The electronic structure and metal-metal bonding in the classic d(7)d(7) tetra-bridged lantern dimer [Pt-2(O2CCH3)(4)(H2O2)2](2+) has been investigated by performing quasi-relativistic X alpha-SW molecular orbital calculations on the analogous formate-bridged complex. From the calculations, the highest occupied and lowest unoccupied metal-based levels are delta*(Pt-2) and sigma*(Pt-2), respectively, indicating a metal-metal single bond analogous to the isoelectronic Rh(II) complex. The energetic ordering of the main metal-metal bonding levels is, however, quite different from that found for the Rh(II) complex, and the upper metal-metal bonding and antibonding levels have significantly more ligand character. As found for the related complex [W-2(O2CH)(4)], the inclusion of relativistic effects leads to a further strengthening of the metal-metal sigma bond as a result of the increased involvement of the higher-lying platinum 6s orbital. The low-temperature absorption spectrum of [Pt-2(O2CCH3)(4)(H2O2)(2)](2+) is assigned on the basis of X alpha-SW calculated transition energies and oscillator strengths. Unlike the analogous Rh(II) spectrum, the visible and near-UV absorption spectrum is dominated by charge transfer (CT) transitions. The weak, visible bands at 27 500 and 31 500 cm(-1) are assigned to Ow --> sigma*(Pt-2) and OAc --> sigma*(Pt-2) CT transitions, respectively, although the donor orbital in the latter transition has around 25% pi*(Pt-2) character. The intense near-UV band around 37 500 cm(-1) displays the typical lower energy shift as the axial substituents are changed from H2O to Cl and Br, indicative of significant charge transfer character, From the calculated oscillator strengths, a number of transitions, mostly OAc --> sigma*(Pt-O) CT in nature, are predicted to contribute to this band, including the metal-based sigma(Pt-2) --> sigma*(Pt-2) transition. The close similarity in the absorption spectra of the CH3COO-, SO42-, and HPO42- bridged Pt(III) complexes suggests that analogous spectral assignments should apply to [Pt-2(SO4)(4)(H2O)(2)](2-) and [Pt-2(HPO4)(4)(H2O)(2)](2-). Consequently, the anomalous MCD spectra reported recently for the intense near-UV band in the SO42- and HPO42- bridged Pt(III) complexes can be rationalized on the basis of contributions from either SO4 --> sigma*(Pt-O) or HPO4 --> sigma*(Pt-O) CT transitions. The electronic absorption spectrum of [Rh-2(O2CCH3)(4)(H2O)(2)] has been re-examined on the basis of X alpha-SW calculated transition energies and oscillator strengths. The intense W band at similar to 45 000 cm(-1) is predicted to arise from several excitations, both metal-centered and CT in origin. The lower energy shoulder at approximately 40 000 cm(-1) is largely attributed to the metal-based sigma(Rh-2) --> sigma*(Rh-2) transition.