Diphosphite rhodium hydride complex

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Under syn gas pressure the rhodium acac precursors were converted to the catalytically active hydride complexes HRh(CO)2(L-L). The complexes are generally assmned to have a trigonal bipyramidal structure and two isomeric structures of these complexes are possible, containing the diphosphite coordinated in a bisequatorial (ee) or an equatorial-apical (ea) fashion. The structure of the complexes can be elucidated by (high pressure) IR and NMR data (Table 3). In the carbonyl region of the infrared spectrum the vibrations of the ee and ea complex can be easily distinguished. The ee complexes typically show absorptions around 20 15 and 2075 cm [24, 26,... 

For ligand 4b an X-ray crystal structure of the hydride rhodium dicarbonyl complex could be obtained [24]. The P-Rh-P angle in HRh(CO)2( 4b) is 116° and the complex is indeed a trigonal bipyramid with the diphosphite in the equatorial plane. The structure is somewhat distorted the larger ligands in the equatorial plane bend toward the small hydride. This explains why the coupling constants between phosphorus and hydrogen are... 

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