Gas-Phase Reaction of Tetraborane(10) and Ethyne: Molecular Structure of nido-1,2-C(2)B(3)H(7) in the Gas Phase.

The molecular structure of nido-1,2-C(2)B(3)H(7), 1, the principal volatile carborane generated in the quenched gas-phase reaction of B(4)H(10) and ethyne at 70 degrees C, has been determined by a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio computations at the CCSD(T)/TZP' level. The structure is consistent with a geometry having C(s)() symmetry, similar to that of pentaborane(9). The apical position is occupied by a carbon atom, displaced toward B(4) from a position directly above the B(5).B(3) vector, and hydrogen atoms asymmetrically bridge the B-B bonds. The basal atoms are almost coplanar, C(2) lying ca. 2 degrees below the B(3)-B(4)-B(5) plane. Important experimental structural parameters (r(alpha) degrees /pm, angle(alpha)/ degrees ) are r[C(1)-C(2)] = 162.6(6); r[C(1)-B(3)] = 161.4(3); r[C(2)-B(3)] = 154.3(2); r[C(1)-B(4)] = 157.4(5); r[B(3)-B(4)] = 185.7(3); <B(3)-B(4)-B(5) = 80.9(1). In addition to this and the other previously reported carboranes, 2,3-C(2)B(4)H(8), 2-Me-2,3,4-C(3)B(3)H(6), and 4-Me-2-CB(5)H(8), several new derivatives have been identified among the volatile products. These include the dicarbahexaboranes 2,4-Me(2)-2,3-C(2)B(4)H(6) and 5-Et-2,3-C(2)B(4)H(7) (the major volatile products obtained when the reaction is allowed to go to completion, previously reported as tricarbahexaboranes) and the derivatives 2,5-Me(2)-2,3-C(2)B(4)H(6), 4-Et-2,3-C(2)B(4)H(7), 1-Me-2,3,4-C(3)B(3)H(6), 2-Me-2-CB(5)H(8), 3-Me-2-CB(5)H(8), and 2,3,4,5-C(4)B(2)H(6). The complex mechanism of the reaction is discussed in light of these new results.