Treatment of (η 4 -s-trans-butadiene)tantalocene cation (3, with methyltris(pentafluorophenyl)borane anion) with tert-butyl isocyanide (4 h, at 45 °C in bromobenzene) yields the [(η 2 -butadiene)(CtNCMe 3 )TaCp 2 + ] complex 4a (70% isolated, ν ˜(CtNR) 2164 cm -1 ). Likewise treatment with n-butyl or cyclohexyl isocyanide gives the analogous products 4b and 4c, respectively. The cyclohexyl isocyanide adduct 4c was characterized by X-ray diffraction. It shows the η 2 -butadiene ligand oriented at the front of the Cp 2 Ta + bent metallocene wedge with the noncoordinated butadiene vinyl group located in the "vinyl-inside" position. The Ta-CtN-R unit is almost linear (angles Ta-C-N ) 177.2(5)°, CtN-R ) 174.0(8)°; d(CtN) ) 1.152(6) Å). Photolysis of [(butadiene)TaCp 2 + ] (3) with excess cyclohexyl isocyanide followed by thermal treatment gave rise to the formation of [bis(κC-cyclohexyl isocyanide)-TaCp 2 + ] (8, with [CH 3 B(C 6 F 5 ) 3 -] counteranion; >50% isolated, ν ˜(CtNR) 2134 and 2088 cm -1 ). Complex 8 was characterized by an X-ray crystal structure analysis. It shows a pseudotetrahedral [Cp 2 Ta(CtNR) 2 + ] cation (R ) C 6 H 11 ). The Ta-CtN-R units are both slightly bent at nitrogen (angles CtN-R ) 169.0(13) and 163.7(8)°; the corresponding Ta-C-N angles are 179.4(9) and 177.4(7)°). The bonding situations of complexes 4 and 8 were analyzed by DFT calculations. The computational study shows that metal to isonitrile back-bonding does not contribute significantly to the Cp 2 Ta + -CtNR bonding interaction in these complexes. The bonding between Cp 2 Ta + and the CtNR ligands is dominated by electrostatic effects.

η 4 -Butadiene)(η 5 -cyclopentadienyl)tantalum dichloride (6), synthesized by treatment of CpTaCl 4 (5) with (butadiene)magnesium (7), was treated with 2 molar equiv of sodium cyclopentadienide to yield Cp 3 Ta(butadiene) (9). The spectroscopic analysis revealed the (η 5 -Cp) 2 (η 1 -Cp)(η 2 -C 4 H 6 )Ta structure of 9. Treatment of 9 with 1 molar equiv of the salt [(Cp 2 Zr-CH 3 ) + (CH 3 B(C 6 F 5 ) 3 ) -] (11) resulted in the clean transfer of a cyclopentadienide ligand from tantalum to zirconium with formation of the neutral zirconium compound Cp 3 -Zr-CH 3 (12) and the cation Cp 2 Ta(butadiene) + (10; with CH 3 B(C 6 F 5 ) 3 -counteranion). The spectroscopic characterization and an X-ray crystal structure analysis has revealed that 10 is a cationic (s-trans-η 4 -butadiene)bis(η 5 -cyclopentadienyl)tantalum complex. It reacts cleanly with a variety of ketones at 60°C to form the respective 7,7-disubstituted tetrahydrotantalaoxepines 13. These cationic seven-membered metallacyclic σ-allyl complexes undergo a rapid topomerization process on the NMR time scale (∆G q top ≈ 12 kcal/mol -1 ). Organic nitriles add similarly at 60°C to the (s-trans-η 4 -butadiene)tantalocene cation (10) to yield the respective 7-substituted 3,4-dihydrotantala-6H-azepines (14) that exhibit a similar dynamic NMR behavior due to conformational equilibration. Two examples of the seven-membered σ-allyl metallacyclic cation complexes 14 were characterized by X-ray crystal structure analyses (14a, R ) CH 3 , and 14d, R ) CH 2 -p-C 6 H 4 CH 3 ). In contrast, 2-butyne adds to the (s-trans-η 4 -butadiene)tantalocene cation in a 1:1 ratio to yield the corresponding metallacyclic (π-allyl)metallocene cation complex 15a (characterized by X-ray diffraction). The addition of 1-pentyne gave a 60:40 mixture of the respective regioisomers 15b and 15c, exhibiting the n-C 3 H 7 substituent at the ring position C5 and C6, respectively.

Keywords: Diene complexes / η 3 -Cyclooctatetraene metal complex / B(C 6 F 5 ) 3 addition / Tantalum / Bicyclo[5.1.0]octadienyl metal complexThe [(η 4 -diene)(η 5 -R Cp)TaCl 2 ] complexes 6a-e (diene = Cp*)Ta] 7a. In solution, the cot ligand of all the complexes 7 is fluxional. Two isomers [supine and prone orientation of the butadiene, isoprene or 2,3-dimethylbutadiene; R Cp = pentamethylcyclopentadienyl, methylcyclopentadienyl or (diene)CpTa subunit] were found for the (η 4 -butadiene)(η 3cot)CpTa system (7e/7eЈ = 95:5). Complex 7e cleanly adds cyclopentadienyl) react with cyclooctatetraene dianion (employed as [(cot)Li 2 ]) to yield the (η 4 -diene)(η 3 -the organometallic Lewis acid B(C 6 F 5 ) 3 to yield the betaine system [(η 4 -C 4 H 6 ){C 8 H 8 B(C 6 F 5 ) 3 }Cp*Ta] 8, which contains a cyclooctatetraene)(η 5 -R Cp)tantalum complexes 7. The η 3 -cot coordination was confirmed by an X-ray crystal structure substituted bicyclo[5.1.0]octadienyl ligand. analysis of the compound [(supine-s-cis-η 4 -C 4 H 6 )(η 3 -cot)(η 5 -

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