Diamagnetic Group 6 Tetrakis(di-<i>tert</i>-butylketimido)metal(IV) Complexes

The addition of 4 equiv of LiNC-<i>t</i>-Bu₂ to CrCl₃, MoCl₅, and WCl₆ in diethyl ether produced the complexes M(NC-<i>t</i>-Bu₂)₄ (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual <i>D</i>_2<i>d</i> symmetry in the solid state. ¹H and ¹³C NMR spectra indicated that the complexes are diamagnetic, and a qualitative MO analysis showed that the orthogonal π-donor and -acceptor orbitals of the ketimide ligand cooperatively split the d_<i>xy</i> and d_z2 orbitals sufficiently to allow spin pairing in the d_<i>xy</i> orbital. A more sophisticated quantum-mechanical analysis of Cr(NC-<i>t</i>-Bu₂)₄ using density functional/molecular mechanics methods confirmed the qualitative analysis by showing that the singlet state is 27 kcal/mol more stable than the triplet state.