Optical power limiting and luminescence properties of two Pt(II) complexes with thiophenyl and phenyl groups in the ligands, trans-Pt(P(n-Bu)(3))(2)(CCAr)(2), where Ar = C4H2SCC-p-C6H4-n-C5H11 (1) and p-C6H4CCC4H3S (2), have been investigated. The fluorescence lifetimes were found to be on the sub-nanosecond time scale, and the quantum yields were low, in accord with fast intersystem crossing from the excited singlet to triplet manifold. The phosphorescence lifetimes of 1 and 2 were shorter than that of a Pt(II) complex having two phenyl groups in the ligands. In order to elucidate the C-Pt bonding nature in the ground state, the C-13 NMR chemical shift of the carbon directly bonded to Pt, the coupling constants (1)J(PtC), (2)J(PtC), and (1)J(PtP), and IR nu(CC) wavenumbers were obtained for 1, 2, and three other trans-diarylalkynyl Pt(II) complexes. X-ray diffraction data of 1 and 2 and density functional theory calculated geometries of models of 1, 2, and trans-Pt(P(n-Bu)(3))(2)(CC-p-C6H4CCC6H5)(2) (3) show that 1 preferably exists in a different conformation from that of 2 and 3. The variations in photophysical, NMR, and IR data can be rationalized by differences in geometry and pi-backbonding from Pt to the alkynyl ligand.