An Intermediate Spin Nickel Hydride Complex Stemming from De localized Ni2(μ-H)2 Bonding

The nickel hydride complex [Cp'Ni(mu-H)](2) (1, Cp' = 1,2,3,4-tetraisopropylcyclopentadienyl) is found to have a strikingly short Ni-Ni distance of 2.28638(3) angstrom. Variable temperature and field magnetic measurements indicate an unexpected triplet ground state for 1 with a large zero-field splitting of +90 K (63 cm(-1)). Electronic structure calculations (DFT and CASSCF/CASPT2) explain this ground state as arising from half occupation of two nearly degenerate Ni-Ni pi* orbitals.