New silicon , germanium , and tin containing imido alkyl molybdenum complexes (ArN) 2 Mo(CH 2 EMe 3 ) 2 (Ar is 2,6 diisopropylphenyl; E = Si (1), Ge (2), Sn (3)) were prepared in the crystalline state in 58-66% yields by the reactions of the (ArN) 2 MoCl 2 (DME) com plex with alkyllithium derivatives Me 3 ECH 2 Li (E = Si or Ge) or the Grignard reagents Me 3 ECH 2 MgCl (E = Ge or Sn). The structures of complexes 1-3 and the known analog (ArN) 2 Mo(CH 2 Bu t ) 2 (4) were established by X ray diffraction analysis. Complexes 1-3 were found to be isostructural. The coordination environment about the Mo atom can be described as a distorted tetrahedron. Complex 4 has a similar structure. The Mo-C distance tends to decrease with increasing electron donating ability of the EMe 3 group.

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The tin-containing molybdenum π-complexes (R 3 SnCH=CH 2 )Mo(N-2,6-Pr i 2 C 6 H 3 )(OCMe 2 CF 3 ) 2 (R = Me, Et, Ph) were synthesized by reaction of PhMe 2 CCH=Mo(N-2,6-Pr i 2 C 6 H 3 )(OCMe 2 CF 3 ) 2 with organotin vinyl reagents R 3 SnCH=CH 2 . The structure of compounds I-III was determined by NMR spectroscopy. Complexes I-III are active initiators of the norbornene metathesis polymerization.

3 ) were synthesized by the reaction of the R´-CH=Mo(NAr)(OR) 2 compounds (R´ = Bu t or PhMe 2 C) with silicon containing vinyl reagents. The structures of complexes 1 and 2 and the known PhMe 2 C-CH=Mo(NAr)(OCMe 2 CF 3 ) 2 compound were established by X ray dif fraction. The catalytic properties of the silicon containing carbene complexes in homometathesis of hex 1 ene and metathesis polymerization of cyclooctene were studied. The catalytic activity of these complexes and the stereoregularity of the resulting polyoctenamers substantially de pend on the nature of the substituent at the carbene carbon atom.

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