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.

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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3 ) were studied in homometathesis of hex 1 ene, metathesis polycondensation of deca 1,9 diene, and ring opening metathesis polymerization of cy clooctene. The nature of the carbene fragment in the tungsten catalysts substantially affects their catalytic activity. Silicon containing catalysts 1 and 2 were found to be 3-5 times less active than their hydrocarbon analogs 3 and 4. Metathesis polymerization of cyclooctene in the bulk with initiators 1-4 completed within a few minutes to form a block. Stereoregularity of the formed polyoctenamers depends to a considerable extent on the nature of the carbene fragments in the starting initiators. Initiators 1-2 lead to polyoctenamers mainly containing the cis units, whereas the use of complexes 3 and 4 affords polyoctenamers mainly containing the trans units. The structures of novel compound 2 and known complexes 1, 3, and 4 were determined by X ray diffraction analysis.

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