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)M^II(OTf)₂] with NaBH₄ forms [(L)M^II(BH₄)]⁺ (M = Fe, Co, Ni) that can be detected by electrospray ionization mass spectrometry. Energy-resolved collision-induced dissociations of [(L)M^II(BH₄)]⁺ provide threshold energies (ΔE_CID) for the formations of [(L)M^II(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 ΔE_CID and the M–H stretching vibration frequencies. The ΔE_CID also correlates with reaction rates for hydride transfer reactivity in the gas phase and known reactivity trends in the solution phase.