Laser-ablated and thermally evaporated Pt atoms react with H-2 to form PtH2, which is characterized by infrared spectra of PtH2, PtHD, and PtD2 in solid argon and neon, and density functional theory calculations. The insertion reaction of Pt into dihydrogen is spontaneous on annealing solid neon to 6-8 K and solid argon to 20-30 K. This observation requires spin-orbit coupling of the ground D-3(3) and excited S-1(0) states of Pt in the curve-crossing region to give the stable PtH2 ((1)A(1)) molecule. Laser-ablated Pt atoms with excess energy also react with Hz to produce PtH. Further reactions of PtH with H-2 or PtH2 with H atoms give evidence for a new asymmetric H2PtH molecule, which is also characterized by isotopic substitution and frequency calculations. (C) 2001 American Institute of Physics.