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The singly tucked-in titanocene [(η5-C5Me5)TiIII{η5:η1-C5Me4(CH2)}] (1) reacted with one equivalent of 1-tert-butylpropyne to yield the propenyl-tethered regioisomer [(η5-C5Me5)Ti(η5:η1-C5Me4CH2C(t-Bu) = κCMe)] (3). The exclusive formation of paramagnetic 3 was proved experimentally and steric reasons were clarified by DFT calculation of transition state which involves both π-systems on the alkyne for bonding interaction with the metal and with sp2 carbon atom of exomethylene group of 1. The obtained compound 3 is by 40 kJ/mol lower in energy than its hypothetical isomer having t-Bu group on Cα of the tether.

New titanocene dihydrosulfide compounds [(C 5 Me 4 CH 2 Ph) 2 Ti(SH) 2 ] ( 6 ) and [(C 5 Me 4 t -Bu) 2 Ti(SH) 2 ] ( 7 ) were obtained by addition of hydrogen sulfide to the corresponding doubly tucked-in titanocenes, and titanocene hydrosulfide compounds [(C 5 Me 4 CH 2 Ph) 2 TiSH] ( 8 ) and [(C 5 Me 4 t -Bu) 2 TiSH] ( 9 ) by H 2 S-induced protonolysis of σ-Ti–C bonds in [(η 5 -C 5 Me 4 CH 2 Ph)Ti(III)(η 5 :η 1 -C 5 Me 4 CH 2 - o -C 6 H 4 )] and [(C 5 Me 4 t -Bu)Ti(III)(η 5 :η 1 -C 5 Me 4 CMe 2 CH 2 )], respectively. The crystal structures of 6 , 8 , and 9 and electronic absorption spectra of 6 – 9 in hexane solution highly resemble those of corresponding [Cp* 2 Ti(SH) 2 ] ( 1 ) and [Cp* 2 TiSH] ( 2 ), however, compounds 6 and 7 strongly differ in their sensitivity to sunlight mutually and with respect to 1 . The sunlight photolysis of 6 in toluene proceeded similarly to the process described previously for 1 except that about three times longer exposition (300 h) was required to obtain the cyclopentadienyltitanium sulfide cage cluster [{(C 5 Me 4 CH 2 Ph)Ti} 4 S 6 ] ( 10 ) in 48% yield. In contrast, compound 7 photo-decomposed very efficiently to give compound 9 as the only isolated titanium-containing product in 87% yield. The formation of 10 can be accounted for the redox elimination of the cyclopentadiene followed by elimination of hydrogen sulfide in intramolecular condensation reaction whereas the formation of 9 requires the dissociation of SH radicals. Both the processes were recognized by Rosenthal and Beweries to concur in photodecomposition of [Cp* 2 Ti(OH) 2 ].

Sunlight photolysis of decamethyltitanocene dihydrosulfide [Cp*2Ti(SH)2] (1, Cp* = η5-C5Me5) in toluene affords the titanium sulfide clusters [(Cp*Ti)6(μ3-S)8] (4) and [(Cp*Ti)4(μ3-S)3(μ2-S)3] (5), the former in very low yield and the latter in good yield. The crystal structures of both compounds are toluene solvates and are based on a nearly regular titanium octahedron for 4 and a trigonal pyramid for 5. The [(Cp*Ti)3(μ3-S)(μ2-S)3] (6) complex isolated in a trace amount from the photolysis of 1 in hexane could be a precursor of 4. A plausible mechanism for the formation of 5 and 6 follows a mechanism recently proposed for photolysis of [Cp*2Ti(OH)2] by U. Rosenthal et al.

Decamethylchromocene [Cr(η5-C5Me5)2] reacted with cyclopentadienyltitanium trichlorides [Ti(η5-C5H5-nMen)Cl3] (n = 0–5) in toluene to precipitate yellow finely crystalline ion pairs [Cr(η5-C5Me5)2]+[Ti(η5-C5H5-nMen)Cl3]− (1–6). Evidence for the chromocenium cation and titanate anions was obtained by paramagnetic 1H NMR spectra of 1–6 in CD2Cl2. The resonance of the cation was invariant at δ 6.9 ppm (Δν1/2 ~ 0.4 kHz) whereas resonances of the titanate anions were shifting in response to the number of methyl groups (δ 10.5–8.4 ppm (CpMe) and δ 26.6–39.0 ppm (CpH)). The presence of toluene of crystallization in 1–6 was observed by 1H NMR spectra, and in the crystal structure of 6. Crystallization of 6 from tetrahydrofuran afforded solvent-free crystals of 6A. In crystal structures of both 6 and 6A the [Ti(η5-C5Me5)Cl3]– titanate anion possessed shorter Ti–Cg distances and longer Ti–Cl bonds than the neutral species.

Complex [Zr{η5-C5Me4(SiMe2H)}2Cl2] (1) was prepared by the reaction of lithium salt of 2,3,4,5-tetramethyl-1-(dimethylsilyl)cyclopenta-2,4-diene with [ZrCl4] in boiling THF. The reduction of 1 with excess magnesium in THF in the presence of excess bis(trimethylsilyl)acetylene (btmsa) afforded the bivalent metal ansa-disilylene complex with π-coordinated btmsa [Zr(η2-btmsa){η5-C5Me4(SiMe2)}2] (2). The dehydrocoupling of SiMe2H groups was accompanied by a hydrogen transfer to releasing btmsa to give a mixture of cis- and trans-1,2-bis(trimethylsilyl)ethene. Chlorination of 2 with PbCl2 afforded ansa-[Zr{η5-C5Me4(SiMe2)}2Cl2] (3) and liberated btmsa. The crystal structures of complexes, 1, 2 and 3 have been determined by X-ray crystallography.

The new ansa -titanocene dichloride [{(SiMePh)(η 5 -C 5 H 4 ) 2 }TiCl 2 ] ( 1 ) was prepared by one pot reaction, whereas synthesis of its methylated analogue [{(SiMePh)(η 5 -C 5 Me 4 ) 2 }TiCl 2 ] ( 3 ) was performed in two steps with isolation of corresponding silane intermediate SiMePh(HC 5 Me 4 ) 2 ( 2 ). The reaction of 1 and 3 with TiCl 4 afforded the dinuclear complexes [(SiMePh){(η 5 -C 5 R 4 )TiCl 3 } 2 ] (R = H ( 4 ) and R = Me ( 5 )). The catalysts formed from 4 and 5 after their activation with excess MAO exhibited a modest activity in ethylene polymerization. The polymer products consisted of high molar mass linear polyethylenes with a broad molar mass distribution. The presence of three paramagnetic titanium species in the mixture 4 /MAO was revealed by EPR spectroscopy. All new prepared compounds 1 – 5 were characterized by multinuclear NMR, EI-MS, IR, and solid-state structures of 1 , 3 and 5 were determined by X-ray single crystal diffraction.

