Several platinum cis-dihydride phosphine complexes, (diphos)PtH2 [4, diphos = iPr(2)P(CH2)(2)PiPr(2) (dippe); 5, diphos = Cy(2)P(CH2)(2)PCy(2) (dcype); and 6, diphos = tBu(2)P(CH2)(2)PtBu(2) (dtbpe)], have been found to be unstable in solution, reversibly losing H-2 and forming binuclear complexes of the general formula [(P-P)PtH](2). The extent of dimerization in solution is directly related to the steric size of the phosphine ligand, larger phosphines imparting kinetic stability to the cis-dihydride monomers. The dimeric complexes possess terminal hydride ligands on the basis of their IR spectra; however, the room-temperature H-1, P-31{H-1} and Pt-195{H-1} NMR spectra show equivalence within the sets of H, Pt, and P nuclei, indicating that the dimers are fluxional in solution. This fluxional process can be stopped at low temperature for the (dippe) and (dcype) derivatives, 1 and 2, but not for the (dtbpe) derivative, 3. The complex 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1) in a cell of dimensions a = 9.245(2) Angstrom, b = 16.966(2) Angstrom, c = 22.690(6) Angstrom, and Z = 4. The structure shows square planar geometry about each Pt(I) center, the coordination sphere being composed of one bidentate phosphine ligand, a terminal hydride, and a direct, unbridged bond to the other Pt center. The angle between the two square planes is 93.2(1)degrees, and the Pt-Pt distance is 2.5777(5) Angstrom. The complex 3 crystallizes as a toluene solvate in the triclinic space group P (1) over bar with a = 11.552(3) Angstrom, b = 13.123(3) Angstrom, c = 17.694(4) Angstrom, alpha = 81,20(2)degrees, beta = 89.20(2)degrees, gamma = 66.55(2)degrees, and Z = 2. The structure is similar to that of 1; however, there are several distortions resulting from steric crowding caused by the larger tert-butyl groups (Pt-Pt distance = 2.6094(6) Angstrom). The spectral and structural details, the solution properties, and the nature of the fluxional process are discussed.