3 )(THF) can be prepared in high yields by a σ-bond metathesis reaction between Y(CH 2 SiMe 3 ) 3 (THF) 2 and amino-functionalized cyclopentadienes or indene. The structure of Y(η 5 :η 1 -C 5 Me 4 SiMe 2 NCMe 2 Et)(CH 2 SiMe 3 )(THF) was shown by single-crystal X-ray diffraction to be that of a three-legged piano stool. Reaction of Y(CH 2 -SiMe 3 ) 3 (THF) 2 with the tridentate linked amido-cyclopentadienyl ligands (C 5 Me 4 H)SiMe 2 -NHR (R ) CH 2 CH 2 OMe, CH 2 CH 2 NMe 2 , CH 2 CH 2 CH 2 OMe, CMe 2 CH 2 OMe), which contain an additional donor site, results in the cleavage of the silicon-cyclopentadienyl bond and the formation of the tetramethylcyclopentadienyl complexes Y(η 5 -C 5 Me 4 H){N(SiMe 2 CH 2are prepared in good yields by hydrogenolysis of the corresponding alkyl complexes. Variable-temperature 1 H, 13 C, 29 Si, and 89 Y NMR spectroscopic data show that the hydrido complexes retain their dimeric structure in solution on the NMR time scale but that they undergo fluxional processes which include THF dissociation and cis-trans isomerization. The presence of monomeric species is inferred from 1 H NMR spectroscopic detection of the crossover product [Y(η 5 :η 1 -C 5 Me 4 SiMe 2 NCMe 3 )(THF)(µ-H) 2 Y(η 5 :η 1 -C 5 Me 4 -SiMe 2 NCMe 2 Et)(THF)], which forms within minutes at 50 °C when [Y(η 5 :η 1 -C 5 Me 4 SiMe 2 -NCMe 2 R)(THF)(µ-H)] 2 with R ) Me and Et are mixed in C 6 D 6 . Ethylene is polymerized with moderate activity by the hydrido complexes, whereas styrene derivatives and 1-hexene are cleanly converted into the monoinsertion products. 1,5-Hexadiene reacts with the hydrido complexes to give the monomeric cyclopentylmethyl complex Y(η 5 :η 1 -C 5 Me 4 SiMe 2 NCMe 3 ){CH 2 -CH(CH 2 ) 4 }(THF). The 1-phenylethyl complexes Y(η 5 :η 1 -C 5 Me 4 SiMe 2 NCMe 3 ){CH(CH 3 )(C 6 H 3 -2-R-4-R′)}(THF) (R ) R′ ) H; R ) H, R′ ) OMe; R ) R′ ) Me), selectively formed by secondary insertion of the corresponding styrene derivatives, show fluxional η 3 -coordination in solution. A monomeric structure with a weak η 3 -interaction was confirmed by single-crystal X-ray diffraction of Y(η 5 :η 1 -C 5 Me 4 SiMe 2 NCMe 2 Et){CH(CH 3 )(C 6 H 4 -4-tBu)}(THF). The insertion product of 1-hexene is dimeric in solution, but partially loses THF under vacuum. This product initiates the polymerization of styrene to give atactic polystyrenes with narrow molecular weight distributions (M w /M n 1.10-1.23) and microstructures enriched in syndiotacticity (rr ≈ 70%).

Models get choosey: Structurally characterized mono‐ and dicationic yttrium methyl complexes (3 and 4) are accessible by protonolysis of the hexamethylate complexes (1 and 2) and function as models for the homogeneous catalysts for 1,4‐cis‐selective polymerization of 1,3‐dienes by the industrially applied multicomponent catalyst.

A series of titanium dichloro and di(isopropoxy) complexes with a 1,4-dithiabutanediyllinked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}X2] (X ) Cl, 2a-i, OiPr, 3a-i; R1 ) H, Me, iPr, tBu, 2-phenyl-2-propyl; R2 ) H, Me, tBu, OMe, 2-phenyl-2-propyl) were synthesized by reacting the corresponding linked bis(phenol) (HOC6H2-6-R1-4-R2)2{S(CH2)2S} (1a-i) with the titanium precursor TiX4. The NMR spectra of the dichloro complexes Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}Cl2] (2a-d) with small ortho substituents R1 ) H, Me, iPr are in agreement with a C2-symmetrical helical structure, but the complexes become fluxional at higher temperatures. The corresponding di(isopropoxy) complexes [Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}(OiPr)2] (3a-d) are fluxional in solution at room temperature due to rapid interconversion between the delta and lambda isomers. In contrast, both dichloro and di(isopropoxy) complexes 2e-i and 3e-i with bulky ortho substituents R1 ) tBu and 2-phenyl- 2-propyl exhibit a rigid C2-symmetrical helical structure in solution up to 100 °C. The helical structure with trans-O,O, cis-S,S, cis-Cl,Cl (“alpha-cis”) is confirmed by the single-crystal structure analysis of two dichloro complexes [Ti(OC6H2-6-R1-4-R2)2{S(CH2)2S}Cl2] (2e, R1 ) tBu, R2 ) Me; 2i, R1, R2 ) 2-phenyl-2-propyl). Related dichloro and di(isopropoxy) complexes that contain a 1,5-dithiapentanediyl-linked bis(4,6-disubstituted phenolato) ligand [Ti(OC6H2-6-R1-4-R2)2{S(CH2)3S}X2] (R1 ) tBu; R2 ) Me, tBu, OMe; X ) Cl, 5a-c; OiPr, 6a-c) were synthesized in an analogous fashion and shown by NMR spectroscopy to be conformationally flexible at room temperature. This fluxionality can be explained by a twisting of one fivemembered chelate ring, converting the trans-O,O (“alpha-cis”) to the cis-O,O (“beta-cis”) isomer. A crystal structure determination of ligand [Ti(OC6H2tBu2-4,6)2{S(CH2)3S}(OiPr)2] (6b) revealed the trans-O,O, cis-S,S, cis-Cl,Cl (“alpha-cis”) isomer. Upon activation with methylaluminoxane, the stereorigid derivatives 2e-i efficiently polymerize styrene to give isotactic polystyrene, whereas the conformationally flexible complexes 2a-d produce atactic polystyrene with negligible activity. Under the same conditions, complexes with the 1,5-dithiapentanediyllinked ligand 5a,b and 6b,c polymerized styrene syndiospecifically with low activity

Large magnesium hydride aggregates [Mg13 (Me3 TACD)6 (μ2 -H12 )(μ3 -H6 )][A]2 ((Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane; A=AlEt4 , AlnBu4 , B{3,5-(CF3 )2 C6 H3 }4 ) were synthesized stepwise from alkyl complexes [Mg2 (Me3 TACD)R3 ] (R=Et, nBu) and phenylsilane in the presence of additional Mg(II) ions. The central magnesium atom is octahedrally coordinated by six hydrides as in solid α-MgH2 of the rutile type. Further coordination to six magnesium atoms leads to a substructure of seven edge-sharing octahedra as found in the hexagonal layer of brucite (Mg(OH)2 ). Upon protonolysis in the presence of 1,2-dimethoxyethane (DME), this cluster was degraded into a tetranuclear dication [Mg2 (Me3 TACD)(μ-H)2 (DME)]2 [A]2 .