H NMR Studies of Metal Carbonyl Hydrides

The presence of hydride ligands may affect the CP/MAS spectra in different ways. The most straightforward effect is detected on the relative intensity ratio of the CO resonances according to the extent of dipolar interaction 

In addition to investigations this important class of compound may also be studied by means of MAS NMR spectroscopy. In fact we have shown that sharp resonances may be obtained from solid state MAS NMR of molecular hydrido carbonyl complexes without recourse either to the multiple pulse methods [22] or to isotopic dilution [23]. This is due to the fact that in these compounds the protons are intramolecularly diluted. We considered again H20s3(CO)io which crystallizes with one molecule as the asymmetric unit, as determined by a neutron diffraction study [24], and displays an interproton distance of 2.38 A. This interproton distance affords a dipolar coupling constant of 8.91 kHz. Now, rotation of the sample at 8.1 kHz allows 

In HRu3(CO)9C2 Bu, the proton relaxation times T are 0.2 s and 2.9 s for t-butyl and for the hydride resonance, respectively. The latter value is significantly shorter than that found for H20s3(CO)io and indicates that dipolar coupling between the methyl groups and the hydride ligand does provide an efficient spin-lattice relaxation pathway. 

In solution, there is just one resonance and one resonance down to 153 K [30]. The solid state H MAS NMR spectrum [31] at ambient temperature shows a single resonance in the hydride region, whereas on the basis of the X-ray findings we would have expected to observe four resonances. Thus, the observation of just one peak suggested the possibility of there being a 

METAL CARBONYLS CONTAINING tc-BONDED AROMATIC LIGANDS 

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