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The cyclic triaminal 1,3,5-tri- tert-butyl-1,3,5-triazacyclohexane (T tBuTAC) was treated with tBuLi, but instead of the expected deprotonation at a CH2 unit, the [(T tBuTAC)( tBuLi)] ( 2) adduct was formed. This is a monomeric tetracoordinate form of tBuLi; its molecular structure was determined by single-crystal X-ray diffraction (XRD) and it is likely to be monomeric in C6D6 solution according to 1H NMR, 13C NMR, and 7Li NMR spectroscopy. T tBuTAC reacts with nBuLi to give a laddertype [(T tBuTAC)( nBuLi)2]2 aggregate of four nBuLi units terminated by two T tBuTAC ligands. This adduct was characterized by NMR spectroscopy and XRD. It slowly decays in solution, and minor quantities of a [T tBuTAC-Li(μ- nBu)]2[Li3( nBu)2N( tBu)( n-pent)] product were obtained and characterized by XRD. This compound contains amide units from the reaction of the formal monomer of T tBuTAC, Me3C-N=CH2, with nBuLi, which demonstrates the different types of reactivity of trisaminals such as T tBuTAC. [ABSTRACT FROM AUTHOR]

Tris(tetramethylaluminate)s of the lanthanoids cerium ( 1), holmium ( 2), and erbium ( 3) were synthesized by treating lanthanoid amide complexes of the type [Ln(NMe2)3(LiCl)3] with an excess amount of AlMe3. These compounds were characterized by single-crystal X-ray diffraction, elemental analysis, and NMR spectroscopy. Paramagnetic NMR spectroscopic studies were undertaken to investigate the influence of the compounds on the solvent chemical shifts. NMR parameters for the 1H, 13C, and 27Al nuclei of all the reported lanthanoid tris(tetramethylaluminate)s were measured. The reactions of tris(tetramethylaluminate)s of samarium and holmium with an excess amount of tetrahydrofuran led to [LnMe2(thf)5][AlMe4] {Ln = Ho ( 4), Sm ( 5)}. NMR spectroscopic investigations of the paramagnetic holmium and samarium methyl compounds (i.e., 4, 5) as well as the known diamagnetic analogue [YMe2(thf)5][AlMe4] revealed methyl group exchange processes in solution. [ABSTRACT FROM AUTHOR]

The salt (η5-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 °C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 °C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si-Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si-Si bond (2.168 Å) with efficiently packed, rigid s-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an η2-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si-Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation. [ABSTRACT FROM AUTHOR]

The compounds CI3SiOCH2CH2NMe2 (1) and Cl2HSiOCH2CH2NMe2 (2) were prepared by reactions of lithium 2-(dimethylamino)ethanolate with SiCl4 and HSiCI3. The analogous reaction with H2SiCI2 gave .ClH2SiOCH2CH2NMe2 (3), but only in a mixture with Cl2HSiOCH2CH2NMe2 (2), from which it could not be separated. All compounds were characterized by IRand NMR (1H, 13C, 29Si) spectroscopy, 1 and 2 by elemental analyses and by determination of their crystal structures. CI3SiOCH2CH2NMe2 (1) and Cl2HSiOCH2CH2NMe2 (2) crystallize as monomeric ring compounds with pentacoordinate silicon atoms participating in intramolecular Si-N bonds [2.060(2) Å (1), 2.037(2) Å (2)]. The dative bonds in 1 and 2 between the silicon and nitrogen atoms could also be proven to exist at low temperatures in solution in 1H,29Si-HMBC-NMR experiments by detection of the scalar coupling between the 29Si and the protons of the NCH2 and NCH3 groups. A function describing the chemical shift δexp29Si dependent on the chemical shifts of the individual equilibrium components, the temperature, and the free enthalpy of reaction was worked out and fitted to the experimental VT-NMR data of 1 and 2. This provided values of the free reaction enthalpies of ΔG = -28.8 ± 3.9 kJ · mol-1 for 1 and ΔG = -22.3 ± 0.4 kJ · mol-1 for 2 and estimates for the chemical shifts of open-chain (index o) and ring conformers (index r) for 1 of δr = -94 ± 2 ppm and δo = -36 ± 5 ppm and for 2 of δr = -82 ± 1 ppm and δo = -33 ± 4 ppm. The value of δr for 1 is very close to that obtained from a solid-state 29Si MAS NMR spectrum. Quantumchemical calculations (up to MP2/TZVPP) gave largely differing geometries for 1 (with a Si ··· N distance of 3.072 Å), but well reproduced the geometry of 2. These differences are due to Cl ··· H and Cl ··· C repulsions and solid state effects, which can be modeled by conductor-like screening model calculations and also rationalized in terms of the topology of the electron density, which was analyzed in terms of the quantum theory of atoms in molecules. [ABSTRACT FROM AUTHOR]

