Paramagnetic Transition Metal Hydride Complexes

The structures, bonding, energetics, preparation, characterization, and reactions, both stoichiometric and catalytic, are categorized for all paramagnetic hydride complexes (PHC) with terminal hydrides and then bridging hydrides: 58 and 177 crystallographically characterized, respectively, including 49 mixed-valence complexes. Common methods of synthesis are documented and trends in PHC properties are revealed. The tabulated experimentally and theoretical bond energies indicate that PHC with similar ligands have weaker M-H bonds than diamagnetic hydrides. Magnetometry- and EPR-related results are tabulated and interpreted in terms of the bonding. Most complexes with bridging hydrides have reduced magnetic moments due to antiferromagnetic coupling. Hyperfine coupling constants range from small for the Ni^III-H-Fe^II Ni_aC state of [NiFe] and the Fe-H-Fe E₄(4H) state of [MoFe] nitrogenase to 293 MHz (for matrix-isolated NH(CO)₃₎ depending on the orbitals involved in bonding and the Fermi contact term. Hydride ¹H NMR resonances were detected for ten compounds with populated paramagnetic states. Reactions featured include oxidation, proton loss, hydrogen atom transfer, hydrogen loss and substitution, hydride transfer and reduction. Brief details of more than 40 homogeneous catalytic processes involving suspected PHC are provided. Since PHC are mainly found for earth-abundant 3<i>d</i> metals, there continues to be much interest in their properties and uses in sustainable catalysis.