In the presence of a catalytic amount of F-(CsF), Me3SiRf (R-f = CF3 and C6F5) exchanges R-f with fluoride of the 16-electron complexes MHF(CO)L-2 (M = Ru, Os; L = (PPr3)-Pr-i, (PBu2Me)-Bu-t) to give Me3Si-F and the unsaturated pentafluorophenyl complexes, MH(C6F5)(CO)L-2, or (when R-f = CF3) saturated fluorocarbene complexes, MHF(CF2)(CO)L-2, via alpha-fluorine migration. X-ray crystal structure and solution F-19 NMR studies reveal that, in the ground state, the three atoms of the CF2 group lie in a plane perpendicular to the P-Ru-P axis so that the pi-back-donation is maximized and the carbene substituents are inequivalent. Having hydride trans to the CF2 ligand, MHF(CF2)(CO)L-2 is a kinetic product, which converts to a thermodynamic isomer. For Ru, the final product is a 16e complex, RuF(CF2H)(CO)L-2, formed by combination of CF2 and hydride. For Os, the product is an 18e complex, OsF2(=CFH)(CO)L-2, resulting from exchange of one carbene fluoride with the hydride. The distinct difference between Os and Ru demonstrates the principle that third-row transition metals show a pronounced tendency toward a higher oxidation state. The isomerization mechanism involves phosphine dissociation as a slow step. Coordinatively saturated RuHF(CF2)(CO)L-2 reacts with CO within the time of mixing to give the F and CF2 recombination product, RuH(CF3)(CO)(2)L-2. This unexpectedly fast carbonylation reaction, as well as F-19 spin saturation transfer experiments, reveals the existence of a fast alpha-flourine migration equilibrium between RuHF(CF2)(CO)L-2 and RuH(CF3)(CO)L-2 in solution. In sharp contrast, the Os analogue does not have such a fast equilibrium, and therefore it does not react with CO at room temperature. At higher temperature, reaction occurs forming the hydride and fluoride exchanged product, Os(CHF2)(F)(CO)(2)L-2, The contrasting behavior of Ru vs Os regarding stability of fluoroalkyl and fluorocarbene is discussed on the basis of the theoretical calculations, which also provide insight into the isomerization of RuHF(CF2)(CO)L-2. Hydrogenolysis of Ru(CF2H)F(CO)L-2 liberates CH2F2, forming RuHF(CO)L-2.