For a Lewis acid, its affinity for a hydride ligand is called its hydricity: Since hydride does not exist as a stable anion in solution, this equilibrium constant (and its associated free energy) are calculated from measurable equilibria. The reference point is the hydricity of a proton, which in acetonitrile solution is calculated at −76 kcal mol−1: Relative to a proton, most cations exhibit a lower affinity for H−. Some examples include: These data suggest that [HPt(dmpe)2]+ would be a strong hydride donor, reflecting the relatively high stability of [Pt(dmpe)2]2+. The ionic bond character in transition metal hydrides can be measured for D ligands via deuterium quadrupole coupling constants. The rates of proton-transfer to and between metal complexes are often slow. Many hydrides are inaccessible to study through Bordwell thermodynamic cycles. As a result, kinetic studies are employed to elucidate both the relevant thermodynamic parameters. Generally hydrides derived from first row transition metals display the most rapid kinetics followed by the second and third row metal complexes. The determination of structures of metal hydrides can be challenging since hydride ligands do not scatter X-ray well, especially in comparison to the attached metal. Consequently M-H distances are often underestimated, especially in early studies. Often the presence of a hydride ligand was deduced by the absence of a ligand at an apparent coordination site. Classically, the structures of metal hydrides was addressed by neutron diffraction since hydrogen strongly scatters neutrons. Metal complexes containing terminal hydrides are common. In bi- and polynuclear compounds, hydrides usually are bridging ligands . Of these bridging hydrides many are oligomeric, such as Stryker's reagent. [(Ph3P)CuH]6 and clusters such as [Rh6(PR3)6H12]2+. The final bonding motif is the non-classical dihydride also known as sigma bond dihydrogen adducts or simply dihydrogen complexes. The [W(PR3)2(CO)3(H2)] complex was the first well characterized example of both a non-classical dihydride and sigma-bond complex in general. X-ray diffraction is generally insufficient to locate hydrides in crystal structures and thus their location must be assumed.