Fluoromethyl Cations and Group XIV Congeners AHnF3-n+ (A = Si, Ge, Sn, Pb; n = 0-2): from Covalent Structures to Ion-Molecule Complexes

Ab initio calculations at the MP2 and CCSD(T) level of theory (triple-zeta quality basis sets for Si and Ge and effective core potentials plus DZ valence basis sets for tin and lead) have been performed to investigate the geometries, harmonic frequenciesand relative stabilities of the (Si,H,F)+, (Ge,H,F)+, (Si,F3)+, (Ge,F3)+ and (A,Hn,F3 – n)+ (A = Si, Ge, Sn, Pb; n = 1 and 2) isomeric ions. While the H–Si–F+, F–SiH2+, H–SiF2+ and SiF3+ covalent structures are invariably predicted as themost stable isomers, the Ge+–(HF), FA+–(H2) and FA+–(HF) complexes (A = Ge, Sn, Pb) are the global minima on the potential energy surfaces, more stable than the corresponding covalent structures by up to some tens of kilocalories per mole. For tin and lead, the HA+–(HF) isomeric ions are also significantly more stable than F–AH2+ (A = Sn and Pb). Compared with the parent group XIV hydrides (A,H3)+, investigated so far by Schleyer and co-workers (J. Am. Chem. Soc. 1996, 118, 12154–12158) and found to possess, for A = Sn and Pb, energetically favoured HA+–(H2) connectivities, the structural switch from covalent structures to ion-dipole complexes occurs, therefore, even earlier. This suggests that the “regular” covalent connectivity of even the simplest AH2X+ or AHX2+ cations of group XIV cannot be taken for granted, and, especially for germanium, tin and lead, the role of ionmolecule complexes must be carefully investigated.