a Reevaluation of Distortional Isomerism

The blue and green forms of MoOCl_2 (PMe_2Ph)_3 and (LWOCl_2) PF _6 (L = 1,4,7-trimethyltriazacyclononane) are not examples of distortional isomerism. For both systems, the blue form is pure and the green form is a mixture of the blue complex and other components. Characterizations of the green material of each indicates that green "MoOCl _2(PMe_2Ph) _3" is a mixture of MoOCl _2(PMe_2Ph) _3 and MoCl_3(PMe _2Ph)_3 and that green "(LWOCl_2) PF _6" is a mixture of (LWOCl_2 ) PF_6 and two other species; one is a W(IV) and the other a W(VI) compound. Both solid state (XPS, Raman, infrared, powder X-ray diffraction) and solution phase (^1H NMR, UV -visible, EPR, cyclic voltammetry) measurements were applied to the respective green materials, presenting a consistent description of their composite nature. A single crystal X-ray structure determination of a crystal of green "MoOCl _2(PMe_2Ph) _3" allowed the compositional disorder to be partially resolved in this case, due to the high composition of MoOCl_3(PMe _2Ph)_3 in the sample (ca. 0.30 mole percent by XPS and visible spectroscopies). The single crystal X-ray diffraction data from the original single crystal study of green "(LWOCl_2 ) PF_6" were reinvestigated to ascertain whether compositional disorder could also be implicated in this system. Several composite models were applied to these data, resulting in improved fits to the observed data. This indicated that the original data collected for the crystal of green "(LWOCl _2) PF_6" could not support distortional isomerism in green "(LWOCl _2) PF_6". X-ray structure calculations on models of the compositional disorder in both systems illustrated the pronounced effect of disordering chlorine electron density on the derived metal-oxo bond lengths. A calibration curve of Mo-Cl bond length versus composition calculated from these models resulted in a favorable comparison of this present theoretical work with other experimental X-ray studies of the MoOCl_2 (PMe_2Ph)_3 system. Other examples of mixtures mistaken for pure materials are also discussed as well as a recent resurgence of distortional isomerism in certain niobium complexes. The nature of the compositional disorder characterized for the MoOCl_2(PMe_2 Ph)_3 and (LWOCl _2) PF_6 systems has profound implications for metal-ligand distances determined by X-ray crystallography.