The phosphinito-bridged Pt(I) complex [(PHCy2)Pt(mu-PCy2){kappa P-2,O-mu-P(O)Cy-2)Pt(PHCy2)](Pt-Pt) (1) reversibly adds H-2 under ambient conditions, giving cis-[(H)(PHCy2)Pt-1(mu-PCy2)(mu-H)Pt-2(PHCy2){kappa P-P(O)Cy-2}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy2)(H)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappa P-P(O)Cy-2}](Pt-Pt) (3). DFT calculations indicate that the reaction of 1 with H-2 occurs through an initial heterolytic splitting of the H-2 molecule assisted by the phosphinito oxygen with breaking of the Pt-O bond and hydrogenation of the Pt and O atoms, leading to the formation of the intermediate [(PHCy2)(H)Pt(mu-PCy2)Pt(PHCy2){kappa P-P(OH)Cy-2}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H center dot center dot center dot H-O-P-Pt ring. Compound D is a labile intermediate which easily evolves into the final dihydride complex 2 through a facile (9-15 kcal mol(-1) depending on the solvent) hydrogen shift from the phosphinito oxygen to the Pt-Pt bond. Information obtained by addition of para-H-2 on 1 are in agreement with the presence of a heterolytic pathway in the 1 -> 2 transformation. NMR experiments and DFT calculations also gave evidence for the nonclassical dihydrogen complex [(PHCy2)(eta(2)-H-2)Pt(mu-PCy2)Pt(PHCy2){kappa P-P(O)Cy-2}](Pt-Pt) (4), which is an intermediate in the dehydrogenation of 2 to 1 and is also involved in intramolecular and intermolecular exchange processes. Experimental and DFT studies showed that the isomerization 2 -> 3 occurs via an intramolecular mechanism essentially consisting of the opening of the Pt-Pt bond and of the hydrogen bridge followed by the rotation of the coordination plane of the Pt center with the terminal hydride ligand.