Elimination of methane during thermolysis of title compounds results in the formation of σ-Ti–C bond to t-butyl or benzyl group. The t-butyl-containing titanocene methyl compound [Ti(III)Me(η5-C5Me4t-Bu)2] (5) eliminates methane at 110 °C to give cleanly [Ti(III)(η5:η1-C5Me4CMe2CH2)(η5-C5Me4t-Bu)] (6). The methyl derivative of analogous benzyl-containing titanocene [Ti(III)Me(η5-C5Me4CH2Ph)2] was not isolated because it eliminated methane at ambient temperature to give [Ti(III)(η5:η1-C5Me4CH2-o-C6H4)(η5-C5Me4CH2Ph)] (7) with one phenyl ring linked to titanium atom in ortho-position. The corresponding titanocene dimethyl compound [TiMe2{η5-C5Me4t-Bu)}2] (9) eliminates two methane molecules at 110 °C to give the singly tucked-in 1,1-dimethylethane-1,2-diyl-tethered titanocene [Ti{η5:η1:η1-C5Me3(CH2)(CMe2CH2)}(η5-C5Me4t-Bu)] (11). In distinction, the analogous benzyl derivative [TiMe2(η5-C5Me4CH2Ph)2] (10) eliminates at 110 °C only one methane molecule to afford [TiMe(η5:η1-C5Me4CH2-o-C6H4)(η5-C5Me4CH2Ph)] (12) containing one phenyl group attached to titanium in o-position and one methyl group persisting on the titanium atom. This compound is stable at 150 °C for at least 3 h. The crystal structures of 5, 6, 7, and 10 were determined.

Thermolysis of [TiMe 2 (η 5 -C 5 Me 4 Ph) 2 ] ( 4 ) at 145 °C for 5 h afforded the singly tucked-in paramagnetic titanocene [Ti(III)(η 5 -C 5 Me 4 Ph){η 5 :η 1 -C 5 Me 3 Ph(CH 2 )}] ( 9 ). In distinction to the singly tucked-in permethyltitanocene [Ti(III)(η 5 -C 5 Me 5 ){η 5 :η 1 -C 5 Me 4 (CH 2 )}] ( 1 ) which was found crystallographically disordered [J.M. Fischer, W.E. Piers, V.G. Young, Jr., Organometallics 15 (1996) 2410] the single crystal X-ray diffraction analysis of 9 afforded molecular parameters with nearly by one order better precision as measured by esd-values.

Singly tucked-in titanocene [Ti(III)(η5-C5Me5)(η5:η1-C5Me4CH2)] (1) reacted readily with di-tert-butyl ketone and 2,4,6-trimethylacetophenone to give permethyltitanocene derivatives [Ti(III)(η5-C5Me5){η5-C5Me4CH2C(t-Bu)2O-κO}] (2) and [(Ti(III)(η5-C5Me5)-{η5-C5Me4CH2CMe(C6H2Me3)O-κO}] (3) with the tetramethylcyclopentadienyl ring tethered to the titanium atom by ethoxy group disubstituted at Cα. Crystal structures of 2 and 3 showed that the Ti–O bond is only slightly inclined to the tethered cyclopentadienyl ligand, and the bulky mesityl or tert-butyl substituents did not impose an observable steric hindrance into molecules of 2 and 3. Whereas the Ti–O bond lengths in 2 and 3 do not significantly differ, a higher wavenumber of the 1a1 → b2 electronic transition for 2 (7326 cm–1) compared with that for 3 (6666 cm–1) indicates a more efficient oxygen-to-titanium π-electron back donation in 2. This is induced by two electron-donating tert-butyl groups on Cα compared with a combined effect of electron-donating methyl and electron-attracting mesityl substituents in 3.

Half-sandwich pentamethylcyclopentadienyltitanium chloroisopropoxides [Ti(η5-C5Me5)-Ti(Oi-Pr)nCl3-n] (n = 1 (1) or 2 (2)) and methylisopropoxides [Ti(η5-C5Me5)Ti(Oi-Pr)nMe3-n] (n = 1 (3) or 2 (4)) were prepared and characterized by spectroscopic methods, and compound 1 by single crystal X-ray diffraction analysis. Compounds 3 and 4 were reacted with an equimolar quantity of B(C6F5)3, however, only compound 4 afforded a crystalline product of limited stability at ambient temperature. Its single crystal X-ray diffraction analysis revealed that it is the zwitterionic complex [(η5-C5Me5)Ti(Oi-Pr)2]+[(μ-Me)B(C6F5)3]- (5) and its crystal structure is very similar to the recently reported tert-butoxy complex [(η5-C5Me5)-Ti(Ot-Bu)2]+[(μ-Me)B(C6F5)3]- (6). The 1H, 13C, and 19F NMR spectra in C6D6 solution also showed the features analogous to those of 6. 1D NOESY experiments proved a close through space interaction of irradiated C5Me5 or OCHMe2 protons with the bridging Ti···Me-B group. The NMR data indicate the inner sphere ion pair structure of 5 in C6D6, like for 6. The crystal structure geometry of the C-H bonds of the Ti···Me-B group fulfils conditions for their agostic interaction with the titanium centre. Although structurally similar to 6, compound 5 is less thermally stable apparently due to a lower bulkiness of Oi-Pr group compared to that of Ot-Bu group.

Reactions of double tucked-in permethyltitanocene [Ti{η3:η4-C5Me3(CH2)2}(η5-C5Me5)] (1) with tert-butanol or propargyl alcohol in toluene proceeded sluggishly even with excess of the alcohol and at elevated temperature. The alcoholysis of 1 with tert-butanol afforded [Ti(η5-C5Me5)(Ot-Bu)3] (3) as the major product and Ti(Ot-Bu)4 as the minor one. The other product of the alcoholysis was pentamethylcyclopentadiene. The only isolated crystalline product was the very minor hydrolytic byproduct [{TiIII(η5-C5Me5)2}(μ-O){TiIV(Ot-Bu)3}] (2). The alcoholysis of 1 containing traces of LiI with HOCH2C≡CH, containing apparently more water than t-BuOH, gave an inseparable mixture of oily products from which several crystals of an unintended hydrolytic byproduct [{Ti(η5-C5Me5)(OCH2C≡CH)(μ-O)}3LiI] (4) were isolated. X-ray crystal structures of 2 and 4 were determined.

