The structures of [Pt-2(pop)(4)](4-), [Pt-2(pcp)(4)](4-), and related species [Pt-2(pop)(4)X-2](4-) and [Pt-2(pop)(4)](2-) in the ground states (pop = P2O5H22-, pcp = P2O4CH42-, and X = I, Br, and Cl) were optimized using the second-order Moller-Plesset perturbation (MP2) method. It is shown that the Pt-Pt distances decrease in going from [Pt-2(pop)(4)](4-) to [Pt-2(pop)(4)X-2](4-) to [Pt-2(pop)(4)](2-). This is supported by the analyses of their electronic structures. The calculated aqueous absorption spectra at the time-dependent density functional theory (TD-DFT) level agree with experimental observations. The unrestricted MP2 method was employed to optimize the structures of [Pt-2(pop)(4)](4-) and [Pt-2(pcp)(4)](4) in the lowest-energy triplet excited states. The Pt-Pt contraction trend is well reproduced in these calculations. For [Pt-2(pop)(4)](4-), the Pt-Pt distance decreases from 2.905 angstrom in the ground state to 2.747 angstrom in the excited state, which is comparable to experimental values of 2.91 - 2.92 angstrom and 2.64 - 2.71 angstrom, respectively. On the basis of the excited-state structures of such complexes, TD-DFT predicts the solution emissions at 480 and 496 nm, which is closer to the experimental values of 512 and 510 nm emissions, respectively.