(1)J(H-D), T-1 min and k(1) for H-2 dissociation from OsHX(H-2)(CO)L(2) have been measured for X = Cl, I, H (L = P(t-Bu)(2)Me or P(i-Pr)(3)), as well as for OsCl2(H-2)(CO)(P(i-Pr)(3))(2). For comparison, new data (including previously unobserved coupling constants) have been reported for W(HD)(CO)(3)(P(i-Pr)(3))(2). A comprehensive consideration of T-1 min data for over 20 dihydrogen complexes containing only 1-2 phosphines cis to H-2, together with a consideration of the shortest "conceivable" H-H distance for H-2 bound to a d(4) or d(6) metal, is used to argue that the "fast spinning" model is not appropriate for determining r(H-H) in such complexes. Regarding OsHX(H-2)(CO)L(2) the stronger electron-donor (lighter) halide, when cis to H-2, facilitates loss of H-2. The complete absence of pi-donor ability when X = H renders H-2 loss most difficult. However, a pi-donor trans to H-2 also makes H-2 loss unobservable. Within the series of isoelectronic, structurally analogous Os complexes, a longer H-H bond shows a larger Delta G(double dagger) for H-2 loss. However, this correlation does not continue to W(H-2)(CO)(3)(P(i-Pr)(3))(2), which has r(H-H) comparable to that of OsH(halide)(H-2)(CO)(P(i-Pr)(3))(2), but a significantly higher Delta G(double dagger). This may originate from lack of a pi-donor ligand to compensate as H-2 leaves W.