Thermally stable zwitterionic complex [(η5-C5Me5)Ti(Ot-Bu)2](δ+)[(μ-Me)B(C6F5)3](δ−) (5) was formed by mixing equimolar quantities of [(η5-C5Me5)TiMe(Ot-Bu)2] and [B(C6F5)3]. The compound was recrystallized from hot toluene and has been stable so far for 1 year at ambient temperature in the solid state and in toluene solution. The 1H, 19F and 1D NOESY NMR spectra in C6D6 solution proved the inner sphere ion pair structure of 5. The X-ray crystal structure of 5 revealed that the bridging methyl group is σ-bonded to boron, and all its C–H bonds fulfil criteria for agostic bonding interaction with the titanium atom. In contrast to the stable solution in C6D6 compound 5 decomposed in CD2Cl2 solvent within hours at room temperature.

Reduction of isopropyldimethylsilyl-substituted titanocene dichloride [TiCl2(η5-C5Me4SiMe2Pri)2] (1) by excess magnesium in the presence of excess bis(trimethylsilyl)ethyne (btmse) in tetrahydrofuran at 60 °C yielded a mixture of products amongst them only the trinuclear Ti–Mg–Ti hydrido-bridged complex Mg[Ti(μ-H)2(η5-C5Me4SiMe2Pri)]2 (3) was isolated and characterized. The precursor of titanocene, [Ti(η5-C5Me4SiMe2Pri)2(η2-btmse)] (6), was obtained from the identical system which, after initial formation of [TiCl(η5-C5Me4SiMe2Pri)2] (2), reacted at −18 °C overnight and then the solution was rapidly separated from the remaining magnesium. Titanocene [Ti(η5-C5Me4SiMe2Pri)2] (7) was obtained by thermolysis of 6 at 75 °C in vacuum. Crystal structures of 1, 2, 3, 6, and 7 were determined.

The [TiCl 2 (η 5 -C 5 Me 5 ) 2 ]/Mg excess /tetrahydrofuran system reacts with but-2-yne in excess to give paramagnetic (η 5 :η 1 -1,2,3,4-tetramethyl-5-(2,3,4,5-tetramethylpenta-2,4-dien-1,5-diyl-κC5)cyclopentadienyl)(η 5 -pentamethylcyclopentadienyl)titanium(III) ( 1 ) as the main product. The crystal structure of 1 shows a minimum steric hindrance between the methylated cyclopentadienyl ligands and non-coplanar conjugated double bonds in the C 5 tether.

Dinuclear half-sandwich complexes [{TiCl3(η5-C5Me4)}2SiMe2] (2) and [{TiCl3(η5-C5Me4SiMe2CH2)}2] (4) were prepared by synproportionation of ansa-titanocene dichlorides [TiCl2{(η5-C5Me4)2SiMe2}] (1) and [TiCl2(η5-C5Me4SiMe2CH2)2] (3), respectively, with TiCl4. Compound 3 was synthesized from 1,2-bis(chlorodimethylsilyl)ethane, 1,2,3,4-tetramethylcyclopentadienyllithium, and [TiCl3(thf)3] by a common procedure. Crystal structures of 2, 3, and 4 were determined by X-ray diffraction analysis.

Single tucked-in titanocene [Ti(η5-C5Me5)(η5:η1-C5Me4CH2)] adds to B(C6F5)3 to give the paramagnetic cationic complex [Ti(η5-C5Me5){η5-C5Me4CH2B(C6F5)3}] (1). Doubly tucked-in titanocene [Ti(η5-C5Me5){η3:η4-C5Me3(CH2)2}] reacts with B(C6F5)3 by one methylene group to give [Ti(η5-C5Me5){η5:η1-C5Me3(CH2)CH2B(C6F5)3}] (6). The crystal structure of 6 revealed that one hydrogen of the methylene group attached to boron agostically interacts with Ti(IV).

Reduction of silyl-substituted titanocene dichloride [TiCl2{η5-C5Me4(SiMe2CH2CH2CF3)}2] (1) with one molar equivalent of magnesium afforded a mixture of products, thus precluding the isolation of the possibly formed titanocene [Ti{(η5-C5Me4(SiMe2CH2CH2CF3)}2]. The presence of isolable monochloride [TiCl{η5-C5Me4(SiMe2CH2CH2CF3)}2] (2) in the mixture indicates that the mangnesium is consumed in concurrent reactions, that produce various titanocene compounds of which some were obtained by the reduction of 1 with excess magnesium. Those include the trinuclear TiIII-MgII-TiIII hydride-bridged complex [Ti{η5-C5Me4- (SiMe2CH2CH2CF3)}2(μ-H)2]2Mg (3) and a dimeric dinuclear Ti-Mg complex 4 containing the [TiIII(μ-H)2Mg(μ-X)]2 core where, however, the nature of the bridging moiety X remains unknown. The reduction of 1 with excess magnesium in the presence of bis(trimethylsilyl)ethyne afforded the product of C-H activation [Ti{η5-C5Me4(SiMe2CH2CH2CF3)}- {η5:η1-C5Me3(CH2)(SiMe2CH2CH2CF3)}] (5) in 47% yield. This compound reacted rapidly with tert-butylethyne to give the TiIII-acetylide complex [Ti(η1-C≡CCMe3){η5-C5Me4- (SiMe2CH2CH2CF3)}2] (6). All the reductions of 1 at molar ratios Mg:Ti ≥ 1 gave mixtures, where a good deal of the reduction products remained in the mother liquors unidentified. The structures of 1, 2, 3, 5, and 6 were determined by X-ray diffraction analysis and, for 2, 3, 4, 5, and 6, further corroborated by ESR spectra.

[TiCl2(η5-C5Me4t-Bu)2] (1) was prepared by the reaction of [TiCl3(thf)3] with 2 equivalents of Li(η5-C5Me4t-Bu) (made by adding BuLi to a 1.33 molar excess of a mixture of tert-butyltetramethylcyclopentadiene isomers in THF) and subsequent oxidation of the Ti(III) intermediate to 1 by adding aqueous HCl. Neglecting the presence of cyclopentadiene isomers not forming the cyclopentadienide anion in the isomer mixture used for the synthesis of 1 resulted in the formation of hydrolytic product [{TiCl2(η5-C5Me4t-Bu)}2(μ-O)] (2) and a considerable decrease of yield of 1. The half sandwich complex [TiCl3(η5-C5Me4t-Bu)] (3) was obtained by synproportionation of 1 and TiCl4. Crystal structures of 1, 2, and 3 were determined. The electronic absorption spectra of 1 and 3 indicate a stronger electron-donor effect of tert-butyl group compared with methyl group.

