The hydridodicarbonyliridium(1) complexes of formula IrH (CO),L, where L = PPh3, P(C6H,F),, PEtPh,, and AsPh, have been prepared by the interaction of solutions of the corresponding chlorides, trans-lrCI(C0) L, , with sodium borohydride in the presence of carbon monoxide. Although the complexes appear to have a single structure in the solid state, i.r. and n.m.r. spectra show that in solutions in organic solvents two isomeric pentaco-ordinate species in equilibrium are formed. The equilibria and the spectra depend not only on temperature but on the nature of the solvent. The form of the n.m.r. spectra requires that the *lP-H coupling constants in the two isomers, w.hich are undergoing rapid intramolecular interconversion, are opposite in sign. The precise nature of the species in solution is discussed.IN order to assist the interpretation of spectral and reactions of hydridodicarbonylbis(tripheny1phosphine)rhodium@), which appears to be the main species involved in the hydroformylation reaction of alkenes at atmospheric temperature and pressure, we prepared the more thermally stable and chemically less reactive iridium analogue, IrH(CO),(PPh,),. The compound has been prepared independently4$ by a different method, but the n.m.r. spectrum was not determined; the unusual i.r. spectroscopic behaviour described here t No reprints available.$ It is likely that the complex was obtained earlier,5 and not recognised as such since on treating IrH(C0) (PPh,), with carbon monoxide it is unlikely that the monocarbonyl IrH(C0) (PPh,), is formed by this route as claimed. Further, the reported i.r. spectra with bands ' 1980-1920 ' agree with the dicarbonyl formulation.was not observed, as the spectra were measured in solution only in chloroform where solvent broadening of the lines occurs. Similar complexes of AsPh,, (FC,H,),P, and Ph,EtP have also been prepared for spectroscopic comparisons.Preparation and Some Chemical Properties of Hydridodicarboutybbis (tripheny1phosphine)iridiurn ( I ) .-The complex is conveniently prepared by the action of sodium