The protonation of the dinuclear phosphinito bridged complex [(PHCy2)Pt(mu-PCy2){kappa(2)P,O-mu-P(O)Cy2}Pt(PHCy2)] (Pt-Pt) (1) by Brønsted acids affords hydrido bridged Pt-Pt species the structure of which depends on the nature and on the amount of the acid used. The addition of 1 equiv of HX (X = Cl, Br, I) gives products of formal protonation of the Pt-Pt bond of formula syn-[(PHCy2)(X)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappaP-P(O)Cy2}] (Pt-Pt) (5, X = Cl; 6, X = Br; 8, X = I), containing a Pt-X bond and a dangling kappa P-P(O)Cy2 ligand. Uptake of a second equivalent of HX results in the protonation of the P(O)Cy2 ligand with formation of the complexes [(PHCy2)(X)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappaP-P(OH)Cy2}]X (Pt-Pt) (3, X = Cl; 4, X = Br; 9, X = I). Each step of protonation is reversible, thus reactions of 3, 4, with NaOH give, first, the corresponding neutral complexes 5, 6, and then the parent compound 1. While the complexes 3 and 4 are indefinitely stable, the iodine analogue 9 transforms into anti-[(PHCy2)(I)Pt(mu-PCy2)(mu-H)Pt(PHCy2)(I)] (Pt-Pt) (7) deriving from substitution of an iodo group for the P(OH)Cy2 ligand. Complexes 3 and 4 are isomorphous crystallizing in the triclinic space group P1 and show an intramolecular hydrogen bond and an interaction between the halide counteranion and the POH hydrogen. The occurrence of such an interaction also in solution was ascertained for 3 by (35)Cl NMR. Multinuclear NMR spectroscopy (including (31)P-(1)H HOESY) and density-functional theory calculations indicate that the mechanism of the reaction starts with a prior protonation of the oxygen with formation of an intermediate (12) endowed with a six membered Pt(1)-X...H-O-P-Pt(2) ring that evolves into thermodynamically stable products featuring the hydride ligand bridging the Pt atoms. Energy profiles calculated for the various steps of the reaction between 1 and HCl showed very low barriers for the proton transfer and the subsequent rearrangement to 12, while a barrier of 29 kcal mol(-1) was found for the transformation of 12 into 5.

As a result of the strong electrophilic character of the Pt bound to O, the phosphinito-bridged PtI complex [(PHCy2)Pt(micro-PCy2){kappa2P,O-micro-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1) undergoes attack at the O-bound Pt atom by molecules such as di- and tricyclohexylphosphane, dicyclohexylphosphane oxide, and dicyclohexylphosphane sulfide. Thus, reaction of 1 with PHCy2 gives the symmetric PtI dimer [(PHCy2)Pt(micro-PCy2)]2(Pt-Pt) (2), while the hydrido-bridged complex syn-[(PHCy2){kappaP-P(O)Cy2}Pt(micro-PCy2)(micro-H)Pt(PHCy2){kappaP-P(O)Cy2}](Pt-Pt) (4) is obtained from reaction of 1 with P(O)HCy2; the thiophosphinito complex [(PHCy2)Pt(micro-PCy2){kappa2P,S-micro-P(S)Cy2}Pt(PHCy2)](Pt-Pt) (8) forms selectively in reaction of 1 with P(S)HCy2. For comparison, the reaction with PCy3 results only in ligand substitution, affording [(PCy3)Pt(micro-PCy2){kappa2P,O-micro-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (5). DFT studies confirmed the remarkable electrophilicity of the oxygen-bound Pt and shed light on the nature of the metal-metal bond in Pt dimers.

The phosphinito-bridged Pt(I) complex [(PHCy(2))Pt(mu-PCy(2)){kappa(2)P,O-mu-P(O)Cy(2)}Pt(PHCy(2))](Pt-Pt) (1) reversibly adds H(2) under ambient conditions, giving cis-[(H)(PHCy(2))Pt(1)(mu-PCy(2))(mu-H)Pt(2)(PHCy(2)){kappaP-P(O)Cy(2)}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy(2))(H)Pt(mu-PCy(2))(mu-H)Pt(PHCy(2)){kappaP-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 [(PHCy(2))(H)Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-P(OH)Cy(2)}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H...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 [(PHCy(2))(eta(2)-H(2))Pt(mu-PCy(2))Pt(PHCy(2)){kappaP-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.

The reactivity of the phosphinito bridged Pt(I) complex [(PHCy(2))Pt(1)(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(2)(PHCy(2))](Pt-Pt) (1) towards Au(I) and Ag(I) electrophiles was explored. Treatment of 1 with AuCl yielded the dichloro Pt(II) complex [(Cl)(PHCy(2))Pt(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(Cl)(PHCy(2))] (4), while [Au(PPh(3))Cl] in thf (or toluene) caused ligand exchange resulting in the formation of [(PPh(3))Pt(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(PHCy(2))](Pt-Pt) (7) and [(PPh(3))Pt(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(PPh(3))](Pt-Pt) (8). With [Au(PPh(3))OTf] (independently from the solvent) or with [Au(PPh(3))Cl] (only in dichloromethane), reaction with 1 gave [(PHCy(2))Pt(1)(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(2){μ-Au(PPh(3))}(PHCy(2))]X(Pt-Pt) ([6]X, X = OTf, Cl) clusters in which the [Au(PPh(3))] moiety bridges the μP-Pt(2) bond. The [Ag(PPh(3))](+) electrophile attacks complex 1 selectively at the Pt(2)-μP bond to afford, at low T, the cationic cluster [(PHCy(2))Pt(1)(μ-PCy(2)){κ(2)P,O-μ-P(O)Cy(2)}Pt(2){μ-Ag(PPh(3))}(PHCy(2))](+)(Pt-Pt) (10(+)) in which the [Ag(PPh(3))](+) moiety bridges the μP-Pt(2) bond. Clusters analogous to 10(+), but without PPh(3) bonded to Ag, are obtained from reactions of 1 with AgOTf, AgBF(4), AgClO(4) and AgCl.