Paramagnetic titanocene complexes containing the unsaturated carbyl group which consists of one and half molecule of 1,4-bis(trimethylsilyl)buta-1,3-diyne (BSD) are formed by the reduction of titanocene dichlorides with one molar equivalent of magnesium in the presence of 1.5 molar equivalent BSD in tetrahydrofuran (THF) for titanocene moieties Ti(η5-C5H5 − nMen)2 (n = 5 (1), 4 (2), and 3 (3)) and Ti{Me2Si(η5-C5Me4)2} (4). The non-methylated titanocene moiety affords under identical conditions known diamagnetic bis(η5-cyclopentadienyl)-2,4-bis(trimethylsilylethynyl)-3,5-bis(trimethylsilyl)titanacyclopenta-2,4-diene (5) as the major product. Crystal structures of 3 and 4 show the same bonding scheme for the 1,4,6-tris(trimethylsilyl)hex-3-ene-1,5-diyne-3-yl ligand as previously found for compound 1 [P.-M. Pellny, F.G. Kirchbauer, V.V. Burlakov, A. Spannenberg, K. Mach, U. Rosenthal, Chem. Commun. (1999) 2505]. Compound 1 is stable against weak proton donors like methanol or alk-1-ynes even at 90 °C, however, it undergoes retroreaction when oxidized by PbCl2 in THF, yielding nearly quantitatively BSD and [TiCl2(η5-C5Me5)2].

Bis{η5-(N,N-dimethylaminodimethylsilyl)tetramethylcyclopentadienyl}dichlorotitanium(IV), [TiCl2{η5-C5Me4(SiMe2NMe2)}2] (1), reacts with sulfane under cleavage of both Si–N and Ti–Cl bonds to give sulfido-tethered titanocene compound bis{η5-1,2,3,4-tetramethyl-5-(dimethylsilylsulfido-κS)cyclopentadienyl}titanium(IV), [Ti{η5-C5Me4(SiMe2S- κS)}2] (4), and dimethylamine hydrochloride as the byproduct. The crystal structure of compound 4 has been determined by single-crystal X-ray diffraction.

Reduction of methyl-substituted titanocene dichlorides bearing pendant double bonds [TiCl2{η5-C5Me4(CH2CMeCH2)}2] (1) and [TiCl2{η5-C5Me4(SiMe2(CH2)2CHCH2)}2] (2) with magnesium yielded diamagnetic Ti(IV) compound [Ti{η1:η1:η5-C5Me3(CH2)(CH2CH(Me)CH2)}{η5-C5Me4(CH2C(Me)CH2)}] (4) and paramagnetic Ti(III) compound [Ti{η5-C5Me4(SiMe2CH2CHCHMe)}(μ-η3,η1:η5,η1(Ti:Mg){C5Me3(CH2)(SiMe2CHCHCMe)})Mg(OC4H8)2] (6), respectively. The reluctance of titanocene intermediates to undergo intramolecular cyclization to cyclopentadienyl-ring-tethered titanacycles (as typically observed) can be explained by a shortness of the 2-methylallyl group and steric hindrance of its double bond in the former case and, in the latter case, by an attack of magnesium on the titanocene intermediate, faster than cyclization reactions. The crystal structures of 4 and 6 were determined by single-crystal X-ray diffraction.

Paramagnetic complexes ansa-[Ti(C CCMe3){η5:η5-C5Me4SiMe2CH CHCH2CH2SiMe2C5Me4}] (1) and ansa-[Ti(OH){η5:η5-C5Me4SiMe2CH2CH CHCH2SiMe2C5Me4}] (2) were prepared by reactions of ansa-[Ti(III){η3:η5:η5-C5Me4SiMe2CHCHCHCH2SiMe2C5Me4}] (3) with t-butylethyne or water, respectively. The X-ray single crystal diffraction analysis of 1 and 2 showed that the cis-double bond is situated asymmetrically in the ansa-bridge of 1 whereas, in 2, the double bond is trans, and lies in the bridge centre. ESR and UV-near IR spectra indicated a weaker oxygen π-electron donation into the Ti–O bond of 2 compared to analogous methoxide ansa-[Ti(III)(OMe){η5:η5-C5Me4SiMe2CH2CH CHCH2SiMe2C5Me4}] (4) or decamethyltitanocene hydroxide (5).

An easy and inexpensive three-step synthesis of new 2,3-dimethyl-1,4-diphenylcyclopentadiene (3) ligand and the titanium and zirconium homometallocene dichlorides [TiCl2(η5-C5H-2,3-Me2-1,4-Ph2)2] (4), [ZrCl2(η5-C5H-2,3-Me2-1,4-Ph2)2] (5), and the mixed ligand zirconium complex [ZrCl2(η5-C5H-2,3-Me2-1,4-Ph2)(η5-C5H5)] (6) prepared thereof are described. The polymerization of ethene using 4–6/MAO catalysts revealed that zirconocene complexes 5 and 6 displayed moderate and high activity, respectively, whereas the titanium catalyst 4/MAO was inactive. The crystal structures of 4 and 5 were determined by X-ray crystallography.

Fully methylated titanocene hydroxide [Ti(OH) (η5-C5Me5)2] (1) was obtained by the reaction of single tucked-in decamethyltitanocene [Ti(η5-C5Me5){η5:η1-C5Me4(CH2)}] (2) with one equivalent of water dissolved in toluene. The X-ray single crystal diffraction of 1 revealed the bent Ti–O–H bonds (112(4)°) where the oxygen atom is only 0.038(4) A away from the Cg(1), Ti, Cg(2) (Cg – centroid of the cyclopentadienyl ring) plane, and the hydroxyl bond is directed to the cyclopentadienyl ring C(1–5). The Ti–O bond length (1.889(2) A) and energy of 1a1→b2 transition (6770(50) cm−1) indicate that the π-donation effect of the hydroxyl group is weaker compared to alkoxy groups.

Complex [Ti(III)(η8-C8H8)(η5-C5Bn5)] (1) was prepared by salt metathesis reaction of [TiCl(η8-C8H8)(thf)]2 with LiC5Bn5 and characterized by ESR and UV-near-IR spectra. Its crystal structure is symmetric with respect to a mirror plane involving the titanium atom and one of the benzyl groups.

Two novel half-sandwich Ti(IV) complexes, [η5:η1-O-C5(CH3)4CH2CH2OCH3]TiCl3 (3) and (η5:η1-P-C5H4CH2CH2PPh2)TiCl3 (6), were prepared and structurally characterised. At elevated temperatures, complex 3 undergoes a conversion into [η5:σ-C5(CH3)4CH2CH2O–]TiCl2 (4). The dynamic behaviour of complexes 3 and 6 in solutions has been studied by variable-temperature 1H, 13C and 31P NMR spectroscopy. Thermodynamic parameters of the intramolecular dissociation-coordination processes for 3 and 6 were elucidated by the numerical analysis of the δ(T) dependencies. The intramolecular M(IV)←E (M=Ti, Zr; E=O, P) coordination in half-sandwich cyclopentadienyl complexes is discussed.

