Diamagnetic Group 6 Tetrakis(di-tert-butylketimido)metal(IV) Complexes

The addition of 4 equiv of LiNC-t-Bu₂ to CrCl₃, MoCl₅, and WCl₆ in diethyl ether produced the complexes M(NC-t-Bu₂)₄ (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual D_2d 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_xy and d_z2 orbitals sufficiently to allow spin pairing in the d_xy orbital. A more sophisticated quantum-mechanical analysis of Cr(NC-t-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.