6282 J. Am. Chem. zyxwvut SOC. zyxwvu 1985, 107, 6282-6286 surprising since the inorganic functionality zyxwvutsrq [ Zr02CH] can be related to an ester, which is very easily reduced by complex zyxwvutsr 15b,20321 in a related reaction. Step d is proven by having carried out the reaction between I and formaldehyde leading to [cp2Zr(C1)- (OMe)], while addition of a C=N multiple bond to the Zr-H bond in complex I was recently reported.22 The oxophilicity of zirconium and the formation of a very stable complex zyxwvut I1 are the real driving force of the reduction of C02 in the reactions outlined above. The present study suggests that reduction of C02 by a M-H (20) Schwartz, J.; Labinger, J. A. Angew. Chem., zyxwvutsrqpon Int. Ed. Engl. 1976, 15, (21) Wailes, P. C.; Weigold, H . J. Orgunomet. Chem. 1970, 24, 413-417. (22) Erker, G.; Fromberg, W.; Atwood, J. L.; Hunter, W. E. Angew. Chem., Int. Ed. Engl. 1984, 23, 68-69; J. Orgunomet. Chem. 1984, 280, 343-354, 355-363. 333-340. bond could be an interesting process provided two fundamental research lines are pursued. The first one is a study on the reduction of the metalloformate functionality, while the second one, which is the most crucial problem in the activation of oxygen containing molecules, is how to react the M-0-M, M-0-R, and M = O inorganic functionalities. Acknowledgment. We thank C N R (Rome) for financial sup- port. Supplementary Material Available: Listing of observed and calculated structure factors, hydrogen coordinates (Table SI), anisotropic thermal parameters (Table SII), nonessential bond distances and angles (Table SIII), torsion angles in cyclohexyl rings (Table SIV), and equation of least-squares planes (Table SV) (26 pages). Ordering information is given on any current masthead page. Axial Ligand Anation and Aquation Reactions in Diplatinum( 111) Complexes. Comparison of Aquation Rates between PtC162- and Diplatinum( 111) Chloro Complexes Having p-Phosphato or p-Pyrophosphito Ligands Ramadan El-Mehdawi, Samuel A. Bryan, and D. M a x Roundhill* Contribution from the Department of Chemistry, Tulane University, New Orleans, Louisiana 701 zyxwvuts 18. Received March 25, 1985 Abstract: The diplatinum(II1) complex [Pt2(p-P0,H)4(H20)2]2-reacts with halide ions X- (X = CI, Br) to give first [Pt2(p-PO4H),X(H20I3- and then [Pt2(p-P0,H)4X2]4-. The reaction establishes an equilibrium situation between the three complexes: Using pseudo-first-order conditions in halide ion at 25 "C, we have used a biphasic analysis to evaluate the rate constants. The values are k, = 0.9 (2) M-' s-,, k2 = 5 (I) X s-' (X = CI) and k, = 0.3 (1) M-I s-l, k2 = 1.5 (3) X s-' (X = Br). Under comparable conditions of temperature (25 "C), pH (3.0), and ionic strength (0.1 M), we have measured the equilibrium constants as K, = 19 (2) M-I, zyxwvutsrqp K2 = 11 (I) M-' (X = CI) and K2 = 21 (2) M-', K2 = 22 (2) M-' (X = Br). In all cases, k, > k2 and k, > k,, and k, = k3 and k2 = k,. The axially coordinated halide ligands at 6-coordinate platinum are substituted some 4 orders of magnitude faster than axial halides in the p-pyrophosphito complexes [Pt2(p-P205H2),X2]& or chloride in PtC162-. This axial halide labilization in [Pt2(p-PO4H),X2l4- (X = CI, Br) as compared to PtCIG2- is believed to be a consequence of a strong intermetallic bond between platinums. s-', k3 = 0.8 (2) M-ls-l, k, = 8 (2) X SKI, k, = 0.4 (I) M-' s-,, k, = 1.9 (3) X The chemistry of metal-metal bonded complexes continues to be a subject of active interest1 A large body of work now exists describing the synthesis of numerous transition-metal complexes having a wide variety of bridging ligands and terminal end groups. Many of these complexes have been structurally characterized, but only a relatively small amount of work has been published correlating differences in substitution patterns and rates with variations in intermetallic bonding between sets of complexes. Such studies are valuable if major changes in reactivity or se- lectivity are to be identified because they confirm that an inter- metallic bond is a variable quantity that can be changed to induce desired differences in reaction chemistry. (I) Cotton, F. A,; Walton, R. A. 'Multiple Bonds between Metal Atoms"; Wiley-Interscience: New York, 1982. 0002-7863/85/1507-6282$01.50/0 Recently three series of diplatinum( 111) complexes have been prepared that allow us to test this premise. One set of complexes has a bridging pyridinato-N,O ligand,2 and a second set has a pyrophosphito-P,P group as a bridge.3 A third group, which we classify as a single set, has either a bridging sulfato-0.0 or hy- (2) Hollis, L. S.; Roberts, M. M.; Lippard, S. J. Inorg. Chem. 1983, 22, 3637-3644 and references therein. (3) Che, C.-M.; Schaefer, W. P.; Gray, H. B.; Dickson, M. K.; Stein, P.; Roundhill, D. M. J. Am. Chem. SOC. 1982, 104, 4253-4255. Bryan, S. A.; Dickson, M. K.; Roundhill, D. M. J. Am. Chem. Soc. 1984, 106, 1882-1883. Che, C.-M.; Herbstein, F. H.; Schaefer, W. P.; Marsh, R. E.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 4604-4607. Stein, P.; Dickson, M. K.; Roundhill, D. M. J. Am. Chem. SOC. 1983,105, 3489-3494. Hedden, D.; Walkinshaw, M. D.; Roundhill, D. M. Inorg. Chem., in press. Alexander, K. A.; Bryan, S. A,; Fronczek, F. R.; Fultz, W. C.; Rheingold, A. R.; Roundhill, D. M.; Stein, P.; Watkins, S. F. Inorg. Chem., in press. Che, C.-M.; Mak, T. C. W.; Gray, H. B. Inorg. Chem. 1984, 23, 4386-4388. 0 1985 American Chemical Society