On Silylated Oxonium and Sulfonium Ions and Their Interaction with Weakly Coordinating Borate Anions

Abstract Attempts have been made to prepare salts with the labile tris(trimethylsilyl)chalconium ions, [(Me3Si)3E]+ (E=O, S), by reacting [Me3Si‐H‐SiMe3][B(C6F5)4] and Me3Si[CB] (CB−=carborate=[CHB11H5Cl6]−, [CHB11Cl11]−) with Me3Si‐E‐SiMe3. In the reaction of Me3Si‐O‐SiMe3 with [Me3Si‐H‐SiMe3][B(C6F5)4], a ligand exchange was observed in the [Me3Si‐H‐SiMe3]+ cation leading to the surprising formation of the persilylated [(Me3Si)2(Me2(H)Si)O]+ oxonium ion in a formal [Me2(H)Si]+ instead of the desired [Me3Si]+ transfer reaction. In contrast, the expected homoleptic persilylated [(Me3Si)3S]+ ion was formed and isolated as [B(C6F5)4]− and [CB]− salt, when Me3Si‐S‐SiMe3 was treated with either [Me3Si‐H‐SiMe3][B(C6F5)4] or Me3Si[CB]. However, the addition of Me3Si[CB] to Me3Si‐O‐SiMe3 unexpectedly led to the release of Me4Si with simultaneous formation of a cyclic dioxonium dication of the type [Me3Si‐μO‐SiMe2]2[CB]2 in an anion‐mediated reaction. DFT studies on structure, bonding and thermodynamics of the [(Me3Si)3E]+ and [(Me3Si)2(Me2(H)Si)E]+ ion formation are presented as well as mechanistic investigations on the template‐driven transformation of the [(Me3Si)3E]+ ion into a cyclic dichalconium dication [Me3Si‐μE‐SiMe2]2 2+.

In 1992, Kira et al. reported NMR data of some [R 1 R 2 R 3 SiOEt 2 ] + ions (R 1,2,3 = alkyl, aryl), for example, [Me 3 SiOEt 2 ] + , [24] while Olah and Prakash et al. already described transientt risilyloxonium ions including [T 3 O] + by solutionN MR techniques. In situ generated[ T 3 O] + was shown to be highly reactive, initiating polymerization of T-O-T,y ieldingd ifferent types of polysiloxanes. [25] As shown in Scheme1[Eq. (2)],b oth [T 3 O] + and [T 3 S] + were generated in situ by treating Me 3 SiH in the presence of one equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ]i nC D 2 Cl 2 at À78 8C, [26] but again no isolation in the solid state wasa chieved.T his prompted us to attemptt he preparation, isolation and full characterization of salts containing [T 3 E] + cations (E = O, S). Therefore, we startedf rom trityl salts with weakly coordinatinganions( wca) [27] as counterions. For example [CHB 11 H 5 Cl 6 ] À ,[ CHB 11 Cl 11 ] À and [B(C 6 F 5 ) 4 ] À [28,29] usuallya llow the isolation of highly reactive cations. [19,20,28,29,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Here we report the straightforward synthesis and full characterization of salts containing the trimethylsilylsulfonium ion [T 3 S] + and aboutt he failure of synthesizings alts with the [T 3 O] + ion that finally led to the isolation of unusual oxoniumb orate salts of the type [T 2 (Me 2 2 [CB] 2 ,r espectively,d epending on the weakly coordinating anion utilized. It should be noted that, as early as 1963, Corey and West [51] used the Lewis acid assisted hydrogen/halogen exchange Bartlett-Schneider-Condon [52] reaction for the first time in silicon chemistry.T hirty years later Lambertu sed ab orate ([B(C 6 F 5 ) 4 ] À ) as weakly coordinating anion in the reactiono fP h 3 C[B(C 6 F 5 ) 4 ] with hydridosilanes (R 3 SiH) and published ag eneral synthetic approacht ot rialkylsilylium cations[ R 3 Si] + for the first time [cf. Scheme 1, Eq. (1)]. [53] However,( 18 years later) Nava and Reed [44] experimentally provedt hat the commonly used, supposed [R 