The reactivity toward H-2 of coordinatively unsaturated Pt(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt(NHC')(NHC))(+) (where NHC' stands for a cyclometalated NHC ligand) react very fast with H-2 at room temperature, leading to hydrogenolysis of the Pt CH, bond and concomitant formation of hydride derivatives [PtH(NHC)(2)](+) or hydrido dihydrogen complexes [PtH(H-2)(NHC)(2)](+). The latter species release H-2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH(IPr)(2)][SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H-2 in complexes [PtH(H-2)(NHC)(2)](+) is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (I(t)B(3)u). This outcome is related to the higher close-range steric effects of the (IBu)-Bu-t ligands. Accordingly, HID exchange reactions between hydrides [PtH(NHC)(2)](+) and D-2 take place considerably faster for IPr and IMes* derivatives than for (IBu)-Bu-t ones. The reaction mechanisms for both H-2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a a-complex assisted-metathesis mechanism in the case of (IBu)-Bu-t,