The scandium hydride complex [(Cp*SiNR)(PMe(3))Sc(mu-H)](2), (1) ((Cp*SiNR) = {(eta(5)-C(5)Me(4))SiMe(2)(eta(1)-NCMe(3))}) is prepared by hydrogenation of (Cp*SiNR)ScCH(SiMe(3))(2) in the presence of trimethylphosphine. The hydride complex is a catalyst precursor for the polymerization of alpha-olefins, yielding atactic products of low molecular weight (M(n) = 3000-7000). GC/MS analysis of volatile, oligomeric products revealed that all scandium centers are active during the polymerization. Selectivity for head-to-tail insertion is high (>99%) and for the tetramer, pentamer, and hexamer formed during propene polymerization, the maximum theoretical numbers of head to-tail stereoisomers are observed by capillary GC. The stoichiometric reaction between 1 and 2 equiv of ethylene produces the unusual ethylene-bridged dimer [(Cp*SiNR)(PMe(3))Sc](2)(mu,eta(2),eta(2)-C2H4) (2) and an equivalent of ethane, whereas the same reaction with propene affords the phosphine-free, alkyl-bridged scandium dimer [(Cp*SiNR)Sc](2)(mu-CH2CH2CH3)(2) (3). The absence of coordinating phosphine allows the latter complex to function as a more active olefin polymerization catalyst precursor. 1 reacts with styrene to form a unique double-insertion product arising from sequential 1,2- and 2,1-styrene insertion. The structure of the catalytic intermediate in solution was determined by low-temperature C-13-NMR studies of the model complexes (Cp*SiNR)[P((CH3)-C-13)(3)]ScCH2CH(CH3)CH2CH2CH3 and (Cp*SiNR)-(PMe(3))(ScCH2CH)-C-13((CH3)-C-13)(2). One phosphine-bound species is observed in equilibrium with only one phosphine-free species. The symmetry properties of the latter indicate that it is a monomeric, hence 12-electron, scandium alkyl complex. Semiquantitative treatment of equilibrium concentration data supports this conclusion.