3 Si][B(C 6 F 5 ) 4 ]s alt (R = Et) does not exist at all, but always exists as ah ydride-bridged silane adduct [R 3 Si-H-SiR 3 ] + ion when [B(C 6 F 5 ) 4 ] À is used as ac ounterion (and R = small substituent, for example, alkyl);a ni ssue that is also addressed in detail in this report. The group of Knapp-Jenne reported the synthesis, spectroscopica nd structural characterization of silylium cations[ R 3 Si] + (R = Me, Et, iPr) stabilized by the perchlorinated weakly coordinating dianion [B 12 Cl 12 ] 2À . [49] Results and Discussion  Olah and Prakashe tal., [26] we have isolated this salt prior to the reaction with T-E-T,b ut not the T + -salt, since [T-H-T][B(C 6 F 5 ) 4 ]a lways forms, when [B(C 6 F 5 ) 4 ] À is the counterion. [44] 4 ], B(C 6 F 5 ) 3 and "C 6 F 4 ", which can be trapped with CS 2 ,h ave already been reported earlier by our group. [12,34] As imilard egradation of the [B(C 6 F 5 ) 4 ] À ionh as been reported before by Müllere tal. in naphthyl-based silylium ions. [31] Cl 11 ] À ), [54] bearing af ormally naked T + ion although strongly stabilized by ad onor-acceptor interaction with the carborate anion (vide infra, see section structure and bonding). We studied both carborate salts, T[CHB 11  [CB]/toluene suspensionh as been stirred for 5minutes and treated with ultrasound prior to the addition of T-O-T.A fter adding of T-O-T,t he two-phase system was gently heated up to 70 8Cf or 30 min. Thermally,a ll [T 3 S] + salts were stable up to over 150 8C, decomposingw ithout melting at this temperature, while 3O, as well as 6O, decomposed already above 90 8C. 29 Si NMR studies for all considered chalconium species are rather difficult or even completely hampered since often highly dynamic equilibria depending on solubility,t emperature, solvent, side-reactions (e.g.,r eactions with the solvent or anion) and concentrationsw ere observed, even when pure crystalsw ere dissolved. 29 Si NMR resonances were observed for T 2 Sb etween 12.9 and 15.3 ppm depending on the solvent (see ESI), which is shifted to lower field for [T 3 S] + (31.7 (2S) À39.3 (4S) ppm) in accord with NMR studies of Olah and Prakash. [26] As expectedahigh-field shift along the series [T 3 S] + (31.7, CD 2 Cl 2 ), T 2 S( 14.6, CD 2 Cl 2 )a nd TS À (À0.9 ppm, thf) was detected. For the reactiono fT -O-T with af ormal "T + "s alt, we carried out as eries of different temperature variable experiments( see NMR experiments 1-9 in the SupportingI nformation file). When isolated crystalso f3Ow ere suspended in toluene, no resonance of the [T 2 (Me 2 (H)Si)O] + ion was detected indicating ar ather low solubility even at ambient temperatures. The same holds true for the reactioni nb enzene (NMR experiments 3-4, see Supporting Information). To increaset he solubility, crystalso f3Ow ere suspended (partly dissolved) in am ixture of toluene/1,2-dichlorobenzene at À20 and 25 8C( experiments 1.1 and 1.2), whichg ave rise to several resonances. On the basis of computations and coupling patterns, we assignedt he signals at 31.5 (dsept, 1 J( 29 SiÀ 1 H) = 230 Hz, 2 J( 29 SiÀ 1 H) = 7.3 Hz) to the Me 2 (H)Si group and 53.8 ppm (multiplet) to both T groups in 3On ext to some unidentified (decomposition) species. However, at 25 8Ca ni ncreasing amounto fM e 4 Si and other side products could be detected (experiment 1.2). More-over,a t2 58C, the couplingp attern of the signal at 53.8 ppm was observed to be ad oublet of ad ecet ( 3 J( 29 SiÀ 1 H) = 2.7 Hz and 2 J( 29 SiÀ 1 H) = 6.8 Hz) as expected for aT 2 group as in 3O. When isolated crystals of 3Ow ere suspended in CD 2 Cl 2 (experiment 2, see Supporting Information), av ariety of signals were detected, which did not allow an unequivocal assignment.  