Protonation and deprotonation effects on the chemistry of the third-row elements: Homolytic versus heterolytic cleavage

Ab initio MO calculations indicate that the effect of protonation of third-row X (X = Ge, As, Se, Br) in CH3XHn, C2H5XHn, C2H3XHn, and C2HXHn is similar to that of first- and second-row X; specifically, both the CX homolytic bond dissociation energies (BDEs) and (except for As and the ethynyl compounds) the CX bond lengths (BLs) increase. Deprotonation decreases the CX BDE for saturated compounds, an electronegativity effect, but increases it for unsaturated ones (except Ge), a resonance effect; correspondingly, the CX BLs increase in saturated and decrease in unsaturated compounds (except Ge). Heterolytic CX dissociation of third-row RCXHn+1+ to RC+ and XHn+1 is often favored over the homolytic process when XHn+1 is electronegative relative to the hydrocarbon moiety (XHn+1 = AsH3, SeH2, BrH); the corresponding dissociation of RCXHn-1- to RC- and XHn-1 similarly may be favored for RC = ethynyl and Xn-1 low in electronegativity (XHn-1 GeH2-, AsH-, Se-), The CC BDEs are also affected by protonation of X; protonation increases the CC BDEs and usually shortens the CC bond, while deprotonation does the opposite.