The reaction of decamethylsilicocene, (Me[sub 5]C[sub 5])[sub 2]Si, with the proton-transfer reagent Me[sub 5]C[sub 5]H[sub 2,sup+] B(C[sub 6]F[sup 5])[sub 4,sup -] produces the salt (Me[sub 5]C[sub 5])Si[sup +] B(C[sub 6]F[sub 5])[sub 4,sup -] (2), which can be isolated as a colorless solid that is stable in the absence of air and moisture. The crystal structure reveals the presence of a cationic π complex with an η[sup 5]-pentamethylcyclopentadienyl ligand bound to a bare silicon center. The [sup 29]Si nuclear magnetic resonance at very high field (δ = -400.2 parts per million) is typical of a π complex of divalent silicon. The (η[sup 5]-Me[sub 5]C[sup 5])Si[sup +] cation in 2 can be regarded as the "resting state" of a silyliumylidene-type (η¹-Me[sub 5]C[sub 5])Si[sup +] cation. The availability of 2 opens new synthetic avenues in organosilicon chemistry. For example, 2 reacted with lithium bis(trimethylsilyl)amide to give the disilene E-{(η¹-Me[sub 5]C[sub 5])[N(SiMe[sub 3])[sub 2]]Si}[sub 2] (3). [ABSTRACT FROM AUTHOR]

Starting from ClSbR F 2 (R F = C 2 F 5 , 3,5-(CF 3 ) 2 C 6 H 3 ) and H(E)P( t Bu) 2 (E = O, S), we prepared the oxy- and sulphanediyl-bridged adducts R F 2 Sb(Cl)-E-(H)P( t Bu) 2 , which are stable against the elimination of HCl. The different electron-withdrawing substituents and chalcogen bridging units influence the size of the Sb-E-P angle. ClSb(C 2 F 5 ) 2 and n Bu 3 SnH react to give HSb(C 2 F 5 ) 2 , which seems to interact weakly with H(O)P( t Bu) 2 in solution as observed via NMR. All products were characterised by NMR spectroscopy and all the stable ones were additionally characterised by X-ray diffraction and elemental analyses.

Flexible poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Hydrosilylation of 4b,8b,12b-triallyltribenzotriquinacene and subsequent exchange of the chlorine substituents with weaker coordinating triflate groups afforded a novel triple silyl-functionalised PLA. By regioselective hydroboration of triallyl-TBTQ with various organoboranes, PLAs with different Lewis acidities were obtained. The synthesised PLAs were combined with neutral bases in host-guest experiments. DOSY NMR spectroscopy was performed to elucidate the complexation process in solution. These experiments revealed a highly dynamic interaction between the boron-functionalised PLA and triazine. However, the addition of one equivalent of tris(dimethylphosphino(methyl))phenylsilane led to the formation of a 1 : 1 adduct, which was confirmed by diffusion experiments.

