Cationic closo carboranes - Promising weakly coordinating ions

The unexplored carbon rich cationic closo carboranes, C3Bn-3Hn+1 (n = 5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calcu lated at the B3LYP/6-31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring-cap orbital overlap co mpatibility along with the number of B-C, C-C, and B-B bonds help in explai ning the stability order in each category. The most stable isomer is the on e with maximum ring-cap orbital overlap and largest number of B-C bonds. Th e order of relative stability among the trigonal bipyramid is 1c > 1b > 1a' , where the stability is proportional to the number of CH caps over the sma ll three-membered ring. The C3B3H6+ isomer with the one allyl C3 group (2b) is more favorable than the one with a cyclopropenyl group (2a). Among the C3B4H7+ isomers the stability order is 3e > 3d > 3c > 3b > 3a, which mostly depends on the ring-cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of r ing cap compatibility and the number of B-C bonds. The order of 5e > 5d > 5 c > 5b > 5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1)-(5) devised for estimating the stability of isomers Of C3Bn-3Hn+ indicate an increase in stability with cage size. T he mono-positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations. (C) 2001 John Wiley & Sons, Inc.