The calculation of the singlet ground-state (SGS) and the lowest-lying triplet-state (LLTS) geometries of [Pt-2(mu-P2O5H2)(4)](4-) in the gas phase using density functional theory (DFT) produces 7% Pt-Pt bond shortening in the LLTS as compared to SGS. The transition from the Pt-Pt antibonding HOMO to the bonding LUMO+1 in the gas phase and to the bonding LUMO in water creates a metal-metal sigma bond in both excited states. According to the molecular orbital population analysis in water performed using the conductor-like polarizable continuum model (CPCM) and the SGS geometry, the Pt-Pt bond arises from the overlap of the metal p orbitals. The singlet excited-state energy of 27 240 cm(-1) in the gas phase is only 40 cm(-1) higher than the experimental absorption energy. The first triplet excited-state energy of 22 730 cm(-1) in the gas phase and 22 810 cm(-1) in water correlates with the experimental phosphorescence excitation energy of 22 100 cm(-1). The energy of the LLTS correlates with the experimental phosphorescence emission energy.