The syntheses of complexes MH2L4 and their protonated analogues [MH3L4](+) (M = Fe (1), Ru (2), Os (3); R = Me (a), Et (b)) are described. The structures of la, Ib, and 3a were determined in X-ray diffraction studies. The solution structures of complexes 1-3 were established by detailed NMR investigations, la, 2b, and 3a form equilibrium mixtures of two isomers in solution. The iron (la) and ruthenium (2b) complexes isomerize between six-coordinate M(II) dihydrogen cis-[MH(H-2)L-4](+) and seven-coordinate M(IV) trihydride [M(H)(3)L-4](+) molecular geometries : the first is a distorted octahedron and the second can be viewed as a hydride-capped M(PR3)(4) tetrahedron. Complex 3a is a pentagonal bipyramidal trihydride cis-[Os(H)(3)(Phe(3))(4)](+) in equilibrium with the hydride-capped tetrahedral structural form. Trihydrides Ib and 3b are exclusively represented by the latter structural type. The cationic molecule 2a corresponds to a dihydrogen complex cis-[RuH(H-2)(PMe3)(4)](+). The metal fragment [MH(PR3)(4)](+) is thus most reactive toward oxidative addition of Hz for osmium and iron, with a notably lower ability to reduce H-2 for ruthenium. This trend and related properties are due to electronic rather than steric factors. All [MH3(PR3)(4)](+) species (1-3) are fluxional in solution. The intramolecular hydride exchanges and isomerizations were studied between 20 and -140 degrees C and quantitatively described in terms of their activation parameters. On the basis of these, mechanistic interpretations are provided. Finally the acid/base properties of the [MH3(PR3)(4)](+)/M(H)(2)(PR3)(4) systems were established in a series of NMR experiments in THF-d(8). The pK(a) values range from 10.3 to 12.9 units and increase in the following order : la (10.3) < 2b (10.7) < 2a (10.9) < 3a (11.2) < 3b (12.9). This series demonstrates a higher acidity than that of the related [MH(H-2))(PP)(2)](+) molecules with bidentate ligands. The complexes with monodentate phosphine ligands [MH3(PR3)(4)](+) (1-3) represent a new and distinguished family with structural, dynamic, and acid/base properties remarkably different from most of the other known [MH3L4](+) representatives.