The η3:η4-allyldiene-(η5-cyclopentadienyl)titanium(II) complex [Ti(η5-C5Me5){η3:η4-C5Me3(CH2)2}] (1) reacts with three molar equivalents of substituted propargylic alcohols FcCCCMe2(OH) (2a) and PhCCCH2OH (2b) at elevated temperature to give (η5-pentamethylcyclopentadienyl)titanium(IV) alkoxides [Ti(η5-C5Me5)(FcCCCMe2O-κO)3] (3a) and [Ti(η5-C5Me5)(PhCCCH2O-κO)3] (3b), respectively. The crystal structure of 3a has been determined by single-crystal X-ray diffraction.

Fully methylated η3:η4-allyldiene(η5-cyclopentadienyl)titanium(II) (1) reacts with alkynylketones RCCC(O)Me (R=SiMe3 (3a) and ferrocenyl (3b)) to give the expected products of the CO group insertion into one Ti–CH2 bond 4a and 4b, respectively. Compound 4a could not be isolated in pure form due to its high solubility; a minor product of two-fold addition (5) was isolated instead. Complex 5, formally [ 1 +2 3a ], was characterized as a monocyclopentadienyl bis(alkoxy)organyl complex, where the alkoxyorganyl ligand arises from CO insertion into one Ti–CH2 and the Ti–C(cyclopentadienyl) bonds. The structures of compounds 4b and 5 were determined by single-crystal X-ray diffraction.

Reduction of isomeric, fully-substituted titanocene dichlorides having a pendant double bond, [TiCl2(η5-C5Me4R)(η5-C5Me5)] (R=CH2CH2CHCH2 (1); R=CH(Me)CHCH2, (2)), with magnesium in THF in the presence of bis(trimethylsilyl)ethyne (btmse) affords different products depending on the alkenyl chain length. Whereas the reduction of 1 yields a compound with intramolecularly η2-coordinated double bond, [Ti(η5-C5Me5){η2:η5-C5Me4(CH2CH2CHCH2)}] (3), compound 2 affords under identical conditions a product with two new TiC bonds, [Ti{(η1:η1:η5-C5Me3(CH2)(CH(Me)CH2CH2)}(η5-C5Me5)] (4). The formation of 3 can be rationalized as a simple intramolecular stabilization of a titanocene intermediate formed by reductive removal of the chloride ligands. The formation of 4 from the respective titanocene, however, requires formally an activation (oxidative addition) of one CH bond at the methyl group adjacent to the unsaturated chain followed by a hydrogen shift (hydrometallation). Under similar conditions, the complex with asymmetric unsaturated ansa-bridge [TiCl2{η5:η5-C5Me4CH2CHCH(CH2)5C5Me4}] (5) is reduced to a titanocene–η2-alkene complex with the double bond shifted to the central position, ansa-[Ti{η2:η5:η5-C5Me4(CH2)3CHCH(CH2)3C5Me4}] (7). In the absence of btmse, analogous complex with saturated bridge ansa-[TiCl2{η5:η5-C5Me4(CH2)8C5Me4}] (6) undergoes a hydrogen abstraction to give 7 as well albeit in a lower yield. The reduction of 6 in the presence of btmse gives η2-alkyne complex ansa-[Ti{η5:η5-C5Me4(CH2)8C5Me4}(η2-Me3SiCCSiMe3)] (8) which is converted upon thermolysis at 150 °C to a mixture of asym- and sym-isomers of the doubly tucked-in complex ansa-[Ti{(η3:η4-C5Me2(CH2)2)(CH2)8(η5-C5Me4)}] (9) in 2:1 ratio; no CH activation involving methylene groups of the ansa tether was observed. The crystal structures of 4, 7, and 8 have been determined by X-ray crystallography.

Reduction of [ZrCl2{η5-C5Me4(SiMe3)}2] (1) with excess magnesium in tetrahydrofuran affords a mixture of the monomeric zirconocene hydride [ZrH{η1:η5-C5Me4(SiMe2CH2)}{η5-C5Me4(SiMe3)}] (2) and the dimeric, tetranuclear Zr(III)− and Zr(IV)−magnesium hydride complexes 3 and 4, respectively. In the presence of bis(trimethylsilyl)ethyne (btmse), a similar reduction yields the η2-alkyne complex [Zr{η5-C5Me4(SiMe3)}2(η2-btmse)] (5) as the major product and compounds 2−4 as minor impurities. Upon thermolysis under vacuum, 5 undergoes a 2-fold hydrogen transfer from trimethylsilyl groups in the zirconocene intermediate to leaving btmse to afford [Zr{η5:η1-C5Me4(SiMe2CH2)}2] (6) and a mixture of alkenes (E)- and (Z)-Me3SiCHCHSiMe3 (E ≫ Z). Crystal structures of compounds 2 and 4−6 were determined by X-ray crystallography.

New tetramethylphenylcyclopentadienyl trinuclear titanocene hydride-magnesium hydride complexes [{Ti(η5-C5Me4R)2(μ-H)2}2Mg], where R = Ph (1) or 4-fluorophenyl (FPh; 2), the dinuclear [Ti(η5-C5Me4Ph)[η5:η1{Ti:Mg}-C5Me4(o-C6H4)](μ-H)2Mg(THF)2] (3) and [Ti(η5-C5Me4Ph)2(μ-H)2MgC(Me)=CHMe] (4) complexes, and the [{Ti(η5-C5Me4Ph)2(η1-C≡CSiMe3)2}-{MgCl(THF)}+] (5) tweezer complex initiated the dimerization of (trimethylsilyl)ethyne (TMSE) or 1-hexyne (HXYN) to exclusively head-to-tail (HTT) dimers at 60 °C with the turnover number ranging from 300 to 500 mol alkyne per mol of the Ti complex. In contrast, all of them were inactive in the dimerization of tert-butylethyne (TBUE). Monitoring of reactions of the 1-5 complexes with the alkynes by electron spin resonance (ESR) method revealed a decay of the initial complexes 1-5 in the dimerizing systems with TMSE and HXYN or a conversion of complexes 3, 4, and 5 into the Ti(III) acetylide [Ti(η5-C5Me4Ph)2(η1-C≡CCMe3)] (6) in systems with TBUE. The acetylide 6 also initiated the dimerization of TMSE and HXYN to HTT dimers only. This fact together with the absence of ESR signals of tweezer complexes and acetylides in the reacting hydride systems allow us to assume that the rate of conversion of complexes 1-4 to tweezer complexes is slower than the rate of dissociation of the tweezer complexes to give the acetylides which are apparently the ultimate catalytic species. Most of the evidence on the catalytic complexes has been obtained from ESR spectra which are reliably characteristic of each type of the complexes. Crystal structures of 4 and 6 were determined.

