Relationship\nbetween Transition-Metal Hydride Bond\nLengths and Stretching Wavenumbers

Here it is demonstrated that there is a linear relationship\nbetween\nthe terminal 3d metal hydride stretching wavenumber ν<sub>MH</sub> and the metal hydride distance <i>d</i><sub>MH</sub> reported\nto date: ν<sub>MH</sub> ∼ (−1.05<i>d</i><sub>MH</sub> + 3.35) × 10<sup>3</sup> cm<sup>–1</sup>. That is, upon moving from Group 3 to Group 10 metals in complexes\nand binary molecules, ν<sub>MH</sub> (cm<sup>–1</sup>) increases as the metal–hydrogen distance <i>d</i><sub>MH</sub> (Å) decreases as determined by X-ray diffraction,\nmolecular spectroscopy, and, in one case, neutron diffraction. This\ntrend contrasts with the relatively constant bond dissociation free\nenergy (BDFE) of the diamagnetic complexes and the usually smaller\nBDFE (by as much as −20 kcal/mol) of paramagnetic ones, thus\nshowing that there is no correlation between either ν<sub>MH</sub> or its force constant with the BDFE. If a positive charge is added\nto the complex by metal-ion oxidation, ν<sub>MH</sub> may increase\nor decrease depending on the coordination number and spin state of\nthe complex.