Properties of Metal Hydrides of the Iron Triad

Metal hydride complexes
are essential intermediates in hydrogenation
reactions. The hydride-donor ability determines the scope of use of
these complexes. We present a new, simple mass-spectrometry method
to study the hydride-donor ability of metal hydrides using a series
of 18 iron, cobalt, and nickel complexes with N- and P-based ligands (L). The mixing of [(L)MII(OTf)2] with NaBH4 forms [(L)MII(BH4)]+ (M = Fe, Co, Ni) that can be detected by electrospray
ionization mass spectrometry. Energy-resolved collision-induced dissociations
of [(L)MII(BH4)]+ provide threshold
energies (ΔECID) for the formations
of [(L)MII(H)]+ that correlate well with the
hydride donor ability of the metal hydride complexes. We studied the
vibrational and electronic spectra of the generated metal hydrides,
assigned their structure and spin state, and demonstrated a good correlation
between ΔECID and the M–H
stretching vibration frequencies. The ΔECID also correlates with reaction rates for hydride transfer
reactivity in the gas phase and known reactivity trends in the solution
phase.