29 Si spectra were recorded at different temperatures from À60 to 25 8C. In no case, we were able to verify the results by Olah and Prakash who reported ar esonance at 51.1 ppm in CD 2 Cl 2 at À70 8Cf or [T 3 O] + .I na ll of our experiments,w eo nly observed the startingm aterials T-O-T as well as T-Ha tl ow temperatures besides the fact that the solubility of any oxonium salt should be rather low.O nincreasing temperatures, the amount of T-Hd ecreases while the formation of Me 4 Si is dramatically increased due to decomposition and formationo f3 + .T or ule out the strong influence of the [B(C 6 F 5 ) 4 ] À ion and the excess of T-Hf rom the [T-H-T] + salt formation on the decompositionp rocess, we also used the carborate salts but again no resonancefor a[T 3 O] + salt could be detected but only T-O-T (experiment 8) caused by ab ad solubility of all considered carborate salts even at 25 8C. When CH 2 Cl 2 wasu sed to increaset he solubility of the carborates alts (experiment 9), also no resonance for a[ T 3 O] + ion was detected but slow decomposition. In conclusion, we believe that it is not possible to generate larger amountso f[ T 3 O] + in solution duet oa ratherb ad solubility,r eaction with the solvent( e.g.,c hloride abstraction from CH 2 Cl 2 )a nd its tendency to decompose (see formation of 3Oa nd [T-F-T] + )a sw ell as the transformation to [T-mO-SiMe 2 ] 2 [CB].T he latter is only formed upon raising the temperature up to 70 8C. When crystalso f6Ow eres uspended in dmso, which is neededt od issolve at least al ittle amount of 6O, four main resonances were detected [d( 29 Si) = À17.4 (septet, Si(CH 3 ) 2 ), 42.6 (decet, Si(CH 3 ) 3 ,a nd 1.5 (septet, Si(CH 3 ) 2 ), and 9.0 (decet,S i(CH 3 ) 3 ,],w hichm ight indicateamonomerdimer equilibrium.

X-ray structure analysis [T 3 S][B(C 6 F 5 ) 4 ]a nd [T 3 S][CHB 11 H 5 Cl 6 ]·toluene:[ T 3 S][B(C 6 F 5 ) 4 ] crystallizes in the monoclinics pace group P2 1 /c and [T 3 S]
[CHB 11 H 5 Cl 6 ]·toluene in P2 1 /n,b othw ithf ourf ormula units per cell. In both salts, there are neither significant cation-anion nor anion-anion contacts. The observed molecular structure exhibits the expected slightly distorted trigonal pyramidal coordination environmenta round the sulfur atom with Si-S-Si angles between 1078 and 1118 (Table 1, Figure 1, cf. 1088 in T-S-T), [55] which is also supported by the sum of all Si-S-Sia ngles with 329.08 and 326.68,r espectively.T he SiÀSb ond lengths of both salts (ranging between 2.24-2.31 ,a verage 2.256 and 2.251 ) are in good agreement with those observed in T-S-T (2.152(2) )a nd TÀS À (2.05-2.07, [56,57] cf. Sr cov (SiÀS) = 2.19 ). [58] Interestingly,there are two slightly different SiÀSbondl engths, which, according to computations, can be attributed to small cation-anion interactions (see below,T able1and S42).  