REACTIONS OF (ETA-C5ME5)TA(PME3)(H)2(ETA-2-CHPME2) AND THE NATURE OF THE ETA-2-CHPME2 LIGAND

Treatment of (eta-C5Me5)Ta(PMe3)(H)2(eta-2-CHPMe2) (1) with tertiary phosphines and phosphites leads to an equilibrium mixture of 1 and (eta-C5Me5)Ta(PR3)(H)2(eta-2-CHPMe2) (3a-d). Equilibrium constants, determined in benzene, are less than unity and decrease in the order PMe2Ph almost-equal-to P(OMe)3 > PEt3 > PMePh2 > P(OPh)3. 1 reacts with dmpe to give the adduct (eta-C5Me5)Ta(eta-1-dmpe)(H)2(eta-2-CHPMe2) (4) followed by (eta-C5Me5)Ta(dmpe)(H)(eta-2-CH2PMe2) (5) arising by a metal --> carbon hydrogen migration. Carbon monoxide reacts with 1 to give a mixture of (eta-C5Me5)Ta(CO)2(PMe3)2 (6) and (eta-C5Me5)Ta(CO)3(PMe3) (7), while dihydrogen reacts to give (eta-C5Me5)Ta(PMe3)2H4 (8). Treatment of 1 with 1 equiv of MeX (X = Cl, Br, I) affords (eta-C5Me5)Ta(PMe3)(H)(X)(eta-2-CHPMe2) (9a-c) and treatment with excess MeX, (eta-C5Me5)Ta(X)2(eta-2-CHPMe2) (11a-c). A comparison of the bonding in (eta-C5Me5)NbCl2(eta-2-CHPMe2) with that in (eta-C5Me5)NbCl2(HC = CH) using Fenske-Hall quantum chemical calculations suggests that the eta-2-CHPMe2 ligand is best described as a phosphinocarbene rather than a phosphaalkyne.