cis-Dialkyl complexes of tantalum, Ta(CH
2Ph)
2Cp*(
4-C
4H
6) (
2a) (Cp* =
5-C
5Me
5),TaMe(CH
2SiMe
3)Cp*(
4-butadiene) (
6), and TaMe(CH
2CMe
3)Cp*(
4-butadiene) (
7), werefound to be catalyst precursors for ring-opening metathesis polymerization (ROMP) ofnorbornene to give poly(norbornene) with a high
cis-vinylene double-bond (97-99%) content,while an
o-xylylene complex Ta(CH
2Ph)
2(
4-
o-(CH
2)
2C
6H
4)Cp* (
9) was also an initiator togive poly(norbornene) with a high
trans-vinylene double-bond (92-95%) content. When aCp-butadiene complex Ta(CH
2Ph)
2Cp(
4-C
4H
6) (
2b) was used as an initiator, we obtainedpoly(norbornene) with no selectivity (1:1 mixture of
trans- and
cis-vinylene bonds). Thefactors
controlling these stereoselectivities have been investigated, and we obtained thefollowing results: (1) We isolated benzylidene complexes and found that the proportion of
anti- and
syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalumcenter, i.e., 1,3-butadiene or
o-xylylene. Thermolysis of
2a in the presence of PMe
3 resultedin the formation of a benzylidene complex Ta(=CHPh)Cp*(
4-C
4H
6)(PMe
3) (
3a) as an
anti-rotamer, which has been characterized by X-ray crystal structure analysis, while a similartreatment of
2b afforded Ta(=CHPh)Cp(
4-C
4H
6)(PMe
3) (
3b), also in the
anti-rotamer form,as revealed by a comparison of the chemical shift value of the benzylic proton of
3b withthat of
3a. In sharp contrast to the
anti-geometry, a benzylidene complex Ta(=CHPh)(
4-
o-(CH
2)
2C
6H
4)Cp* (
12), bearing an
o-xylylene ligand instead of the butadiene ligand, wasobtained by thermolysis of
9. The X-ray crystal structure analysis of
12 revealed that it isa
syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes areconsidered as intermediates during the propagation step. We could not isolate anymetallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane
Cp*(
4-o-xylylene)Ta[CH(Ph)CH(C10H6)CH] (
15) was isolated in 18% yield when acenaphthylene,instead of the norbornene, was added to
12. The
trans-phenyl geometry of the metallacyclobutane ring system, which indicated the retention of the Ta=C bond stereochemistry,was determined by the X-ray analysis of
15, whose structural features were compared withthat of
Cp*(
4-butadiene)Nb[CH2CH(C10H6)CH] (
14). (3) In contrast to high
cis/
transstereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the
13C NMR spectra ofthe hydrogenated derivatives was found to be almost
atactic. This suggests that thealkylidene species would be a resting state during the propagation, and thereby monomercould be added to both faces of the Ta=C bond, leading to the
atactic polymer. Consequently,the high
cis and
trans stereoselectivity thus demonstrated is ascribed to the huge
congestionbetween the Cp* ligand and auxiliary ligands 1,3-butadiene or
o-xylylene.