Comparative study of the hypercoordinate carbonium ions and their boron analogs: A challenge for spectroscopists

Theoretical and related experimental studies have shown that carbon can extend its bonding from Kekule's tetravalent methane to five, six or seven coordinated protonated methanes, i.e., CH ₅⁺, CH ₆²⁺ and CH ₇³⁺ and their analogs. Ab initio calculations indicate that higher coordinated, highly charged carbonium ions could be viable species despite the substantial charge-charge repulsion. Structure and bonding of such carbonium ions and the isoelectronic boron analogs were studied and are discussed. These contribute to our general understanding of the electrophilic reactivity of C-H and C-C single bonds.