Figure S1). Therefore, ar efinement without any restraints was not possible. Another indication for the [12,34] is that also the occupation of toluene, whichi s always presenti nc onjunction with the oxoniumc ation, correlates with the occupation of the [T-F-T] + ion. So if the slightly larger [T-F-T] + ion is included, one of the SiMe 3 groups is approximately located at the toluene position, leaving no accessible void for the toluene molecule ( Figure S1). Also, the [T 2 (Me 2 (H)Si)O] + ion is strongly disordered. [T-mO-SiMe 2 ] 2 [CHB 11 Cl 11 ] 2 :C ompound [T-mO-SiMe 2 ] 2 [CHB 11 Cl 11 ] 2 (6O) crystallizes in the triclinic space group P " 1a st oluene solvate. While the toluenei sc oordinatedb yt he one Ha tom of the [CHB 11 Cl 11 ] À in a h 6 manner,t he closest SiÀCl distances amountst o3 .9958(6) indicative for aw eak van der Waals type interaction (Sr vdW (SiÀCl) = 3.85 ). [59] Them olecular structure of the centrosymmetric cyclic cation (Figure 1b (6) )and al arge Si-O-Si angle (142.2(3)8). [60] Thermodynamic and kinetic considerations of the chalconium ion formation depending on the counterion  4 ]. This different reaction behavior led to am ore detailed investigation of this problem by quantum mechanicalc alculations at the PBE1PBE/aug-cc-pVDZl evel of theory including dispersion correction. [61] As depicted in Scheme 3, there are two ways how the [Me 2 (H)Si] + ion can be produced, which is necessary to form the [T 2 (Me 2 (H)Si)E] + ion in the reactionw ith T-E-T.A sa strongL ewis acid, naked T + reacts with any possible neutral donor, which is the reason, why always bridged adducts such as [T-H-T] + (7)a nd [T-Me-Si(H)Me 2 ] + (8;F igure 2) are formed in exergonic reactions with free Me 3 SiH in almost barrier-free reactions (Table 2, Scheme 3, equilibria A, C,a nd D). Also conceivable would be the formation of T + ·toluene adducts as startingm aterials, which is not shown in Scheme 3f or clarity but the Gibbs energies are also listed in Table 2. Thesed ata clearly suggest that toluene adduct formation plays an essential role in the equilibrium chemistry of silyliumi on reactions. Startingf rom [T·toluene] + ions the formation of 7 as wella s9 are true equilibria with Gibbs energiesc lose to zero (À0.35 À8.56 kcal mol À1 )or[T·toluene] + ions (C). Besides, also an intramolecular process (B)v ia a4 -membered cyclic transition state, which is associated with ab arrier of 23.8 kcal mol À1 ,w as found for the generation of 8.T hermodynamically,t he formation of 7,a sw ell as 9,i sf avored over 8 by 12.8 and 8.6 kcal mol À1 ,r espectively.H owever,t hese resultsd on ot explain the difference in the reaction behavior of T-E-T with 1 affording either 2Eo r 3Edepending on the chalcogen (Scheme1). To understand the difference in product formation, one has to look more closely at the thermodynamic data of the formation of [T 3 E] + and [T 2 (Me 2 (H)Si)E] + (Scheme 4, Table 2). As expected, all reactions startingf rom the naked, toluene-and the Me 3 SiH-coordinated cations are exergonic [ Table 2, Eq. (4)- (7) 7): À14.53 kcal mol À1 ], in accord with our experimental observations. Additionally,t he formation of either 2So r3O, respectively,a nd its precipitation as [B(C 6 F 5 ) 4 ] À salt superimposes the described exchange equilibria (Schemes 3, 4) leadingt oa new equilibrium adjustment. The different thermodynamic stability of 2Ev ersus 3Ef or the chalcogens oxygen and sulfur is because of as ignificantly strongerP auli repulsion in 2O. This leads to ap reference for species 3Ow ith reduced repulsion due to the smaller substituent (H versus Me, vide infra). For the significantly larger sulfur atom, this plays as ubordinate role.