Chalice-shaped tridentate poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Stannylation of the alkyne units, attached via phenyl-spacers to the benzhydrylic positions to the TBTQ scaffold, with Me 2 NSnMe 3 afforded the trimethyltin substituted TBTQ derivative. Replacement of these tin functions with other elements resulted in rigid boron- and aluminium-functionalised PLAs. More flexible PLAs were obtained by hydrometallation reactions of the terminal alkyne groups of 4b,8b,12b-tris((ethynyl)phenyl)tribenzotriquinacene. The resulting poly-Lewis acids were tested for their acceptor abilities in host-guest experiments with suitable bases. Preliminary tests with pyridine led to the synthesis of a large tridentate base with three pyridyl groups attached to a TBTQ backbone. Complexation of this Lewis base with the PLAs resulted in the formation of aggregates, which were studied in solution in more detail by 1 H DOSY NMR experiments regarding their size. Further experiments were performed with the tridentate bases 1,4,7-trimethyl-1,4,7-triazacyclononane and tris((dimethylphosphino)methyl)phenylsilane., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Visible light can be used to shift dynamic covalent imine assemblies out of equilibrium. We studied a fluorinated azobenzene building block that reliably undergoes geometric isomerism upon irradiation. The building block was used in combination with two different amines, ethylenediamine and R,R-1,2-diaminocyclohexane, to create a library of imine macrocycles. Whereas the simple amine can be used to access a polymeric state and a defined bowl-shaped macrocycle, the chiral amine gives access to a rich network of macrocycles that undergo both isomerisation as well as interconversion between different macrocyclic species, thereby allowing for control over the number of monomers involved in the cyclo-oligomerization; 1 H- and 19 F-DOSY NMR, MALDI-MS measurements, and UV/Vis spectroscopy were used to study the processes., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The geminal frustrated Lewis pair (FLP) (F 5 C 2 ) 3 SnCH 2 P(tBu) 2 (2) was prepared by reacting (F 5 C 2 ) 3 SnCl with LiCH 2 P(tBu) 2 . It is neutral and contains an extremely electronegatively substituted, but relatively soft (hard-soft acid-base, HSAB) acidic tin function. Its FLP-type reactivity was proven by reaction with a variety of small molecules (CO 2 , SO 2 , CS 2 , PhNCO, HCl, (Ph 3 P)AuCl). However, it shows no reaction in H/D scrambling experiments with H 2 /D 2 mixtures and binds CO 2 reversibly, as was observed by VT-NMR spectroscopy. Compound 2 and all its adducts were completely characterized by means of multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction experiments., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The first C5-protonated abnormal N-heterocyclic carbene (aNHC), PhC{N(2,6-iPr 2 C 6 H 3 )} 2 CHC: (4) is readily accessible by C4-deprotonation of [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH]X (3 a-X) (Ar=Ph, X=Br or I) with a base. The aNHC 4 is stable at 298 K in [D 8 ]THF solution and has been spectroscopically characterized. The facile availability of 4 enables the synthesis of a series of main-group compounds as well as transition-metal complexes featuring a new phosphorus-aNHC hybrid ligand. Double deprotonation of [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 CHCH]X (Ar=Ph, 3 a-X (X=Br or I); 4-Tol, 3 b-Br; 4-DMP, 3 c-Br; Tol=MeC 6 H 4 , DMP=Me 2 NC 6 H 4 ) with nBuLi yields the C4,C5-ditopic carbanionic aNHCs, [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 (C:) 2 ]Li(THF) n (Ar=Ph, 13 a; 4-Tol, 13 b; 4-DMP, 13 c), which on treatment with Ph 2 PCl affords cationic vicinal bisphosphine derivatives [ArC{N(2,6-iPr 2 C 6 H 3 )} 2 {C(PPh 2 } 2 ]X (Ar=Ph, 14 a-X, X=Br or I; 4-Tol, 14 b-Br; 4-DMP, 14 c-Br)., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Bidentate gallium Lewis acids were prepared by the reaction of diethynyldiphenylsilane with neat trimethyl- or triethylgallium. Bis[(dimethylgallyl)ethynyl]diphenylsilane (1) and diethylgallyl derivative 2 were characterized as Et 2 O or pyridine adducts by NMR spectroscopy; 2⋅2Py was isolated. Lewis acids 1 and 2 form host-guest adducts with bidentate nitrogen bases, but defined cyclic 1:1 adducts are only formed between 1 and bases with matching N⋅⋅⋅N distances: 4,4'-dimethyl-3,3'-bipyridinylacetylene (3), bis[(pyridin-3-yl)ethynyl]diphenylsilane (4), and bis[(2-methylpyridin-5-yl)ethynyl]diphenylsilane (5). The structures of adducts 1⋅3, 1⋅4, and 1⋅5 were established by X-ray diffraction experiments. 