Framework Bonding and Coordination Sphere Rearrangement in the M 2 X 2 Cores of Synthetic Analogues of Oxyhemocyanin and Related Cu and Pt Complexes Xiang-Yang Liu, † Ana A. Palacios, † Juan J. Novoa, ‡ and Santiago Alvarez † Departament de Quı ´mica Inorga `nica and Departament de Quı ´mica Fı ´sica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain ReceiVed May 23, 1997 The electronic structures of oxo- and peroxo-bridged binuclear copper compounds analogous to the active site of oxyhemocyanin are analyzed in terms of their framework electron counts with the help of density functional and extended Hu ¨ckel calculations. Through-ring bonding in the Cu 2 O 2 framework is discussed by means of a topological analysis of the electron density for the model compounds [(NH 3 ) 3 Cu(µ-η 2 :η 2 -O 2 )Cu(NH 3 ) 3 ] 2+ , [(NH 3 ) 3 Cu(µ- O) 2 Cu(NH 3 ) 3 ] 2+ , and [(PH 3 ) 2 Cu(µ-H) 2 Cu(PH 3 ) 2 ]. The existence of isomeric peroxo- and bis(oxo)-bridged Cu complexes can be rationalized in light of the framework electron counting rules by taking into account that two electrons can be localized in the metal 3d orbitals in the former but delocalized through framework bonding molecular orbitals in the latter. An analysis of the theoretical and experimental structural data indicates that a reorganization of the Cu coordination sphere that can be affected by the nature of the terminal ligands is important for the relative stability of the two isomeric forms. In particular, the peroxo-bridged structure is favored by tridentate ligands, whereas the oxo-bridged isomer is favored by bidentate ones. The stability of the two isomers is also compared for analogous complexes with different metal or bridging atoms for which only one isomeric form is known. One of the most interesting results in the field of model compounds of bioinorganic interest is the recent discovery of binuclear copper compounds with a side-on (µ-η 2 :η 2 ) bridging peroxo group, obtained by reacting [L 3 Cu I L′] (L 3 is a tridentate, N-donor ligand, and L′ is a labile ligand such as acetonitrile) with oxygen, 1-9 closely related to the structure of the subunit II of Limulus polyphemus oxyhemocyanin. 10,11 Not only the existence and reactivity of these compounds, but also the discovery by Tolman and Que et al. that the oxygen-oxygen bond can be reversibly cleaved while the M 2 X 2 framework is kept intact is interesting. 3,12 All of the available experimental data points to a formal description of the two alternative structures as a (µ-η 2 :η 2 -peroxo)dicopper(II) compound with a strong antiferromagnetic coupling (1a) and a bis(µ-oxo)- dicopper(III) complex (1b). A structural database search 13 reveals the existence of three analogous complexes of Cu or Ni with dichalcogenide bridges (X ) S, Te) 14-16 with a structure of type 1a for which the corresponding 1b isomers are not known. Conversely, there are many compounds of the general formula [L 2 M(µ-X) 2 ML 2 ] (M ) Rh(I), Ir(I), Pd(II), Pt(II), † Departament de Quı ´mica Inorga `nica. ‡ Departament de Quı ´mica Fı ´isica. (1) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto, S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 1277. (2) Lynch, W. E.; Kurtz, D. M., Jr.; Wang, S.; Scott, R. A. J. Am. Chem. Soc. 1994, 116, 11030. (3) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young, V. G. Jr.; Que Jr., L.; Zuberbu ¨hler, A. D.; Tolman, W. B. Science 1996, 271, 1397. (4) Mahapatra, S.; Halfen, J. A.; Wilkison, E. C.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1994, 116, 9785. (5) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.; Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1996, 118, 11555. (6) Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg. Chem. 1995, 34, 952. (7) Klein Gebbink, R. J. M.; Martens, C. F.; Feiters, M. C.; Karlin, K. D.; Nolte, R. J. M. J. Chem. Soc., Chem. Commun. 1997, 389. (8) Karlin, K. D.; Kaderli, S.; Zuberbu ¨hler, A. D. Acc. Chem. Res. 1997, 30, 139. (9) Tolman, W. B. Acc. Chem. Res. 1997, 30, 227. (10) Magnus, K. A.; Hazes, B.; Ton-That, H.; Bonaventura, C.; Bonaven- tura, J.; Hol, W. G. J. Proteins: Struct., Funct., Genet. 1994, 19, 302. (11) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96, 2239. (12) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1995, 117, 8865. (13) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8, 31. (14) Mealli, C.; Midolini, S. Inorg. Chem. 1983, 22, 2785. (15) Di Vaira, M.; Peruzzini, M.; Stoppioni, P. J. Chem. Soc., Chem. Commun. 1986, 374. (16) Fujisawa, K.; Moro-oka, Y.; Kitajima, N. J. Chem. Soc., Chem. Commun. 1994, 623. 1202 Inorg. Chem. 1998, 37, 1202-1212 S0020-1669(97)00629-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/28/1998