The structural geometry, electronic structure, photophysical properties, and the fluxional behavior of a series of A-frame diplatinum alkynyl complexes, [Pt-2(mu-dPPm)(2)(mu-C equivalent to CR)(C equivalent to CR)(2)](+) [R = Bu-t (1), C6H5 (2), C6H4Ph-p (3), C6H4Et-p (4), C6H4OMe-p (5); dppm = bis(diphenylphosphino)methane], have been studied by density functional theory (DFT) and time-dependent TD-DFT associated with conductor-like polarizable continuum model (CPCM) calculations. The results show that the Pt ... Pt distance strongly depends on the binding mode of the alkynyl ligands. A significantly shorter Pt ... Pt distance is found in the symmetrical form, in which the bridging alkynyl ligand is sigma-bound to the two metal centers, than in the unsymmetrical form where the alkynyl ligand is sigma-bound to one metal and pi-bound to another. For the two structural forms in 1-5, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels show a dependence on the nature of the substituents attached to the alkynyl ligand. The energies of the HOMO and LUMO are found to increase and decrease, respectively, from R = Bu-t to R = Ph and to R = C6H4Ph-p, because of the increase of the pi-conjugation of the alkynyl ligand. On the basis of the TDDFT/CPCM calculations, the low-energy absorption band consists of two types of transitions, which are ligand-to-ligand charge-transfer (LLCT) [pi(alkynyl) -> sigma*(dppm)]/metal-centered MC [d sigma*(Pt-2) -> p sigma(Pt-2)] transitions as well as interligand pi -> pi* transition from the terminal alkynyl ligands to the bridging alkynyl ligand mixed with metal-metal-to-ligand charge transfer MMLCT [d sigma*(Pt-2) -> pi*(bridging alkynyl)] transition. The latter transition is lower in energy than the former. The calculation also indicates that the emission for the complexes originates from the triplet interligand pi(terminal alkynyls) -> pi*(bridging alkynyl)/MMLCT [d sigma*(Pt-2) -> pi*(bridging alkynyl)] excited state. In terms of the fluxional behavior, calculations have been performed to study the details of the mechanisms for the three fluxional processes, which are the sigma,pi-alkynyl exchange, the ring-flipping, and the bridging-to-terminal alkynyl exchange processes.