Rhodium-hydrides vibration

In an effort to assign the bands to ee and ae isomers the (thixantphos)Rh(CO)2D complex was measured for comparison. Upon H/D exchange, only Vi and v3 shift to lower wavenumbers (respectively 18 and 14 cm-1), and therefore, these two bands are assigned to the carbonyl frequencies of the ee complex. The two bands that do not shift, v2 and v4, belong to the ae complex. From the disappearance of a low-frequency shoulder upon H/D exchange, it can be concluded that one of the rhodium hydride vibrations is partly hidden under v4. 

The rhodium-hydride vibration disappears upon deuteration of the complex as the rhodium-deuteride vibration appears in the fingerprint region. The large frequency shift of the highest energy absorption is indicative of a trans-CO geometry [40]. In solution IR, the rhodium hydride vibration and the lowest energy CO vibration overlap, which results in only two absorptions. 

No uu—u stretching vibrations can be detected in the infrared spectra of the cobalt and rhodium complexes, but the presence of hydride ligands is confirmed by the appearance of a quintet pattern in the high-field n.m.r. spectra of these derivatives. The apparent equivalent coupling of the hydride ligand to the four 31P nuclei has been tentatively explained in terms of a nonrigid structure.2... 

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,... 

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