The boron-atom insertion reaction of nido-9,11-I-2-7,8-C2B9H92-, with the HBCl2:SMe2 complex yields closo-4,7-I(2)1,2-C2B10H10, 1, in excellent yield. Although the two boron atoms (133 and 136) nearest to the carbon atoms in 1 are equally available for attack by nucleophiles, the boron-degradation reaction of 1 with alkoxide ion occurs only at the B6 vertex, yielding regioselectively [(CH3)(3)NH][nido-2,4-12-7,8-C2B9H10], 2. The molecular structures of 1 and 2 have been determined by X-ray diffraction studies. Crystallographic data are as follows. For 1, monoclinic, space group P2(1)/n, a = 6.9199(19) angstrom, b = 23.9560(7) angstrom, c = 7.2870(2) angstrom, beta = 94.081(4)degrees, V = 1204.9(6) angstrom(3), Z = 4, rho(calcd) = 2.18 g cm(-3), R = 0.020, R-w = 0.0610; for 2, orthorhombic, space group Pca2(l), a = 14.1141(7) angstrom, b 7.0276(4) angstrom, c = 16.4602(g) angstrom(3), V = 1632.7(15) angstrom(3), Z = 4, rho(calcd) = 1.81 gcm(-3), R = 0.022, R-w = 0.0623.