The chloro−imido complex [Nb(η5-C5H4-SiMe2Cl)(CH2Ph)Cl(NtBu)] undergoes selective substitution of the Nb−Cl bond when it is reacted with LiNHtBu, giving [Nb(η5-C5H4-SiMe2Cl)(CH2Ph)(NHtBu)(NtBu)], whereas preferential reaction at the Si−Cl bond occurs with NH2 tBu, giving [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)Cl(NtBu)], which is then transformed into [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)(NHtBu)(NtBu)] on further reaction with NH2 tBu. Chemical reactivity studies demonstrate that a spontaneous transformation of [Nb(η5-C5H4-SiMe2Cl)(CH2Ph)(NHtBu)(NtBu)] into [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)Cl(NtBu)] takes place, via an intermolecular reaction, by attack of the silicon-bonded chlorine at the unsaturated niobium center. Benzylation of [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)Cl(NtBu)] with Mg(CH2Ph)2·2THF gives [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)2(NtBu)], which also results from reaction of the chloro−silyl derivative [Nb(η5-C5H4-SiMe2Cl)(CH2Ph)2(NtBu)] with NH2 tBu. The dichloro−imido complex [Nb(η5-C5H4-SiMe2Cl)Cl2(NtBu)], readily obtained from [Nb(η5-C5H4-SiMe2Cl)Cl4], also reacts preferentially at the Si−Cl bond when treated with NH2 tBu to give [Nb{η5-C5H4-SiMe2(NHtBu)}Cl2(NtBu)], which is further transformed into [Nb{η5-C5H4-SiMe2(NHtBu)}Cl(NHtBu)(NtBu)]; the latter is alternatively prepared by reaction of [Nb(η5-C5H4-SiMe2Cl)Cl4] with a large excess of NH2 tBu. The tetrachloro compound reacts with excess LiNHtBu to give the constrained-geometry complex [Nb{η5-C5H4-SiMe2(η1-NtBu)}(NHtBu)(NtBu)], which is converted into the chloro−niobium derivative by reaction with SiMe3Cl. These constrained-geometry complexes may also be obtained from the dichloro derivative [Nb(η5-C5H4-SiMe2Cl)Cl2(NtBu)], by reaction with 3 or 2 equiv of LiNHtBu, respectively. Thermal treatment of the benzyl compounds [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)X(NtBu)] (X = Cl, CH2Ph) at 160 °C results in formation of the silyl−amido complexes [Nb{η5-C5H4-SiMe2(η1-NtBu)}X(NtBu)] (X = Cl, CH2Ph) with elimination of toluene, whereas a similar transformation of the related iminoacyl complex [Nb{η5-C5H4-SiMe2(NHtBu)}{η2-[C(CH2Ph){N(2,6-Me2C6H3)}](CH2Ph)(NtBu)] produces a simultaneous isomerization to give the vinylamido compound [Nb{η5-C5H4-SiMe2(η1-NtBu)}{N(CHCHPh)(2,6-Me2C6H3)}(NtBu)]. Insertion of CN(2,6-Me2C6H3) into the niobium−benzyl bonds of complexes [Nb{η5-C5H4-SiMe2(NHtBu)}(CH2Ph)X(NtBu)] (X = Cl, CH2Ph) and [Nb{η5-C5H4-SiMe2(η1-NtBu)}(CH2Ph)(NtBu)] leads to the iminoacyl compounds [Nb{η5-C5H4-SiMe2(NHtBu)}{η2-[C(CH2Ph){N(2,6-Me2C6H3)}]}X(NtBu)] (X = Cl, CH2Ph) and [Nb{η5-C5H4-SiMe2(η1-NtBu)}{η2-[C(CH2Ph){N(2,6-Me2C6H3)}]}(NtBu)], respectively. All of the reported new compounds were characterized by elemental analyses and 1H and 13C NMR spectroscopy, and the X-ray molecular structure of [Nb{η5-C5H4-SiMe2(NHtBu)}Cl2(NtBu)] was studied by diffraction methods.

