The protonation of the phosphinito-bridged Pt(I) complex [(PHCy2)Pt(mu-PCy2){kappa P-2,O-mu-P(O)Cy-2}Pt(PHCy2)](Pt-Pt) (1) by aqueous HBF4 or hydrofluoric acid leads selectively to the hydrido-bridged solvento species syn-RPHCy2)(H2O)Pt(mu-PCy2)(mu-H)Pt(PHCy2){kappa P-P(OH)Cy-2}](Y)(2)(Pt-Pt) ([2-H2O]Y-2) {Y = BF4, F(HF)(n)} when an excess of acid was used. On standing in halogenated solvents, complex [2-H2O](BF4)(2) undergoes a slow but complete isomerization to [(PHCy2)(2)Pt(mu-PCy2)(mu-H)Pt{kappa P-P(OH)Cy-2}(H2O)](BF4)(2)(Pt-Pt) ([4-H2O][BF4](2)) having the P(OH)Cy-2 ligand trans to the hydride. The water molecule coordinated to platinum in [2-H2O][BF4](2) is readily replaced by halides, nitriles, and triphenylphosphane, and the acetonitrile complex [2-CH3CN][BF4](2) was characterized by XRD analysis. Solvento sfecies other than aqua complexes, such as [2-acetone-d(6)](2+) or [2-CD2Cl2](2+) were obtained in solution by the reaction of excess etherate HBF4 with 1 in the relevant solvent. The complex [2-H2O](Y)(2) [Y = F(HF)(n)] spontaneously isomerizes into the terminal hydrido complexes [(PHCy2)Pt(mu-PCy2){kappa P-2,O-mu-P(O)Cy-2}Pt(H)(PHCy2)](Y)(Pt-Pt) ([6](Y)). In the presence of HP, complex [6](Y) transforms into the bis-phosphanido-bridged Pt(II) dinuclear complex [(PHCy2)(H)Pt(mu-PCy2)(2)Pt{kappa P-P(OH)Cy-2}](Y)(Pt-Pt) ([7](Y)). When the reaction of 1 with HF was carried out with diluted hydrofluoric acid by allowing the HF to slowly diffuse into the dichloromethane solution, the main product was the linear 60e tetranuclear complex [(PHCy2){kappa P-P(O)Cy-2}Pt-1(mu-PCy2)(mu-H)Pt-2(mu-PCy2)](2)(Pt-1-Pt-2) (8). Insoluble compound 8 is readily protonated by HBF4 in dichloromethane, forming the more soluble species [(PHCy2){kappa P-(OH)Cy-2}Pt-1(mu-PCy2)(mu-H)Pt-2(mu-PCy2)](2)(BF4)(2)(Pt-1-Pt-2) {[9][BF4](2)}. XRD analysis of [9][BF4](2)center dot 2CH(2)Cl(2) shows that [9](2+) is comprised of four coplanar Pt atoms held together by four phosphanido and two hydrido bridges. Both XRD and NMR analyses indicate alternate intermetal distances with peripheral Pt-Pt bonds and a longer central Pt center dot center dot center dot Pt separation. DFT calculations allow tracing of the mechanistic pathways for the protonation of 1 by HBF4 and HF and evaluation of their energetic aspects. Our results indicate that in both cases the protonation occurs through an initial proton transfer from the acid to the phosphinito oxygen, which then shuttles the incoming proton to the Pt-Pt bond. The different evolution of the reaction with HF, leading also to [6](Y) or 8, has been explained in terms of the peculiar behavior of the F(HF)(n)(-) anions and their strong basicity for n = 0 or1.