Reduction of fully substituted titanocene dichlorides [TiCl2(η5-C5Me4R)2] (R=SiMe2CH2CH2Ph, 3; SiMe2Ph, 4; and SiMePh2, 5) with magnesium in THF proceeds via the formation of titanocene monochlorides [TiCl(η5-C5Me4R)2] 6–8 to afford monomeric titanocenes, [Ti(η5-C5Me4R)2], 9–11. Titanocene monochlorides give the expected EPR spectra in toluene solution and glass. The structure of 7 was further corroborated by single-crystal X-ray diffraction. Titanocenes 9–11 are EPR silent down to −196 °C but exhibit paramagnetic broadening of the signals in solution NMR spectra. The positions of the NMR signals are temperature-dependent, obeying the Curie Law in the range investigated (0–60 °C). As revealed by X-ray crystallography, titanocenes 9 and 11 possess bent metallocene structures with the cyclopentadienyl rings tilted at an angle of 9.8(1) and 14.4(2)°, respectively. Titanocenes 9–11 are easily oxidized with PbCl2 to the parent dichloride complexes 3–5. Titanocenes 9 and 10 react with bis(trimethylsilyl)ethyne (btmse) only in large excess of the alkyne to give an equilibrium concentration of the respective [Ti(η5-C5Me4R)2(η2-Me3SiCCSiMe3)] complexes. On the other hand, titanocene 11 does not observably react with btmse.

Reduction of dichloro-{1,3-bis(η 5 -tetramethylcyclopentadienyl)-1,1,3,3-tetramethyldisiloxane}titanium(IV), [TiCl 2 {η 5 :η 5 -(C 5 Me 4 SiMe 2 ) 2 O}] ( 1 ), with magnesium in the presence of bis(trimethylsilyl)ethyne affords the ansa -[Ti{η 5 :η 5 -(C 5 Me 4 SiMe 2 ) 2 O}(η 2 -Me 3 SiCCSiMe 3 )] ( 2 ) complex in high yield. Compound 2 is thermally robust, decomposing at 170 °C in high vacuum with liberation of 1,2-bis(trimethylsilyl)ethene to give predominantly an asymmetric, allydiene complex ansa -[Ti{(η 5 -C 5 Me 4 )SiMe 2 OSiMe 2 {η 3 :η 4 -C 5 Me 2 (CH 2 ) 2 }] ( 3 ) as a result of twofold CH bond activation. Crystal structures of 2 and 3 were determined by X-ray diffraction.

The paramagnetic compounds exo,exo-bis(η5-isodicyclopentadienyl)chlorotitanium(III) (3) and its analogue with trimethylsilyl-substituted isodicyclopentadienide (isodiCp) ligand (4), and the similar pair of diamagnetic exo,exo-bis(isodicyclopentadienyl)[η2-bis(trimethylsilyl)ethyne]titanium(II) complexes 5 and 6 were obtained by common reduction procedures from exo,exo-bis(isodicyclopentadienyl)titanium(IV) dichloride (1) and exo,exo-bis[η5-2-(trimethylsilyl)isodicyclopentadienyl]titanium(IV) dichloride (2), respectively. As indicated by ESR spectroscopy compound 3 is a dimer in the solid state and in frozen toluene glass but monomeric in toluene solution. Compound 4 is monomeric in solution as well as in the solid state. As judged from the red shift of the ν(CC) vibration, compound 6 binds bis(trimethylsilyl)ethyne more strongly than compound 5. A comparison of their wavenumbers with those of the [Ti(C5H5−nMen)2(η2-Me3SiCCSiMe3)] (n=2–5) complexes shows that Lewis acidity of the central titanium atom decreases in the order of ligands 1,3-dimethylcyclopentadienyl>isodiCp∼1,2,3-trimethylcyclopentadienyl>(trimethylsilyl)isodiCp∼tetramethylcyclopentadienyl. The crystal structure of the most bulky complex 6 shows a bis-lateral (anti) conformation of the isodiCp ligands with the π-coordinated five-membered rings nearly eclipsed.

A series of three ansa-titanocene monochlorides containing η5-tetramethylcyclopentadienyl ligands bridged by five- or eight-membered aliphatic chains were prepared via reduction of the corresponding dichlorides with half molar equivalent of magnesium and characterized by spectral methods. The solid-state structures of the monochloride complexes ansa-[TiCl{η5:η5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4}] (1a) and ansa-[TiCl{η5:η5-C5Me4(CH2)3CH(Me)CH(Me)CHCHCH2C5Me4}] (4a), and of the bridge-unsaturated titanocene dichloride complex ansa-[TiCl2{η5:η5-C5Me4CH2CHCH(CH2)5C5Me4}] (3) were determined by single-crystal X-ray diffraction. All the compounds show bent metallocene structures with the ansa-chain situated in a side position with respect to Cg, Ti, Cg (Cg=centroid of the cyclopentadienyl ligand) plane. Angles subtended by the least-squares planes of the cyclopentadienyl rings and conformation of the ansa-chains indicates the absence of steric strain in the metallocene framework.

Bis{η5-(N,N-dimethylaminodimethylsilyl)tetramethylcyclopentadienyl}dichlorotitanium(IV), [TiCl2{η5-C5Me4(SiMe2NMe2)}2] (1), easily undergoes hydrolytic cleavage of the Si–N bonds to give dimethylamine and {1,3-bis(η5-tetramethylcyclopentadienyl)-1,1,3,3-tetramethyldisiloxane}dichlorotitanium(IV), ansa-[TiCl2{(η5-C5Me4SiMe2)2O}] (2), in almost quantitative yields. The crystal structures of compounds 1 and 2 were determined by single-crystal X-ray diffraction.

(Amidocyclopentadienyl)dichlorotitanium(IV) complexes of the general formula [TiCl2{η5:η1(N)-C5(1-SiMe2Nt-Bu-2,3,4-Me3-5-R)}], where R = H (6b), Ph (6c), 4-fluorophenyl (6d) and 1-methylallyl (6e) were synthesized and characterized by spectral methods. Solid-state structure of [TiCl2{η5:η1(N)-C5Me4(SiMe2Nt-Bu)}] (6a) and 6d determined by single-crystal X-ray diffraction showed that variation of the ring substituent R has only a negligible effect on the molecular structure of the complexes. The NMR spectra indicate that motion of the bulky substituent R in compounds 6c, 6d and 6e is hindered below ca 50 °C.

