intermediates. The potential required for the oxidation of Pt2(p-P205H2)4C and the reduction of P ~ ~ ( C ( - P Z O ~depends H ~ ) ~onXX. ~ ~By differential pulse polarography at a glassy carbon electrode the peak potential for oxidation is 0.69 V, and with added CI-,Br-, and I- the respective potentials are 0.56, 0.37, and -0.03 V vs. Ag/AgCl. The peak potentials for reduction of Pt2(r.P205H2)4X:- for X = C1, Br, and I are -0.1 1, -0.18, and -0.21 V, respectively. The complex Pt2(p-P205H2)4ereduces at a potential of -0.43 V in the presence of magnesium ion. The cyclic voltammograms in aqueous solution with added halide ion show irreversible behavior with E , - E , values of 0.86, 0.65, and 0.29 V for X = C1, Br, and I. Rapid scan (1000 V/s) cyclic voltammetry in acetonitrile solvent with no added halide ion shows irreversible behavior with oxidation and reduction waves at 1.12 and 0.61 V. The electrochemical data are explained on the basis of halide ion adsorption at the electrode surface.
The diplatinum(I1) complex anion Ptz(p-P205H2)4khas attracted interest primarily because of its intense luminescence in aqueous solution at ambient temperature.' The ground- and excited-state chemistry of this complex can be rationalized on the basis of Gray's simplified molecular orbital model for binuclear complexes? In this model the close separation between the planar d8 metal centers causes the dzz and pz orbitals to split into bonding and antibonding pairs. The ground-state electron configuration of Ptz(pP205H2)4e is d$du*2, and the excited-state configuration is du2du*'pu'. Electron-transfer reactions with these two states has been the source of much interest, but there is no published work describing the electrochemistry of these complexes. The Pt2(II) complex Pt2(p-P205H2)4e will undergo tweelectron oxidation to give products having Pt2(III) centers. If this oxidation is effected with halogens X2 (X = C1, Br, I) the product is Pt2(p-P20SH2)4X2.4-Alternatively the oxidation can be effected with one-electron oxidants such as ceric ion or hexachloroiridate(1V). In the presence of halide ion the product is again Pt2(pPz05H2)4X:-, but with Ce4+in the absence of added halide ion (1) Sperline, R. P.; Dickson, M. K.; Roundhill, D. M. J . Chem. SOC., Chem. Commun. 1977, 62-63. (2) Mann, K. R.;Gordon, J. G., 11.; Gray, H. B. J. Am. Chem. SOC.1981, 103, 7061-7064.
the complex Pt2(p-P205H2)4(H20)22is formed. In reverse, the conversion of Pt2(p-Pz05H2)4X24to Pt2(p-P205H2)44can be accomplished by using acidified zinc, ascorbic acid, sulfite ion, or hydrogen as reductants. No intermediates have been observed in these transformation^.^ By contrast the reduction of Pt2(pP205H2)44- to either the Pt2(I,II) or the Pt2(1) complexes Pt2(pP ~ O S Hand ~ Pt2(p-P205H2)46 ) ~ ~ has been reported.4 The former complex has been observed by pulse radiolysis techniques, and the latter anion is reported formed by the chromous ion reduction of the Pt2(11,11) ground state. Electron-transfer reactions with the excited state Pt2(pP205H2)4e* have also been reported. Quenching studies show that this state is both a strong oxidant and reductant, and in the photoinduced addition of aryl halides to Pt2(p-P205Hz)44it has been proposed that the pathway involves an SRNlelectron-transfer mechanism from the excited state to the aryl halide.5 Although (3) Bryan, S. A.; Dickson, M. K.;Roundhill, D. M. J . Am. Chem. SOC. 1984,106, 1882-1883. Che, C.-M.; Butler, L. G.;Grunthaner, P. J.; Gray, H. 8. Inorg. Chem. 1985, 24, 4662-4665. (4) Che, C.-M.; Atherton, S. J.; Butler, L. G.; Gray, H. G.J . Am. Chem. SOC.1984, 106, 5143-5145. Alexander, K. A,; Stein, P.; Hedden, D. B.; Roundhill, D. M . Polyhedron 1983, 2, 1389-1392. ( 5 ) Roundhill, D. M. J. Am. Chem. SOC.1985, 107, 4354-4356.
0002-7863/86/ 1508-5408$01.50/0 0 1986 American Chemical Society
Electrochemical Oxidation of Pt,(p- P20SH2),'-
J . Am. Chem. Sot., Vol. 108, No. 18, 1986 5409
no electrochemical studies have been reported, several estimates of the one-electron redox potentials involved have been made. Overall these estimates suggest that the oxidation of Pt2(pP205H2)44-requires a potential of