2⋅2Py reacts with DABCO to afford polymeric (DABCO-2-) n ., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Frustrated Lewis pairs (FLPs) have a great potential for activation of small molecules. Most known FLP systems are based on boron or aluminum atoms as acid functions, few on zinc, and only two on boron-isoelectronic silicenium cation systems. The first FLP system based on a neutral silane, (C2F5)3SiCH2P(tBu)2 (1), was prepared from (C2F5)3SiCl with C2F5 groups of very high electronegativity and LiCH2P(tBu)2. 1 is capable of cleaving hydrogen, and adds CO2 and SO2. Hydrogen splitting was confirmed by H/D scrambling reactions. The structures of 1, its CO2 and SO2 adducts, and a decomposition product with CO2 were elucidated by X-ray diffraction., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Directed tridentate Lewis acids based on the 1,3,5-trisilacyclohexane skeleton with three ethynyl groups [CH2Si(Me)(C2H)]3 were synthesised and functionalised by hydroboration with HB(C6F5)2, yielding the ethenylborane {CH2Si(Me)[C2H2B(C6F5)2]}3, and by metalation with gallium and indium organyls affording {CH2Si(Me)[C2M(R)2]}3 (M = Ga, In, R = Me, Et). In the synthesis of the backbone the influence of substituents (MeO, EtO and iPrO groups at Si) on the orientation of the methyl group was studied with the aim to increase the abundance of the all-cis isomer. New compounds were identified by elemental analyses, multi-nuclear NMR spectroscopy and in some cases by IR spectroscopy. Crystal structures were obtained for cis-trans-[CH2Si(Me)(Cl)]3, all-cis-[CH2Si(Me)(H)]3, all-cis-[CH2Si(Me)(C2H)]3, cis-trans-[CH2Si(Me)(C2H)]3 and all-cis-[CH2Si(Me)(C2SiMe3)]3. A gas-phase electron diffraction experiment for all-cis-[CH2Si(Me)(C2H)]3 provides information on the relative stabilities of the all-equatorial and all-axial form; the first is preferred in both solid and gas phase. The gallium-based Lewis acid {CH2Si(Me)[C2Ga(Et)2]}3 was reacted with a tridentate Lewis base (1,3,5-trimethyl-1,3,5-triazacyclohexane) in an NMR titration experiment. The generated host-guest complexes involved in the equilibria during this reaction were identified by DOSY NMR spectroscopy by comparing measured diffusion coefficients with those of the suitable reference compounds of same size and shape., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Lithium carbenoids are versatile compounds for synthesis owing to their intriguing ambiphilic behavior. Although this class of compounds has been known for several years, few solid-state structures exist because of their high reactivity and often low thermal stability. Using cryo X-ray techniques, we were now able to elucidate the first solid-state structure of a Li/F alkyl carbenoid, pentafluoroethyllithium (LiC2F5), finally yielding a prototype for investigating structure-reactivity relationships for this class of molecules. The compound forms a diethyl ether-solvated dimer bridged by a rare C-F-Li link. Complementary NMR spectroscopy studies in solution show dynamic processes and indicate rapid exchange of starting material and product. Theoretical investigations help to understand the formation of the observed unusual structural motif., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A series of bis- and tris[(trimethylsilyl)ethynyl]anthracenes (1,5-, 1,8-, 9,10- and 1,8,10-) has been synthesised by multistep (cross coupling) reactions and the behaviour of the SiMe3-functionalised alkynylanthracene derivatives towards UV irradiation was qualitatively studied by NMR spectroscopy. In the case of 9,10-bis[(trimethylsilyl)ethynyl]anthracene we observed a photodimerisation upon UV irradiation; the third example was reported for a symmetrically 9,10-difunctionalised anthracene derivative, besides those with small fluorine- and methyl-substituents. The anthracene dimerisation is completely thermally reversible and the temperature dependence of the cycloelimination reaction was studied by (1)H VT-NMR experiments. The (deprotected) 1,5- and 1,8-diethynylanthracenes were converted with (dimethylamino)trimethylstannane to obtain the corresponding SnMe3-functionalised alkynes, potentially useful as highly conjugated building blocks in Stille cross coupling reactions. The new anthracene compounds were completely characterised by multinuclear NMR spectroscopy, (high resolution) mass spectrometry and - in most cases - by X-ray diffraction experiments.

