Structure and dynamic behavior of (eta(3)-allyl)bromodicarbonylmolybdenum(II) complexes containing polydentate 2-pridylphosphanes or their oxides as chelating ligands: Occurrence of three fluxional processes

The pseudo-octahedral complexes [Mo(eta(3)-allyl)Br(CO)(2)(PPynPh3-n-P,N)] (Py = 2-pyridyl; n = 2, 3) and [Mo(eta(3)-allyl)Br(Co)(2)(OPPymPh3-m-O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depend ing on the chelating ligand. The complexes containing PPhPy2 and PPy3 as ch elating P,N-donors undergo a novel "pivoted double switch" mechanism which scrambles two equatorial coordination sites with racemization, while mainta ining the identity of the phosphorus atom trans to the allyl ligand. The co mplexes containing 2-pyridylphosphane oxides as chelating O,N-donors underg o a nondissociative intramolecular trigonal-twist (or tumstile) rearrangeme nt which maintains the identity of the phosphane oxide oxygen atom coordina ted in an equatorial position. All the complexes containing uncoordinated p yridyls undergo a slow dissociative exchange of pyridyls without coordinati on site scrambling. The structure of the complex [Mo(eta(3)-allyl)Br(CO)(2) (OPPy3-O,N)] has been determined by X-ray diffraction.