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 νMH and the metal hydride distance dMH reported
to date: νMH ∼ (−1.05dMH + 3.35) × 103 cm–1. That is, upon moving from Group 3 to Group 10 metals in complexes
and binary molecules, νMH (cm–1) increases as the metal–hydrogen distance dMH (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 νMH or its force constant with the BDFE. If a positive charge is added
to the complex by metal-ion oxidation, νMH may increase
or decrease depending on the coordination number and spin state of
the complex.