4-CO-ORDINATE VERSUS 5-CO-ORDINATE BIS(IMIDO) ALKENE COMPLEXES OF MOLYBDENUM - THE CONTRASTING EFFECTS OF TERT-BUTYL-IMIDO AND 2,6-DIISOPROPYLPHENYL-IMIDO SUBSTITUENTS

The five-co-ordinate complex [Mo(NC6H3Pr2i-2,6)(2)(PMe(3))(2)(C2H4)] 2a has been prepared via treatment of [Mo(NC6H3Pr2i-2,6)(2)Cl-2(dme)] la (dme = 1,2-dimethoxyethane) with 2 equivalents of EtMgCl in Et(2)O in the presence of excess PMe(3); its propene analogue 2b is unstable at reduced pressure, converting to [Mo(NC6H3Pr2i-2,6)(2)(PMe(3))(2)] 3 with loss of propene. Complex 2a can also be synthesised, and 2b generated in situ, by treatment of 3 with ethene and propene respectively in hydrocarbon solvent; the analogous eta(2)-butadiene complex [Mo(NC6H3Pr2i-2,6)(2)(PMe(3))(2)(eta(2)-C4H6)] 2c has also been generated in solution and identified by NMR spectroscopy. The molecular structure of 2a shows a distorted trigonal-bipyramidal co-ordination geometry with axial phosphines and equatorial imido ligands: the ethene ligand aligns in the P-Mo-P plane and perpendicular to the N-Mo-N plane. Complex 3 can also be prepared directly through the reaction of la with magnesium turnings in tetrahydrofuran (thf) in the presence of excess PMe,. By contrast, the reaction of [Mo(NBu(t))(2)Cl-2(dme)] Ib under identical conditions affords the binuclear imido-bridged complex [{Mo(NBu(t))(mu-NBu(t))(PMe(3))}(2)] 4. Complex 4 does not react with ethene or propene to give the known mononuclear bis(tert-butylimido) derivatives, [Mo(NBu(t))(2)(PMe(3))(C(2)H(3)R)] (R = H 5a or Me = 5b) reflecting the robust nature of the imido bridges.