Composition of catalyst resting states of hydroformylation catalysts derived from bulky mono-phosphorus ligands, rhodium dicarbonyl acetylacetonate and syngas

•In situ spectroscopic study of hydroformylation catalysts at typical reaction concentrations.•Two ligands in the literature found to not coordinate to rhodium in the presence of syngas.•The highly active Rh/1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane catalyst system studied by in situ HPIR spectroscopy and found to be mixture of resting states, with the composition tuneable by reaction conditions.