STRUCTURAL DISTORTIONS IN MAIN-GROUP METALLACARBORANES

The group 13 and 14 metallacarboranes in the pentagonal bipyramidal sy stem show a consistent pattern of structural distortions. The metal is not symmetrically bonded to the C2B3 face of the carborane but is sli pped, along its pseudo-mirror plane. In metallacarboranes where the ca ge carbons are adjacent, slippage is always towards the unique boron a tom in the mirror plane of the bonding face, while in complexes where the cage carbons are separated by a boron atom, slippage is in the opp osite direction. Such slippage is accompanied by a folding of the C2B3 bonding face. The structural determination of -(C8H6N4)(t-CMe(3))Ga-2 ,4-(SiMe(3))(2)-2,4-C2B4H4, described herein, along with the previousl y reported structures of the ''carbons adjacent'' metallacarboranes, s how that the direction of ring folding depends on the magnitude and di rection of metal slippage. This relationship is investigated theoretic ally using semiempirical molecular orbital calculations on the model c omplexes, 1-Sn-2,3-C2B4H6 and 1-Sn-2,4-C2B4H6.