Molecules of the type mer-M(H)(3)(NO)L-2 [M = Ru (1), Os (2); L = PR3] are characterized on the basis of H-1 NMR T-1min values and IR spectra as pseudo-octahedral trihydrides significantly distorted by compression of the cis W-M-H angles to similar to 75 degrees. The distortion, uncharacteristic of six-coordinate d(6) complexes, is rationalized with DFT (B3LYP) calculations as being driven by increased H-to-lvi sigma donation and by the exceptional pi-accepting ability of linear NO+. In both 1 and 2, hydrides undergo intramolecular site exchange with Delta H-HH(double dagger)(1) = 10-11 kcal/mol and Delta H-HH(double dagger)(2) = 16-20 kcal/mol, depending on L, whereas for mer-Ru(H)(3)(NO)((PBu2Me)-Bu-t)(2) (Ib), moderate exchange couplings (up to 77 Hz) are featured in the low-temperature H-1 NMR spectra, in addition to chemical exchange. On the basis of experimental and theoretical results, a dihydrogen intermediate is suggested to mediate hydride site exchange in 1. The cis H-M-H distortion shortens the tunneling path for the exchanging hydrides in 1, thereby increasing the tunneling rate; diminishes the "conflict" between trans hydrides in the mer geometry; and decreases the nucleophilicity of the hydrides. The generality of the observed structural distortion and its dependence on the ligand environment in late transition metal tri- and dihydrides are discussed. A less reducing metal center is generally characterized by greater distortion.