2H-T1 Relaxation and Deuterium Quadrupole Coupling Constants in Transition Metal eta2-H2 Complexes

2H-T1 min values of the classical D and non-classical D2 ligands in the complexes pp3RuD2 ([D2]1), pp3OsD2 ([D2]1), [pp3RuD(D2)]+ ([D3]3), and [pp3OsD(D2)]+ ([D3]4) (pp3=P(CH2-CH2PPh2)3) have been measured by variable-temperature 2H NMR spectroscopy in CH2Cl2. The deuterium quadrupole coupling constants (DQCCs) for the dihydrogen ligands in [D3]3, [D3]4, [OsD(D2)Cl(CO)[P(iPr)3]2] ([D3]8) and [Re(PMe3)4CO(D2)]+ ([D2]10) have been derived from the 2H-T1 min data assuming four different models of internal D2 motion. By estimating the angle α between the direction of the electric field gradient and the motion axis, and the asymmetry parameter η, from MO calculations, we showed that the model adopted can influence the calculation of the DQCC. The DQCCs in the dideuterium complexes ranged between 47 and 86 kHz (or between 56 and 101 kHz when a was close to the magic angle), demonstrating clearly that the DQCC in a D2 ligand is lower than that in comparable, classical deutende systems. This conclusion was supported by independent 2H-71 min experiments carried out on both [Cp*Ru(D2)(dppm)]+ (dppm = PPh2CH2PPh2) containing a D2 ligand that was quite rigid (on the T1 NMR time scale) and its classical analogue [Cp*Ru(D)2(dppm)]+. The results can be interpreted in terms of direct back-bonding interactions between M and H in an M - H2 triangulo system.