Octa-and nona-hydridosiliconium di-and tri-cations (SiH 8 2+ and SiH 9 3+ ) containing eight-and nine-coordinate silicon atoms

Structures of octahydrido silconium dication (SiH 82+ ) and nonahydrido silconium trication (SiH 93+ ) were found to be calculationally viable minima at the MP2/cc-pVTZ level and CCSD(T)/cc-pVTZ levels. Their structure has three and four two-electron three-center (2e-3c) bonds, respectively. The protonation of SiH 7+ to form the dication was found to be slightly endothermic by 4.1 kcal/mol at the CCSD(T)/cc-PVTZ//CCSD(T)/cc-PVTZ + ZPE level. Further protonation to form the trication was found to be highly endothermic by 162.2 kcal/mol. The deprotonation barriers of the ions were also computed

Cao et al. 13 reported the first spectroscopic observation of SiH7 + .Their IR data suggest that SiH7 + is a symmetric complex (H2 … SiH3 + … H2) with two two-electron three-center (2e-3c) bonds and with two twoelectron two-center (2e-2c) bonds.This is in contrast to the species CH7 + , which has been concluded, from both IR spectrum 14 and calculations, 15 to have a structure consisting of a H2 subunit weakly bound to one of the hydrogen atoms of the 2e-3c bond (CH 5 + .H 2 ).Hu et al. 16 have also reported the calculated structures and Infrared spectrum of parent heptacoordiate siliconium ion, SiH7 + .A similar structure has also been reported for GaH 7 + . 17In continuation of our study of hypercoordinate compounds, we have now extended our theoretical investigations to the next higher homologues of SiH7 + i.e.SiH8 2+ and SiH9 3+ ions at the MP2/cc-pVTZ and CCSD(T)/cc-pVTZ levels.

Results and Discussion
Structures of 1 and 2 were optimized in the gas phase at the MP2/cc-pVTZ and CCSD(T)/cc-PVTZ levels.CCSD(T)/cc-PVTZ level structures are discussed throughout unless otherwise stated.Structure 1 was found to be a viable minimum (Figure 1) on the potential energy surface (PES) of SiH8 2+ at the both MP2/cc-pVTZ and CCSD(T)/cc-PVTZ levels.Computed energies are given in Table 1.Structure 1 contains three 2e-3c bonds involving the silicon atom and thee hydrogen molecules and two 2e-2c bond involving the silicon atom and a hydrogen molecule.The ion can be considered as a complex between SiH4 2+ (protonated silicenium dication) 18 and two hydrogen molecules (Scheme 2).The Si-H bond distance (1.995 Å) of the axial 2e-3c bond units is considerably longer than that of the equatorial unit (1.842 Å).The possible stability of the eight-coordinate SiH8 2+ is due to the fact that the silicon can undergo sp 3 d hybridization.In comparison, the eight-coordinate CH8 2+ ion was found not be a minimum on the PES.The dication dissociated into CH6 2+ and H2 upon optimization.This is because, unlike silicon, carbon is unable to undergo sp 3 d hybridization.Computed vibrational frequencies of the structure 1 are given in Table 2.  Protonation of SiH7 + to form 1 was found to be endothermic by 4.8 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level (4.1 kcal/mol at the CCSD(T)/cc-PVTZ//CCSD(T)/cc-PVTZ + ZPE level).
Transition structure, 3TS (Figure 1) for the deprotonation of 1 was also located.The structure 3TS lies 68.3 kcal/mol higher in energy than structure 1.Thus the trication has also a high barrier for deprotonation.MP2/cc-pVTZ and CCSD(T)/cc-PVTZ optimizations show that the nine-coordinate siliconium structure 2 is also a minimum on the potential energy surface of SiH9 3+ .Tricationic structure 2 (Figure 1) contains four 2e-3c bonds involving the silicon atom and four hydrogen molecules and a 2e-2c bond involving the silicon atom and a hydrogen atom.The Si-H bond distances (1.946 and 1.884 Å) of the axial 2e-3c bond units are slightly longer than those of the equatorial units (1.915 and 1.836 Å).Charge-charge repulsions in the trications are substantial.However, the bonding interactions are strong enough to counter charge-charge repulsions rendering them remarkably stable.Dissociation of 2 into 1 and H + was calculated to be very exothermic by 163.5 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level (162.2 kcal/mol at the CCSD(T)/cc-PVTZ//CCSD(T)/cc-PVTZ + ZPE level).The transition structure 4TS for the dissociation lies just 13.5 kcal/mol higher in energy than structure 2. This shows that the trication 2, if formed will dissociate spontaneously into 1 and H + .Potential energy surface of 1 and 2 calculated MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level is depicted in Figure 2. We also searched for any minimum-energy structures of decacoordinate siliconium ion, SiH10 4+ .At the MP2/cc-pVTZ level no minimum could be found on the PES of SiH10 4+ (including a structure with five 2e-3c bonds as shown in Scheme 3).Thus in SiH10 4+ charge-charge repulsion may have reached its prohibitive limit.Scheme 3. Possible structure of SiH10 4+ .

Conclusions
The present calculational study at the MP2/cc-pVTZ and CCSD(T)/cc-pVTZ levels shows that the octahydridosilconium dication (SiH8 2+ ) 1 and nonahydridosilconium trication (SiH9 3+ ) 2 are viable energy minima.Structures 1 and 2 were found to stabilized by three and four 2e-3c bonds, respectively.The protonation of SiH7 + to form 1 was calculated to be slightly endothermic by about 5 kcal/mol.Charge-charge repulsions in these di-and tri-cations are substantial.However, the bonding interactions are strong enough to counter charge-charge repulsions rendering them remarkably stable.

Experimental Section
Calculations Geometry optimizations and frequency calculations were carried out with the Gaussian 09 program. 19ibrational frequencies at the MP2/cc-pVTZ//MP2/cc-pVTZ level were used to characterize stationary points as minima (NIMAG (number of imaginary frequency) = 0 or transition state NIMAG = 1) and to compute zero point vibrational energies (ZPE), which were scaled by a factor of 0.96. 20CCSD(T)/cc-pVTZ optimizations and frequency calculations calculations have been performed with the CFOUR program. 21,22

Table 2 .
Calculated frequencies a (cm -1 ) and IR intensities (km/mol) of 1 a Computed frequencies were not scaled.