Transition metal carbonyls and carbonyl hydrides

Carbon monoxide is an important ligand. It coordinates to most transition metals and is implicated in many catalytic processes. Transition metal carbonyl complexes have been studied for many years. Assigmnents for the CO stretch region are available for many complexes but complete vibrational assignments [69] are limited to the simpler compounds such as [M(CO)e] (M = Cr, Mo, W), [Fe(CO)s] and [Ni(CO)4]. Even for these, some of the fundamentals are known only 

INS can provide complementary information. The CO stretch region around 2000 cm is well-suited to investigation by infrared and Raman spectroscopies. However, the M-C stretch, M-C=0 bend and C-M-C bend vibrations are much weaker and more difficult to access. Additionally, some of the modes are inactive. 

Metal carbonyls have many derivatives obtained by (formal) substitution of a carbonyl for another ligand, including hydride. The interest in the complexes arose because they were considered as potential models for hydrogen on metal surfaces ( 7.3.1). In general this is valid, in that the frequencies fall in similar ranges to those found on metal surfaces, the detailed interaction with the surface is distinctly different from that in a transition metal cluster and the analogy is imperfect. 

The INS spectra of metal carbonyl hydrides [My(CO)zHJ (x,y,z 1) are dominated by the hydrogen motion. However, this motion is coupled more or less strongly to the carbonyl modes and the result is a rainbow at 

The INS spectrum of [Co(CO)4H] [72] Fig. 11.20a illustrates this point. The intense band at 696 cm is the doubly degenerate Co-H bend and the features at 330 and 430 cm are M-C=0 bending modes that involve the axial CO. The assignments are confirmed by a DFT calculation of the isolated molecule [73], Fig. 11.20b. Fig. 11.20c is an estimate of the spectrum that would be observed on TOSCA and demonstrates the huge strides in instrument performance that have occurred in the 30 years since Fig. 11.20a was recorded on a low-bandpass spectrometer at a reactor source. 

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