Fully substituted trichloro(η5-cyclopentadienyl)titanium(IV) complexes with pendant alkenyl groups, [TiCl3(η5-C5Me4R)] (R = CH(Me)CH=CH2 (3a), (CH2)2CH=CH2 (3b), and (CH2)3CH=CH2 (3c)), were synthesized by the reaction of the corresponding trimethylsilyl or trimethylstannyl substituted cyclopentadienes C5Me4R(EMe3), where E = Si or Sn, with TiCl4. In all cases, the pendant double bonds did not exhibit any interaction with the titanium atoms as indicated by 1H, 13C NMR or IR spectra as well as by the solid-state structure of 3c. The catalytic systems prepared by mixing complexes 3a-3c, [TiCl3(η5-C5Me5)], [TiCl3(η5-C5H5)], and similar complexes with R = butyl (4), phenyl (5), 4-fluorophenyl (6) with methylalumoxane (MAO) at the molar ratio Al/Ti = 500 catalyzed polymerization of styrene to syndiotactic polystyrene. Their polymerization activities increase in the order: [TiCl3(η5-C5Me5)] < 3a < 3b < 3c < 4 < 5 < 6 < [TiCl3(η5-C5H5)], while the molecular weights (Mw) of syndiotactic polystyrene grow in the order: [TiCl3(η5-C5H5)] < 3a <3b < 4 ≈ 6 < [TiCl3(η5-C5Me5)] < 3c ≈ 5 (maximum Mw ca 3.0 · 105). A comparison of the results for the catalytic systems based on 4 with those derived from 3a-3c points to a rather weak competition of the pendant double bonds with monomer for a cationic Ti(III) catalytic centre.

The new class of compounds arising from unconventinal additions of 1,4-disubstituted 1,3-butadiynes to activated permethylmetallocene species of Ti and Zr can be used in facile functionalizations. For example, the chiral product 5, obtained from the addition of tBuC⋮CC⋮CtBu to activated permethyltitanocene, has been derivatized by simple means, e.g., hydrogen chloride, bromine, or dihydrogen, affording practically useful functionalized titanocene compounds containing a chiral auxiliary ligand with an intramolecularly coordinated (compound 7) or a free double bond (compounds 8a,b). The new complexes have been characterized spectroscopically. Additionally, X-ray crystal structure analyses were performed for 7 and 8a,b.

Novel (arene)titanium(II) haloaluminate complexes (η6-arene)Ti[(μ-X)2AlX2]2 (X = Cl, Br) were obtained from (η6-benzene)Ti[(μ-X)2AlX2]2 (X = Cl, Br) complexes by ligand exchange reactions with bis(pentamethylphenyl)methane and dimesitylmethane. The [η6-1,2,3,4,5- pentamethyl-6-(2,3,4,5,6-pentamethylbenzyl)benzene]Ti[(μ-X)2AlX2]2 (X = Cl, Br) and [η6-1,3,5- trimethyl-2-(2,4,6-trimethylbenzyl)benzene]Ti[(μ-Br)2AlBr2]2 complexes containing titanium coordinated to only one of the two phenyl rings were obtained and their crystal structures were determined. (η6-Benzene)Ti[(μ-Cl)2AlCl2]2 and dimesitylmethane afforded the (η6-mesi- tylene)Ti[(μ-Cl)2AlCl2]2 complex which is isotypic with the known bromo analogue. This product shows that Friedel-Crafts scission of aryl-alkyl bonds occurs in such systems even under mild conditions.

The reduction of (C5Me4Ph)2TiCl2 by magnesium in tetrahydrofuran affords a mixture of the diamagnetic doubly tucked-in titanocene complex (C5Me4Ph)[C5Me2(CH2)2Ph]Ti (1), the paramagnetic trinuclear Ti–Mg–Ti hydride bridged complex [(C5Me4Ph)2Ti(μ-H)2]2Mg (2) and the paramagnetic binuclear titanocene hydride–magnesium hydride complex (C5Me4Ph)[C5Me4(o-C6H4)]Ti(μ-H)2Mg(THF)2 (3). The X-ray diffraction analysis of 3 revealed that the magnesium atom forms the σ-bond to the ortho-carbon atom of one of the phenyl rings and binds 2 molecules of THF.

The reduction of {[η8-1,4-(SiMe3)2C8H6]Ti(μ-Cl)}2(THF) by magnesium in the presence of bis(trimethylsilyl)acetylene in tetrahydrofuran affords diamagnetic bis[μ-η5:η5-1,4-bis(trimethylsilyl)cyclooctatetraenetitanium](Ti–Ti) (1) in low yield. The X-ray crystal analysis of 1 revealed that bridging cyclooctatetraene ligands in the centrosymmetric dimer are concave-bent. An extremely short Ti–Ti distance of 2.326(2) A is compatible with the presence of a double Ti–Ti bond.

The reaction between equimolar quantities of TiCl 3 (THF) 3 and 1,4-bis(trimethylsilyl)cyclooctatetraene dianion (COT″) yields a mixture of (COT″) 2 Ti ( 1 ), [(COT″)Ti] 2 (μ-Cl) 3 ( 2 ) and [(COT″)Ti(μ-Cl)] 2 (THF) ( 3 ). Crystalline 2 and 3 were isolated on the basis of different solubility in hexane. Their X-ray diffraction analyses revealed dimeric structures linked by chlorine bridges. In 3 , only one of the Ti atoms coordinates one molecule of THF. The mixed-valency Ti(III)/Ti(IV) compound 2 shows an ESR spectrum typical of mononuclear (COT″)Ti(III) complexes. The ESR spectra of 3 in toluene or 2-methyltetrahydrofuran glasses prove the triplet state of coupled Ti(III) d 1 electrons.

Dichlorobis[η5-tetramethyl(phenyl)cyclopentadienyl]titanium(IV) (1) and chlorobis[η5-tetramethyl- (phenyl)cyclopentadienyl]titanium(III) (2) were prepared by the generally known procedures. The X-ray crystal structures of 1 and 2 showed nearly identical Ti-CE (CE - centroid of the cyclopentadienyl ring) and Ti-Cl distances, the CE-Ti-CE angles and dihedral angles between least-squares planes of the cyclopentadienyl rings to those in [(C5Me5)2TiCl2] and [(C5Me5)2TiCl], respectively. ESR spectra of 2 in 2-methyltetrahydrofuran (MTHF) solutions and glasses revealed that 2 coordinates MTHF to give [(C5Me4Ph)2Ti(Cl)(MTHF)] (2a). The formation of 2a in MTHF solution is completed by lowering temperature to 205 K. A comparison with the known data gives the following order of affinity of [(C5Me4R)2TiCl] towards MTHF: Ph > SiMe3 > H > Me.