Formally,t he transformation of aM e 3 Si group to aM e 2 (H)Si group represents as ilylium ion catalyzed methyl/hydrogen exchange reactiont hat hasb een experimentally observed before in the reactiono fN   were also generated by this catalytic process in accord with our computation (Scheme 3, Ta ble 1, equilibria E and F). As displayed in Scheme3,b oth formation reactions of 9 are exergonic with À31.2 and À19.3 kcal mol À1 in the gas phase (0.0 and 4.2 kcal mol À1 in toluene, Ta ble S8, S9), theoretically manifesting the thermodynamically possible formationo f9 in this dynamic equilibriumc hemistry.S imilar exchange reactions have been observedb yM üller and co-workers [21,62] and Oestreich et al., [63] who described substituent scrambling in the formation of the arene silylium cation [(Mes) 3 4 Si in benzene or toluene indicating substituent redistribution, too. [65] Similar H-silane activation mechanisms by B(C 6 F 5 ) 3 have been reported in literature. [66,67] [CHB 11 Cl 11 ] À À anion:[ T-mE-SiMe 2 ] 2 2 + + versus[T 3

E] + + salt formation
To understand the different reaction channels when carborate anions such as [CHB 11 Cl 11 ] À were utilized as counterion, we need to have ac loser look first at the reaction in the gas phase. First of all, dication 6E 2 + is the dimerization product of 5E + ,w hich can only be formed from 10E + by the release of Me 4 Si (see Figure 3a nd Scheme6 top). In contrast to [T 3 E] + , 10E + ,w hichf eatures no tri-coordinated silyliumi on but a bridging methyl group, is thermodynamically much less favored for both oxygen and sulfur compared to [T 3 E] + (O:1 8.1, S: 29.3 kcal mol À1 ). Starting from T-E-T and T + ,b oth [T 3 E] + and 10E + can be formed in an exergonicprocess without any barrier to overcome. However, when [T 3 E] + is formed it needs to be transformed into 10E + in an endergonicr eactiona nd also the reactiont ot he monocation 5E + as wella st he dimerization affording 6E 2 + are all endergonic.S ince the reaction of T-E-T and T + was carried out in toluene, the reaction profile was also computed utilizingt he corresponding toluenea dducts. In this case the formation of the dication 6E 2 + is still an endergonic process,w hilef or both chalcogens the formation of the [T 3 E] + ion represents the thermodynamically favored reactionb ut the process for the formation of [T 3 O] + is less exergonic compared to [T 3 S] + (O: À13.3 vs. S: À22.7 kcal mol À1 ), which is also the case for the nakedi on reaction (À35.1 vs. À44.5 kcal mol À1 ). As depicted in Figures 4a nd 5, the situation changes significantly, when the whole process is computed utilizing ion pairs with [CHB 11 Cl 11 ] À as counterion (Scheme 6b ottom). Since the cat-ions can be attached to differentp ositions at the carborate anion, [54] many isomers for each class of intermediates were found and activation barriers neededt ob el ocalized for each reactions tep along the reaction path. Moreover,d ifficulties to localize true minima( Figure4)a nd transition states ( Figure 5) arose from the fact that very flat potential energy surfaces were found around the carborate anion, as depicted in the two-dimensional heat map of Figure 6. In the following,o nly the thermodynamically most stable isomers are discussed (for furtheri somers see Supporting Information). The reaction starts with 12Et hat describes the T + /[CHB 11 Cl 11 ] À ion pair along with the weakly bound T-E-T molecule. Both reactants are already very close to each other.T here are also isomers of 12Ew ith much larger distances between T + and T-E-T (see Supporting Information). Now the exergony of the T 3 E[CHB 12 Cl 12 ]( 4E) formation drops strongly forb oth oxygen Scheme5.Lewis acid-catalyzedscrambling process for Me 3 SiH, which occurs,w hen catalytic amountso faLewis acid (LA) are present. [22] Figure 3. Computeds tructures that play an essentialrole in the formation of the monocation 5O + that dimerizes to give the observedd ication 6O 2 + (Figure 1). Selectedstructural data are listed in Ta ble S42. (À2.7 kcal mol À1 )a nd sulfur (À10.7 kcal mol À1 )s peciesb ut is still larger for sulfur.T he most important change, however,i s the factt hat the formationo ft he CB/ dication/ CB ion pair 6E is now thermodynamically favored comparedt o4Ef or oxygen but not for sulfur (O: À5.5v s. S: + 8.5 kcal mol À1 ), in accord with our experimental findings. For this reason, we had a closer look at the reactionp ath along the formation of 6O. Both 4Oa nd 10Oc an be formed directly startingf rom 12O with barriers to overcome of 12.8 (TS3_O) and 19.3 kcal mol À1 (TS4_O), respectively.S ince only 10Oc an decompose affording the monocation stabilized as ion-pair 13O, we also computed the activation barrier (TS5_O) for the transformation of 4Ot o 10Ow hich amounts to 24.8 kcal mol À1 .O nce 10O( with ap reformed Me 4 Si molecule and ab ridging methyl group) is formed, it can easily split one bridging SiÀCb ond affording the monocation and Me 4 Si (13O) in an exergonic process with an activation energy of 11.5 kcal mol À1 (TS6_O). However,t he Me 4 Si is still weakly coordinated to the ion pair.F inally,t he release of Me 4 Si leads to 5O, which can dimerize to give 6Oi n an exergonic process. Therefore, the whole process might be regarded as an anion-mediated transformation.

