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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 Hückel 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(&mgr;-eta(2):eta(2)-O(2))Cu(NH(3))(3)](2+), [(NH(3))(3)Cu(&mgr;-O)(2)Cu(NH(3))(3)](2+), and [(PH(3))(2)Cu(&mgr;-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.

A study of the preferred structures for the M2X2 rings in the binuclear complexes of types [M2(mu-XR2)2L8] and [M2(mu-XR3)2L8] is presented, based on qualitative orbital arguments supported by extended Hückel calculations on Cr compounds. The main conclusions are confirmed by DFT calculations on key compounds of Cr and Mn and agree well with the results of a structural database analysis. With the simplified electron counting scheme deduced, complexes with six or four electrons available for bonding of the M2X2 framework are predicted to have two possible minimum energy structures, with either a short M-M or X-X distance, whereas compounds with eight framework electrons are expected to present no short through-ring distance. Such a behavior is consistent with the framework electron rules reported earlier for compounds with different coordination spheres and provides a general description of the structure and bonding in a variety of compounds with M2X2 diamonds. Metal-metal bonding across the ring can be equally predicted taking into account only the bonding characteristics of the t2g-like orbitals for the XR2- but not for the XR3-bridged complexes. In addition, the framework electron counting scheme has the advantage of being independent of the formal oxidation state assigned to the metal atom.