The reduction of [(η8-C8H8)Ti(THF)(μ-Cl)]2 by magnesium in the presence of bis(trimethylsilyl)acetylene (BTMSA) afforded the diamagnetic complex [(η8-C8H8)Ti]2[μ-η2:η2-Me3SiCCSiMe3] (1) in high yield. The crystal structure of 1 revealed that BTMSA is a four-electron ligand which binds two equivalent (η8-C8H8)Ti moieties under the angle of av. 69°. The average CC distance of 1.51 A in the acetylene ligand is close to that of sp3 carbon atoms, however, a high thermal stability and enormously large down-field chemical shift of the acetylenic carbon atoms (δ 292.8 ppm) points to an unusually high contribution of π-back-bonding to the Ti-acetylene bond. An analogous reduction in the presence of diphenylacetylene afforded a tetraphenylcyclobutadiene complex (η8-C8H8)Ti(η4-C4Ph4) and hexaphenylbenzene.

The zirconocene diacetylide ( η 5 -C 5 HMe 4 ) 2 Zr( η 1 -CCSiMe 3 ) 2 ( 1 ) is reduced by potassium in toluene to give the paramagnetic tweezer complex [( η 5 -C 5 HMe 4 ) 2 Zr( η 1 -CCSiMe 3 ) 2 ] - K + ( 2 ). The presence of Zr(III) follows from the ESR spectrum of 2 ( g =1.9914, a Zr =1.86 mT). The X-ray crystal structures show that the bite angle of tweezer arms of 102.7° in 1 is diminished to 92.8° in complex 2 . The potassium cation is embedded between the acetylide arms, 1.13 A away from the bite angle plane, at the Zr-K distance of 3.818(1) A.

(Trimethylsilyl)tetramethylcyclopentadiene C5HMe4(SiMe3) reacts with one equivalent of (C6H6)Ti(II)(AlCl4)2 to give the paramagnetic trinuclear complex [C5Me4(SiMe3)]Ti(III)(AlCl4)2 (1). At a five-fold molar excess of C5HMe4(SiMe3), the paramagnetic binuclear complex [C5Me4(SiMe3)]2Ti(III)(AlCl4) (2) and the diamagnetic centrosymmetric dimer {[C5Me4(SiMe3)]Ti(III)(AlCl4)(μ-Cl)}2 (3) are obtained as a major and a minor product, respectively. The molecular structures of 1 and 3 have been obtained by X-ray crystal analysis. The structure of 2 has been deduced from its ESR spectrum which shows the unpaired electron coupling to one Al nucleus.

The complexes [(η5-C5Me4Ph)TiCl(μ-Cl)]2 (1) and [(η5-C5Bn5)TiCl(μ-Cl)]2 (2) (Bn = benzyl) were obtained by the reaction of TiCl3 and the corresponding alkali metal cyclopentadienides in a THF-toluene mixture. The X-ray diffraction single-crystal analysis revealed that both compounds form centrosymmetric dimers bridged via a pair of chloride ligands. Compound 1 crystallizes in the monoclinic space group P21/n (No. 14; a = 11.387(3) Å, b = 16.496(2) Å, c = 8.383(2) Å, β = 110.786(15)°, V = 1 472.2(6) Å3, Z = 2), and compound 2 in the monoclinic space group P21/c (No. 14; a = 18.967(5) Å, b = 10.7190(10) Å, c = 17.996(5) Å, β = 116.747(15)°, V = 3 267.2(13) Å3, Z = 2). Common features of the structures differ only negligibly. The Ti-Ti distance (in 1 3.3345(11) Å, in 2 3.374(2) Å) is considerably shorter than in paramagnetic dimeric titanocene monochlorides (3.91-3.95 Å). The compounds are diamagnetic, probably due to spin-paired d1 electrons.

Dimethylsilylene-bridged complexes Me2Si(C5Me4)2TiCl (2), Me2Si(C5Me4)2Ti[η2-C2(SiMe3)2] (3) and Me2Si(C5H4)2Ti[η2-C2(SiMe3)2] (4) have been prepared by the general methods which are known for obtaining of analogous non-bridged titanocene complexes. X-ray crystal structures of Me2Si(C5Me4)2TiCl2 (1), 2, and 3 reveal that the dihedral angle between the least-squares planes of cyclopentadienyl rings increases in the order 2 < 3 < 1. Comparison with the structures of analogous (C5HMe4)2Ti and (C5Me5)2Ti compounds shows that the value of increases in the series (C5Me5)2Ti < (C5HMe4)2Ti < Me2Si(C5Me4)2Ti, e.g. in the bis(trimethylsilyl)acetylene complexes from 41.1° for (C5Me5)2Ti[η2-C2(SiMe3)2] (9) to 50.0° for (C5HMe4)2Ti[η2-C2(SiMe3)2] (8) and to 53.5° for 3. Compounds 3, 8 and 9 induce the head-to-tail dimerization of 1-hexyne with the selectivity of 72%, 21% and ca. 100% respectively. The discrepancy between the selectivities and the values of for 3 and 8 is accounted for by a larger flexibility of the titanocene skeleton in 8, affording a larger space for a non-specific coordination of 1-hexyne. The effects of the μ-Me2Si group in 2 and 3 on some of their properties are compared with the effects of Me and H substituents in the non-ansa compounds with controversial results. For instance, the affinity of 2 to 2-methyltetrahydrofuran approaches that of (C5H2Me3)2TiCl whereas the shift of the v(C≡C) vibration in 3 indicates a stronger metal-acetylene bond than in 9.

Crystal structures of the Ti(III) tweezer complexes [(C5HMe4)2Ti(σ-C≡CSiMe3)2]-[Li(THF)2]+ (2a), [(C5HMe4)2Ti(σ-C≡CSiMe3)2]-Na+ (3) and [(C5HMe4)2Ti(σ-C≡CSiMe3)2]-Cs+ (5) have been determined. In all of them the alkali metal cation is placed away from the Ti-Cα1-Cα2 plane at the distance: Li+ 0.511 Å, Na+ 1.023 Å and Cs+ 0.521 Å. The reason for the deviation of Li+ in 2a is the asymmetrical orientation of the THF ligands in the [Li(THF)2]+ cation with respect to the Ti-Cα1-Cα2 plane, which seems to release the steric congestion between the THF ligands and the trimethylsilyl groups. In 3 and 5, the molecules form polymer chains with a weak intermolecular bonding interaction between the cations and one of the C5HMe4 ligands of the neighbouring molecule in a sandwich manner.