Structure and Bonding
[T-H-T] + + and [T-Me-Si(H)Me 2 ] + + :E ven thoughw eh ave not isolated the bridgingc ationss hown in Figure 2, butd iscussed them mechanistically (see the chapter on the ligand scrambling), it is worthwhile to take ac loser look at af ew structural and bond theory issues. C 2 -symmetric [T-H-T] + features two elongated SiÀHb onds (1.637, cf. Sr cov (SiÀH) = 1.48 ) [58] and a rather large Si-H-Si angle (146.68). Due to the formal hydrogen coordination both Si centers are not planar,e xhibiting an averaged Si-C-Sia ngle of 1168 (S]Si = 348.28). NBO (natural bond orbital) [68] analysis localizes a2 -electron-3-center bond along the Si-H-Sim oiety,w hich, however,i sm ainly located at the bridging Ha tom (67 %) but only with 16.5 %a te ach Si atom, in accord with computed relativelyl arge negative net charge of À0.31 e for the bridging hydrogen atom and MO considerations ( Figure S49, S50, MO = molecular orbital). It should be noted that the hydrideb ridged [Et 3 Si-H-SiEt 3 ] + ion has been reported by Reed and Nava (vide supra). [44] NRT (natural resonance theory) [69][70][71] describes the bondingw ithin [T-H-T] + as a resonance between T-HT + $T + H-T$T + H À T + .W hile the first two formulae are by far the most important ones, Lewis formulae like the last one with ah ydride H À sandwiched between two T + are at least present in the resonance with aw eighto f roughly5 %i ndicating an on-negligible hydride character for the ionic hydrogen bridge.
The [T-Me-Si(H)Me 2 ] + ion (8)s hows also ab ridging bond, however,amethyl group bridging two Si centers in an asymmetricf ashion since both Si centersa re differently substituted ( Figure 2). Hence as horter and as lightly longerS i ÀC bridge distance are observed (2.013 vs. 2.140 ,c f. Sr cov (SiÀC) = 1.91 ), [58] with the shorter bond length to the Si(H)Me 2 group. The SiÀCÀSi unit is slightly bent (177.88)a nd both Si centers, as well as the bridging methyl group, are slightly pyramidal-  [4,6,7] Table S41). [72,73] Of course, also for sulfur,a sc an be seen from the structurald ata, increases the angle sum around the Sa tom with an increasing number of T groups ([H 3 S] + :2 84, [TH 2 S] + 293, [T 2 HS] + 311, and [T 3 S] + 3298), which, however,i ss till far away from 3608 that would indicate ap lanar species. In accordance with these findings, the lone pair located at the chalcogen atom hasl arger s-character (smaller p-character,T able S34) with an increasingn umber of H substituents, which even increases from oxygen to sulfur in accordancew ith Bent's rule. [74,75] Obviously,t he larger s-character of the heavierc halcogen atom sulfur favors the pyramidal structure. As seen by the donor-acceptor energies, the delocalization energies (due to hyperconjugation)i ncrease with the number of Ts ubstituents and in all considered chalconium ions (except [H 3 S] + ), the chalcogen atom Ei sa lwaysn egatively chargedb ut as expected oxygen is more negative than sulfur. With an increasing number of Tg roups, the partial chargea t the chalcogen atoms becomes considerably more negative (cf.  (Table S34). Hence, within the concept of trimethylsilylium (T + )b eing al arge proton( H + ), it should be noted that besides the larger steric strain, which is introduced upon substitution of Hb yT , [16,76] also al arger charge transfer needs to be considered as wella st he factt hat Si is more electropositive than hydrogen with all the implications according to Bent's rule.
