The reaction of ˙NO with (PNP)Os(H)₃, where PNP is N(SiMe₂CH₂P^tBu₂)₂, involves H₂ loss from a 1 : 1 adduct to form the intermediate (PNP)OsH(NO)₂, which then forms coordinated HNO; loss of this triatomic molecule forms the final metal complex product in high yield: (PNP)Os(NO). Under a different concentration of gaseous reagents, a second path leads to the same products. These results are supported by spectroscopic observations at various temperatures, as well as by DFT calculations of the energies of alternative transition states and intermediates.