The six-coordinate molybdenum(IV) oxido isocyanide complex 1 [Delta,Lambda-OC-6-2-3-[MoO(N-p boolean AND N-i)(2)((CNBu)-Bu-t)]; N-p boolean AND N-i = 4-tert-butylphenyl(pyrrolato-2-ylmethylene)amine] is obtained in diastereomerically pure form in the solid state, as revealed by single-crystal X-ray diffraction. In solution, this stereoisomer equilibrates with the Delta,Lambda-OC-6-2-4 diastereomer 2 at ambient temperature. The stereochemistry of both isomers has been elucidated by NMR, IR, and UV/vis spectroscopy in combination with density functional theory (DFT)/polarizable continuum model and time-dependent DFT calculations. The isomerization 1 -> 2 is suggested to proceed via a dissociative trigonal twist with dissociation of the imine nitrogen donor N-i of one chelate ligand (hemilabile ligand) rather than dissociation of the monodentate isocyanide ligand. The isomerization barrier has been experimentally determined as 91 and 95 kJ mol(-1) in tetrahydrofuran and toluene, respectively.