To study the sterici nfluence on the pyramidalization within the [T 3 E] + cations, we first computed the potential energy profile DE tot as af unction of the Si-E-Si angle (between 90 and 1208)i ne xact C 3 symmetry (Figure 7, Ta ble 3, Ta ble 4). Both speciesd on ot adopt exact C 3 symmetry in its lowest-lying      [1] this reason,w eh aved efined the exact C 3 symmetry geometry with 1208 angles of both species to be the reference for the computation of relative energy contributions (Table 3, Ta ble 4 and S38, S39). In accord with experiment, thesec omputations revealed that for [T 3 O] + the optimized structure (geom1)i sf avored over the exact planarr eference state by À0.63 kcal mol À1 ,b ut the trigonal pyramidal geometry (geom2,w ith S](Si-O-Si) = 109.98)i sl ess favored by 11.07 kcal mol À1 ,w hich even further increases the smaller the Si-O-Sia ngles becomes (> 71 kcal mol À1 for 908). This situation changes for [T 3 S] + , which shows am uch flatter potentialw ith as hallow minimum at 109.98 (geom2)t hat lies À5.36 kcal mol À1 below the reference geometry,r epresenting at rue minimum (no imaginary frequencies). Further pyramidalization to ](Si-S-Si) = 908 significantly increases the relative energy to + 15.55 kcal mol À1 .
To determine the origin of the different minimum structures (almostp lanar [T 3 O] + vs. trigonal pyramidal [T 3 S] + ), we performed NBO analyses along the energy profile for the corresponding differentS i-E-Si angles. Natural sterica nalysisa si mplementedi nt he NBO6 program [77] expresses steric exchange repulsion as the energy differenced ue to orbital orthogonalization. [78] The absolutev alues, as well as the relative values, increase with decreasing Si-E-Sia ngle, however,t he sterics train with respect to the reference geometry was calculated to be considerably largerf or the sulfur species( cf. S: geom1:2 .4/ geom2:3 7.0 vs. O: 0.8/ 27.4 kcal mol À1 ,T able 3, Ta ble 4). Obvi-ously,t here must be as econd effect that overcompensates the increased steric strain in the pyramidal sulfur geometry.F or this reason, we looked at delocalization effectsu sing standard NBO deletion techniques.
For oxygen, the localized E(L) value favors the planar coordination over the pyramidal (geom2)b y5 4.2, while the delocalized contribution, E(NL) with À43.2 kcal mol À1 is in favor of the pyramidal structure, which, however,d oes not compensate the localization contribution.T he stability of the planar structure can, therefore, be attributed to the electronicl ocalization energy E(L). Thisp icturec learly changes for the sulfur species for which both contributions, E(L) and E(NL), favor the pyramidal geometry by À2.6 and À2.7 kcal mol À1 .M oreover,b oth energy values are much smallerc ompared to those of the oxygen species( Ta ble 3, Ta ble 4). As expected the differences between the reference geometry and geom1 are much less pronounced for both chalcogen species.
Interestingly,t he delocalization of the lone pairl ocated at the chalcogen atom is the main contributor to the delocalization effect (hyperconjugation), which, however, doesn ot much change upon decreasing the Si-O-Sia ngle from 120 to 1098 (ca. 30 kcal mol À1 ). Hence, hyperconjugation due to lone pair (LP) delocalization is not the main reasonf or the energetically favoredp lanar arrangemento ft he Si 3 Os keleton in [T 3 O] + but the decreaseds teric repulsion and the favourable localization energy.I nc ase of [T 3 S] + for the 1208 reference species, also a value of 30.5 kcal mol À1 was found for the delocalization of the LP,w hich means the hyperconjugative effect is as large as for [T 3 O] + ,b ut this delocalization effect considerably decrease upon pyramidalization by ca. 6.5 kcal mol À1 .B esides wave function-based methods( e.g.,a si nN BO anaylsis)t os tudy the steric influence within am olecule, there are density functional theory( DFT) based methods, which are completely different in their approacha nd may even lead to qualitatively differentr esults as those found by wave function-based methods. In 2007 Shubin Liu introduced an interesting DFT based approach for a new energy decomposition analysis (SLA = Shubin Liu analysis [80] as implemented by Tian Lu in MULTIWFN, [81] Table 3, Ta ble 4) that can be used