Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers

Abstract

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) × 10 3 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.