H-2-T-1 relaxation and deuterium quadrupole coupling constants in transition metal eta(2)-D2 complexes

H-2-T-1 min values of the classical D and non-classical D-2 ligands in the complexes pp(3)RuD(2) ([D-2]1), pp(3)OsD(2) ([D-2[2), [pp(3)RuD(D-2)](+) ([ D-3]3), and [pp(3)OsD(D-2)](+) ([D-3]4) (pp(3)=P(CH2CH2PPh2)(3)) have been measured by variable-temperature N-2 NMR spectroscopy in CH2Cl2,. The deute rium quadrupole coupling constants (DQCCs) for the dihydrogen ligands in [D -3]3, [D-3]4. [osD(D-2)Cl(Co)[P(iPr)(3)](2)] ([D-3]8) and [Re(PMe3)(4)CO(D- 2)](+) ([D-2]10) have been derived from the H-2-T-1 min data assuming four different models of internal D, motion. By estimating the angle a between t he direction of the electric field gradient and the motion axis, and the as ymmetry parameter eta, from MO calculations, we showed that the model adopt ed 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 wa s close to the magic angle), demonstrating clearly that the DQCC in a D-2 l igand is lower than that in cornparable, classical deuteride systems. This conclusion was supported by independent H-2-T-1 min experiments carried out on both [Cp*Ru(D-2)(dppm)](+) (dppm = PPh2CH2PPh2) containing a D-2 ligand that was quite rigid ton the T-1 NMR time scale) and its classical analogu e [Cp*Ru(D)(2)(dppm)](+). The results can be interpreted in terms of direct back-bonding interactions between M and H in an M-H-2 triangulo system.