Spectroscopic evidence of the first example of an isomer with the hydride ligand in the equatorial plane in the HRh(CO)4-x(PR3)x family:: HRh(CO)2(PR3)2 (PR3 = 1,2,5-triphenyl-1H-phosphole, TPP)

A variable-temperature multinuclear NMR study using enriched (CO)-C-13 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.