Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers

Here it is demonstrated that there is a linear relationship between the terminal 3d metal hydride stretching wavenumber ν_MH and the metal hydride distance d_MH reported to date: ν_MH ∼ (−1.05d_MH + 3.35) × 10³ cm^–1. That is, upon moving from Group 3 to Group 10 metals in complexes and binary molecules, ν_MH (cm^–1) increases as the metal–hydrogen distance d_MH (Å) decreases as determined by X-ray diffraction, molecular spectroscopy, and, in one case, neutron diffraction. This trend contrasts with the relatively constant bond dissociation free energy (BDFE) of the diamagnetic complexes and the usually smaller BDFE (by as much as −20 kcal/mol) of paramagnetic ones, thus showing that there is no correlation between either ν_MH or its force constant with the BDFE. If a positive charge is added to the complex by metal-ion oxidation, ν_MH may increase or decrease depending on the coordination number and spin state of the complex.