In dihydrogen complexes

Related studies have made use of 2H NMR spectroscopy in the solid state. The theoretical foundation for the study of dynamics in dihydrogen complexes was developed by Buntkowsky and coworkers.124 Solid-state 2H NMR spectroscopy has been used to study the dynamics of bound D2 in tntns- Ru( D2)CI(dppe)2 PI>, where evidence was found for coherent rotational tunneling, with a barrier of 6.2kcal/ mol.125 In more recent work, solid-state 2H NMR spectroscopy has been used to determine the structure and dynamics of several dihydrogen Ru complexes.126... 

We suggested fast relaxation as a criterion for the presence of close H-H distances in dihydrogen complexes, and Morris et and our own group have detected a substantial excess relaxation for H -H bonding. Making the usual assumptions, the excess in cases such as 5 and 6 can be interpreted in terms of an H- H distance of about 1.8 A in all the cases studied, a value consistent with the structural data in related systems. 

The unusual H-H bonding mode in dihydrogen complexes is of the greatest interest, also from the point of view of DQCC measurements. MO calculations of a simple [Rb-Dj]" model have shown that a transformation of classical dihydride structures to dihydrogen complexes leads to a dramatic increase of DQCC from 50 up to 155 kHz [11]. In addition, the asymmetry parameter T) grows from 0.025 to 0.62 and the orientation of the major axis of EFG (eq ) is remarkably deviated from the D-D vector in the dihydrogen complex (Scheme 1). Similar data have also been obtained in MO calculations of transition metal hydrides (Table 6). 

The infrared stretching frequency has also been used to distinguish dihydrogen from dihydride complexes. The Raman frequency of free H is 4300 cm and typical v j vibrations are found between 1700 and 2300 cm" . The dihydrogen ligand vibrates at a frequency between that of free H and those of metal hydrides. " Typical values for the H-H stretch in dihydrogen complexes lie between 2400 and 3100 cm" . For example, Kubas s compoimd contained a band at 2690 cm" (see Equation 2.19) for the H-H stretch. Although observed in some cases, the H-H stretch is weak and has not been identified in the majority of H complexes. 

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