Spectroscopic Evidence of the First Example of an Isomer with the Hydride Ligand in the Equatorial Plane in the HRh(CO)<sub>4</sub><sub>-</sub><i><sub>x</sub></i>(PR<sub>3</sub>)<i><sub>x</sub></i> Family: HRh(CO)<sub>2</sub>(PR<sub>3</sub>)<sub>2</sub> (PR<sub>3</sub> = 1,2,5-Triphenyl-1<i>H</i>-phosphole, TPP)
enriched 13CO shows that the HRh(CO)2(TPP)2 (TPP = 1,2,5-triphenyl-1H-phosphole) complex, the resting state of the catalytic system for the hydroformylation of styrene by the Rh/TPP mixture, exists in solution as two stereoisomers in equilibrium. The minor isomer (10%) exhibits a geometry where the hydride ligand occupies an equatorial position, a unique case within the HRh(CO)4-x(PR3)x family.