Is bond stretch isomerism in mononuclear transition metal complexes a real issue? The misleading case of the MoCl5/tetrahydropyran reaction system

Distinct batches of orange (1a–e) and green crystals (2a–e) were isolated from the reactions of MoCl5 with tetrahydropyran (thp), respectively at room temperature (in CH2Cl2) and at ca. 80 °C (in ClCH2CH2Cl). Crystals 2a–e are isomorphous to 1a–e and the IR spectra are almost superimposable. 1a–e were identified by X-ray studies as cis-MoCl4(thp)2, in agreement with previous findings. Careful refinement of the X-ray data of 2a–e by modeling one position as disordered between chlorine (minor component) and oxygen (major component) led to the conclusion that 2a–e consisted of a mixture of cis-MoCl4(thp)2 and mer-MoOCl3(thp)2, the latter being largely prevalent.