The formation of host-guest (H-G) complexes between 1,8-bis[(diethylgallanyl)ethynyl]anthracene (H) and the N-heterocycles pyridine and pyrimidine (G) was studied in solution using a combination of NMR titration and diffusion NMR experiments. For the latter, diffusion coefficients of potential host-guest structures in solution were compared with those of tailor-made reference compounds of similar shape (synthesized and characterized by NMR, HRMS, and in part XRD). Highly dynamic behavior was observed in both cases, but with different host-guest species and equilibria. With increasing concentrations of the pyridine guest, the equilibrium H2⇄H2κ(1)-G1⇄HG2 is observed (in the second step a host dimer coordinates one guest molecule); for pyrimidine the equilibrium H2→H1κ(2)-G1⇄HG2 is observed (the formation of a 1:1 aggregate is the second step)., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new class of rare-earth-metal alkynyl complexes has been prepared. The reactions of the tris(tetramethylaluminate)s of lanthanum, praseodymium, samarium, yttrium, holmium, and thulium, [Ln(AlMe₄)₃], with phenylacetylene afforded compounds [Ln{(μ-C≡CPh)₂AlMe₂}₃] (Ln=La (1), Pr (2), Sm (3), Y (4), Ho (5), Tm (6)). All of these compounds have been characterized by NMR spectroscopy, X-ray crystallography, and by elemental analysis. NMR spectroscopic studies of the series of para- magnetic compounds [Ln(AlMe₄)₃] and [Ln{(μ-C≡CPh)₂AlMe₂}₃] have also been performed., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) with [Y(AlMe(4))(3)] resulted in the formation of [(TiPTAC)Y(Me(3)AlCH(2)AlMe(3))(μ-MeAlMe(3))] by C-H activation and methane extrusion. In contrast, the presence of bulkier cyclohexyl groups on the nitrogen atoms in 1,3,5-tricyclohexyl-1,3,5-triazacyclohexane (TCyTAC) led to the formation of the cationic dimethyl complex [(TCyTAC)(2)YMe(2)][AlMe(4)]. The investigations reveal a dependency of the reaction mechanism on the steric bulk of the N-alkyl entity and the solvent employed. In toluene C-H activation was observed in reactions of [Y(AlMe(4))(3)] with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC)(2)YMe(2)][AlMe(4)], [(TMTAC)(2)YMe(2)][AlMe(4)] and [(TiPTAC)(2)YMe(2)][AlMe(4)], could be synthesised by addition of the triazacyclohexane at a later stage. The THF-solvated complex [YMe(2)(thf)(5)][AlMe(4)] could be isolated and represents an intermediate in these reactions. It shows that cationic methyl complexes of the rare-earth metals can be formed by donor-induced cleavage of the rare-earth-metal tetramethylaluminates. The compounds were characterised by single-crystal X-ray diffraction or multinuclear and variable-temperature NMR spectroscopy, as well as elemental analyses. Variable-temperature NMR spectroscopy illustrates the methyl group exchange processes between the cations and anions in solution., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of the donor-functionalised N,N-bis(2-{pyrid-2-yl}ethyl)hydroxylamine and [LnCp3] (Cp=cyclopentadiene) resulted in the formation of bis(cyclopentadienyl) hydroxylaminato rare-earth metal complexes of the general constitution [Ln(C5H5)2{ON(C2H4-o-Py)2}] (Py=pyridyl) with Ln=Lu (1), Y (2), Ho (3), Sm (4), Nd (5), Pr (6), La (7). These compounds were characterised by elemental analysis, mass spectrometry, NMR spectroscopy (for compounds 1, 2, 4 and 7) and single-crystal X-ray diffraction experiments. The complexes exhibit three different aggregation modes and binding motifs in the solid state. The late rare-earth metal atoms (Lu, Y, Ho and Sm) form monomeric complexes of the formula [Ln(C5H5)2{eta2-ON(C2H4-eta1-o-Py)(C2H4-o-Py)}] (1-4, respectively), in which one of the pyridyl nitrogen donor atoms is bonded to the metal atom in addition to the side-on coordinating hydroxylaminato unit. The larger Nd3+ and Pr3+ ions in 5 and 6 make the hydroxylaminato unit capable of dimerising through the oxygen atoms. This leads to the dimeric complexes [(Ln(C5H5)2{mu-eta1:eta2-ON(C2H4-o-Py)2})2] without metal-pyridine bonds. Compound 7 exhibits a dimeric coordination mode similar to the complexes 5 and 6, but, in addition, two pyridyl functions coordinate to the lanthanum atoms leading to the [(La(C5H5)2{ON(C2H4-o-Py)}{mu-eta1:eta2-ON(C2H4-eta1-o-Py)})2] complex. The aggregation trend is directly related to the size of the metal ions. The complexes with coordinative pyridine-metal bonds show highly dynamic behaviour in solution. The two pyridine nitrogen atoms rapidly change their coordination to the metal atom at ambient temperature. Variable-temperature (VT) NMR experiments showed that this dynamic exchange can be frozen on the NMR timescale.

The pentamethylcyclopentadienylsilicon(II) cation, Me5C5Si+, opens up access to novel silicocene derivatives; the penta-iso-propylcyclopentadienylsilicon(II) cation, iPr5C5Si+, is obtained by reaction of the mixed silicocene (iPr5C5)(Me5C5)Si with H(OEt2)2+ Al[OC(CF3)3]4-.

For the intramolecular stabilization of silicenium ions (R 3 Si + ), O, S, and P donors as well as the known nitrogen-containing systems (as in 1 a) are suitable. The silyl cations in 1 a-d show a trigonal-bipyramidal structure; dynamic processes can be proved by NMR spectroscopy for 1 c, d. Calculations on model compounds document substantial differences in the bonding relationships and support the structural findings. Furthermore, preliminary experiments with 1 b-d indicate significant differences in reactivity., (© 1999 WILEY‐VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.)