The hydroformylation catalyst (L)RhH(CO) 2 , with a triptycene-derived bisphosphite ligand, and its iridium analogue were studied in solution, in the solid state, and computationally (DFT) in order to determine their molecular structures and to understand the equilibria between isomers. The predicted intramolecular distance of the two biphenyl side groups of these ligands strongly depends upon the treatment of inter-and intramolecular noncovalent interactions. A balanced treatment of these "weak" interactions is a prerequisite for obtaining reasonable structures and consequently also for determining relative energies of the different P-coordination modes. Low-temperature NMR studies confirm the equilibrium between axial−equatorial and equatorial−equatorial isomers in solution and have been used to estimate their relative stability in the equilibrium.

We report the synthesis and single-crystal molecular structures of two stereoisomers of trigonalbipyramidal hydrido dicarbonyl Ir complexes [(L 2 )Ir(H)-(CO) 2 ] with ae (axial−equatorial) and ee (equatorial− equatorial) ligand P coordination and fluxional behavior in solution. L 2 is a new chelating bisphosphite with unprecedented high selectivity in Rh-catalyzed bis-hydroformylation of butadiene to adipic aldehyde. These Ir analogues are ideal stabilized structural models for nonseparable ae and ee Rhhydroformylation resting state isomers [(L 2 )Rh(H)(CO) 2 ]. With Ir, both stereoisomers with the same ligand could be characterized independently for the first time.

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