Hydride complexes, vibrational data

Table II. Vibrational Data on Metal Hydride Bridged Complexes...

GAMESS . The vibrational data obtained in this way should, however, be used with care, since many weakly bonded systems exhibit large-amplitude motions (especially when simple hydrides are involved) which cannot be accurately modelled at the harmonic level. Indeed, the very concept of the equilibrium structure of such complexes, while formally valid, loses much of its significance, and a much larger section of the surface must be sampled than is customary for ordinary molecules. Appropriate techniques for evaluating vibrational wavefunctions beyond the harmonic level have been reviewed by Le Roy et and very recently by Briels et a/. and the interested reader is referred to these. 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

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,... 

The equilibrium constants determined in Eqs. (8) and (9) are actually Boltzmann factors (statistics not included), but they indicate that in this case the heavier isotope prefers to occupy the hydride site. In contrast, both Field and Crabtree had previously observed the opposite effect, e.g., = 1.3 for ReD2(HD) - ReHD(D2) in [ReH2(H2)(CO)(PMe2Ph)3]+.57,58 However, detailed analysis of chemical shift and coupling constant data as a function of temperature in the Ir complex has not been carried out in these systems. The isotope effect was simplistically interpreted to be a consequence of a greater vibrational zero-point energy difference between Re(i/2-HD) and Re(f/2-D2) relative to Re-H and Re-D.58 These systems are quite complex... 

Table 7 IR data in the range of vco and Vno vibrations (in cm ) for ReHX(CO)(NO)(PR3) hydrides (3) and their hydrogen bonded complexes 4 and 5 (of 1 1 and 2 1 composition.

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