We report the synthesis of 1-(chloromethylphenylsilyl)cyclopentadiene. The reaction of a toluene/CH 2 Cl 2 suspension of NbCl 5 with 1-(SiClMeX)-1-(SiMe 3 )C 5 H 4 (X ) Me, Ph) leads to the monocyclopentadienyl derivatives [M{η 5 -C 5 H 4 (SiClMeX)}Cl 4 ] (X ) Me, Ph) in 90 and 78% yields, respectively. Reaction of 1 equiv of water with [M{η 5 -C 5 H 4 (SiClMe 2 )}Cl 4 ] in toluene takes place with elimination of HCl, resulting in formation of the dinuclear niobium oxo derivative [{(NbCl 2 ) 2 (µ-O)(µ-Cl) 2 }{(η 5 -C 5 H 4 ) 2 (Me 2 SiOSiMe 2 )}]. Reaction of the complexes [M{η 5 -C 5 H 4 (SiClMeX)}Cl 4 ] with LiNH t Bu and NEt 3 in toluene leads to the half-sandwich imido complexes [Nb{η 5 -C 5 H 4 (SiClMeX)}Cl 2 (N t Bu)] (X ) Me, Ph), which react at room temperature with 1 equiv of Mg(CH 2 Ph) 2 (THF) 2 to give the imido dibenzyl complexes [Nb-{η 5 -C 5 H 4 (SiClMeX)}(CH 2 Ph) 2 (N t Bu)] (X ) Me, Ph). The reaction of the chloro imido complexes [Nb{η 5 -C 5 H 4 (SiClMeX)}Cl 2 (N t Bu)] with excess Mg(CH 2 Ph) 2 (THF) 2 on heating for 2-3 days to 80-90 and 50-60 °C, respectively, takes place, affording the totally alkylated complexes [Nb{η 5 -C 5 H 4 [Si(CH 2 Ph)MeX]}(CH 2 Ph) 2 (N t Bu)] (X ) Me, Ph). The chloro benzyl derivative [Nb{η 5 -C 5 H 4 (SiClMe 2 )}(CH 2 Ph)Cl(N t Bu)], which could not be prepared by addition of the stoichiometric amount of the alkylating agent, was isolated in quantitative yield by heating a mixture of the dichloro and dibenzyl complexes in toluene at 120 °C for 4 h. The 18-electron imido η 2 -iminoacyl derivatives [Nb{(η 5 -C 5 H 4 (SiMeXY)}R(N t Bu){η 2 -C(CH 2 Ph)dN-(2,6-Me 2 C 6 H 3 )}] (R ) CH 2 Ph, X ) Cl, Y ) Me, Ph; R ) CH 2 Ph, X ) CH 2 Ph, Y ) Me, Ph; R ) Cl, X ) Cl, Y ) Me) are formed when the isocyanide CN(2,6-Me 2 C 6 H 3 ) is added to a hexane solution of the benzylimido complexes. The molecular structure of the oxo complex [{(NbCl 2 ) 2 -(µ-O)(µ-Cl) 2 }{(η 5 -C 5 H 4 ) 2 (Me 2 SiOSiMe 2 )}] has been determined by X-ray diffraction methods.

Reaction of the amido-niobium complex [Nb{η 5 -C 5 H 4 SiMe 2 (NH t Bu)}(N t Bu)Cl(NH t Bu)] (2) with 1 equiv of CNAr (Ar ) 2,6-Me 2 C 6 H 3 ) in hexane gave the iminocarbamoyl compound [Nb{η 5 -C 5 H 4 SiMe 2 (NH t Bu)}(N t Bu)Cl{η 2 -C(NH t Bu)dNAr}] (5) in quantitative yield at room temperature, whereas heating to 120 °C was required when similar reactions were carried out with the bridged silyl amido compounds [Nb{η 5 -C 5 H 4 SiMe 2 (η 1 -N t Bu)}(N t Bu)Cl] (3) and 8), which rearranges to the silyl-η 1 -amido η 2 -iminoacyl derivative [Nb{η 5 -C 5 H 4 -SiMe 2 (η 1 -N t Bu)}(N t Bu){η 2 -C(CH 2 Ph)dNAr}] (9) on heating its C 6 D 6 solution to 145 °C. Reaction of compound 2 with CO 2 gave the dicarbamato complex [Nb{η 5 -C 5 H 4 SiMe 2 (η 1 -OOCNH t Bu)}(η 2 -OOCNH t Bu)(N t Bu)Cl] (10), whereas the same reaction with [Nb{η 5 -C 5 H 4 -SiMe 2 (NH t Bu)}(N t Bu)Cl 2 ] (1) gave an unidentified mixture of products, which were transformed after heating to 130-140 °C into the oxo derivative [{Nbwhich was further benzylated to give [{Nb(N t Bu)(CH 2 Ph) 2 } 2 -{η 5 -C 5 H 4 SiMe 2 -O-SiMe 2 -η 5 -C 5 H 4 }] (12). Similar reaction of 3 with CO 2 in toluene gave the oxo compounds [Nb(N t Bu)Cl-µ-(η 5 -C 5 H 4 SiMe 2 -η-O)] 2 (13) and [{Nb(N t Bu)Cl} 2 (µ-O){η 5 -C 5 H 4 -SiMe 2 -O-SiMe 2 -η 5 -C 5 H 4 }] (14). The molecular structure of 13 was determined by X-ray diffraction methods. Formation of the oxo complexes 11, 13, and 14 also resulted from hydrolysis of [Nb(η 5 -C 5 H 4 SiMe 2 Cl)(N t Bu)Cl 2 ] in the presence of NEt 3 .

3 )] 5. All compounds have been characterized by IR and NMR techniques as well as FAB ϩ mass spectrometry. Crystal structures of 1, 4, and 5 have been determined by X-ray diffraction methods. The pathways of the thermolysis of 1 to give 2-5 have been investigated.

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