Structure and Dynamic Behavior of (η3-Allyl)bromodicarbonylmolybdenum(II) Complexes Containing Polydentate 2-Pyridylphosphanes or Their Oxides as Chelating Ligands: Occurrence of Three Fluxional Processes

The pseudo-octahedral complexes [Mo(η3-allyl)Br(CO)2(PPynPh3–n-P,N)] (Py = 2-pyridyl; n = 2, 3) and [Mo(η3-allyl)Br(CO)2(OPPymPh3–m-O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depending on the chelating ligand. The complexes containing PPhPy2 and PPy3 as chelating P,N-donors undergo a novel “pivoted double switch” mechanism which scrambles two equatorial coordination sites with racemization, while maintaining the identity of the phosphorus atom trans to the allyl ligand. The complexes containing 2-pyridylphosphane oxides as chelating O,N-donors undergo a nondissociative intramolecular trigonal-twist (or turnstile) rearrangement which maintains the identity of the phosphane oxide oxygen atom coordinated in an equatorial position. All the complexes containing uncoordinated pyridyls undergo a slow dissociative exchange of pyridyls without coordination site scrambling. The structure of the complex [Mo(η3-allyl)Br(CO)2(OPPy3-O,N)] has been determined by X-ray diffraction.