The (PtPt1)-Pt-1 monocarbonyl derivative ((Bu2HP)-H-t)Pt(mu-PBu2t)(2)Pt(CO) (5), with a planar, asymmetrically substituted Pt-2(mu-PBu2t)(2) core, reacts with CF3SO3H to give the new pt(II)pt(II) carbonyl hydride [((Bu2HP)-H-t)Pt(mu-PBu2t)(2)Pt(Co)(H)]CF3SO3 (6a). Complex 6a, in which the proton is terminally attached to the Pt atom bearing the carbonyl ligand, is formed under kinetic control and is stable in well-dried nonbasic solvents. This isomer is converted quantitatively by an external weak base into the thermodynamically favored form, [((PBu2H)-H-t)(H)Pt(mu-PBu2t)(2)Pt(CO)]CF3SO3 (6b), in which the proton is terminally attached to the Pt atom bearing the phosphine group. In the presence of an excess of triflic acid, 6a is further protonated to give [Pt-2(mu-PBu2t)(mu-H)(CO)((PBu2H)-H-t)(eta(2)-(PBu2H)-H-t)](CF3SO3)(2) (7) by the formation of a P-H bond between the hydride and the adjacent phosphido ligand induced by metal protonation. Complex 7 was characterized by multinuclear NMR spectroscopy, which strongly suggests a Pt-H-P agostic interaction. Like 6a, complex 7 is stable in nonbasic media, but yields 6b rapidly and quantitatively in the presence of a base. Experimental data compare well with the results of ab initio calculations on model compounds corresponding to 5, 6a(+), and 6b(+), whose structures have been optimized in the gas phase.