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

Abstract

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.