Geometries and energetics of the borocenium cation, [BCP2](+), and the decamethylborocenium cation, [BCP*(2)](+), have been studied by using density functional theory (DFT) and ONIOM methods. Possible structures with different hapticities were considered for [BCP2](+)' at the B3LYP/6-31+G(d) level, and for [BCP*(2)](+) at the ONIOM(B3LYP/6-31+G(d):HF/STO-3G) level. In the ground-state structure of [BCP*(2)](+) one ring is monohapto and one ring is pentahapto (eta (1)/eta (5)), in good agreement with experiment. The energy difference between the eta (1)/eta (5) (A) and eta (5)/eta (5) (1) structure of [BCp2](+) is 54.5 kcal/mol at the B3LYP/6-311 +G(2d,p)// B3LYP/6-31+G(d)+ZPC level, which is in contrast to the small energy difference (similar to2 kcal/mol) found between the eta (1)/eta (5) and eta (5)/eta (5) structures in the isoelectronic Cp2Be system. The ring exchange process for the [BCP2](+) cation has an activation energy barrier of 14.7 kcal/mol where the hapticity of one ring gradually decreases (eta (5) --> eta (2)(TS) --> eta (2) --> eta (1)(TS) --> eta (1)) and the hapticity for the other ring gradually increases (eta (1) --> eta (1)(TS)--> eta (2) --> eta (2)(TS) --> eta (5)). GIAO/B3LYP/6-31+G(d,p) calculations for the [BCP*(2)](+) B-11 and C-13 chemical shifts are in excellent agreement with experiment. The decamethyl-substituted Cp donates more electron density to boron compared to Cp resulting in a 5.4 ppm downfield B-11 chemical shift for [BCp*(2)](+) relative to [BCP2](+).