to study steric effects as shown by his group in as eries of papers. [80,[82][83][84][85][86][87] According to SLA, the total energy density functionali se xpressed as the sum of steric,e lectrostatic and quantum effects that represent independente nergy contributions: [1]. According to this expression, Liu could demonstrate that the steric effect has to do with the energetic contribution from the minimal space upheldb ya toms in molecules with all other effects (such as electrostatica nd quantum) totally excluded.A ccording to this definition, the steric contribution E s [1]has nothing to do with the Pauli repulsion, since the Pauli energy [79] is included in E q [1], the fermionic quantum energy,w hich includesb oth the potentiala nd kinetic contributions due to the exchange-correlationi nteractions in as ystem.A pplingS LA for [T 3 O] + and [T 3 S] + with ap lanar and trigonal pyramidal Si 3 Eg eometry (  ture of all considered [T 3 E] + geometries is always as tructure with three highly polarized SiÀEb onds (for degree of polarization see Ta ble 3, Ta ble 4) and one lone pair located on the chalcogen in ap ure p-type atomico rbital( > 99 %pc haracter in the planar case). Lewis formula D includes between 55-61 % weight, while structures with an eutralT -E-T and af ormalT + fragment possess less than 12 %( E). Therefore, all these species are indeed formal chalconium ions of the type[ T 3 E] + rather than T-E-T stabilized silylium ions:T 2 E!T + .L ewis representationso ft ype F describe delocalization effects of the lone pair (e.g.,i nto s*(SiÀC) orbitals, vide infra) and amount to 3 (E = S) À5% (E = O).
[T-mE-SiMe 2 E] 2 2 + + :I na greement with the X-ray data, the [T-mO-SiMe 2 O] 2 2 + ion is characterized by ac entrosymmetric planar4membered Si 2 O 2 ring featuring two tricoordinated oxonium atoms in ap lanar environment( see Figure 1, Ta ble 1a nd S42), while the terminal Tg roups are slightly bent out of the ring plane (8.48). Withint he ring, both SiÀOb ond lengths are slightly shorterc ompared to both terminal distances (1.770 vs. 1.859 ). Similar structural features are found for the sulfur species, however,t he bending out of the Si 2 O 2 ring plane of the terminal Tg roups is much strongerp ronounced ( 59.68). Hence, again the oxygen is an almostt rigonal environment( S](Si-O-Si) = 359.98), whiles ulfur prefers ap yramidal arrangement (S](Si-S-Si) = 325.08). The difference in the SiÀEb ond lengths within the ring compared to the terminal distances is beste x-plained by as trong hyperconjugative effecto ft he lone pairs localized at both tricoordinated chalcogen atoms in a p-type atomic orbital as indicated by NBO investigations. Within the ring system,t his delocalization effect [LP(E)!s*(Si-C)] is much stronger compared to that with the terminal SiÀCb onds (O: 35.0 vs. 10.3 and S: 17.8 vs. 7.2 kcal mol À1 )a nd introduces even partial SiÀOd ouble bond character.H owever,t his typeo fh yperconjugation is the main contribution to the overall delocalization effect that is associated with the two chalcogen lone pairs (O:6 5.0 and S: 47.6 kcal mol À1 ). According to NBO analysis, aL ewis representation with three highly polarized SiÀO bondsi sf avored( Scheme9,f ormula G). However,t here are also smaller hyperconjugative effects, which can be associated with Lewis representations such as H and I.I ti st herefore not completely out of place to bring ap ossible donor-acceptor adduct notation (formula J,acyclo-disila-dichalcotane doubly silylated) into play,a lthough Lewis formula G is certainly the best description in the picture of localized bondingo rbitals. [49] It should be noted that there are av ariety of computational studies on oxonium and sulfonium species in literature. [49,66,[72][73][74][87][88][89][90][91][92] Ion-pairs:A sd iscussed before, ion-pair formation stabilizes all silyliumi ons mentioned here. In particular,t he highly reactive T + ion in [T]CB is strongly stabilized as can be seen from the short Si-  [49] which is in the range of at ypical polarized SiÀCl single bond (cf. Sr cov (Si-Cl) = 2.17 and Sr vdW (Si···Cl) = 3.85 ), [59,94] and the rather large charget ransfer to the T + group (0.40 e,T able 5). With respect to the charge Scheme8.Lewis representationso f[T 3 E] + .