New Carbaalanes Containing Clusters of Aluminum and Carbon Atoms:  (AlEt)₈(CCH₂C₆H₅)₅H, (AlMe)₈(CCH₂C₆H₅)₅(C⋮C-C₆H₅), and (AlMe)₇(CCH₂CH₃)₄H₂. Quantum Chemical Characterization

The open carbaalane cluster (AlEt) 7 (C CHC 6 H 5 ) 2 (CCH 2 C 6 H 5 ) 3 H ( 2 ), which was obtained in our group only recently, reacted with an excess of diethylaluminum hydride with hydroalumination of its C C double bonds and formation of the compound (AlEt) 8 (CCH 2 C 6 H 5 ) 5 H ( 4 ). The central cluster of the product comprises a cube of eight Al atoms, five faces of which are occupied by C -benzyl groups; the sixth face is bridged by a hydrogen atom. The carbaalane (AlMe) 8 (CCH 2 C 6 H 5 ) 5 (C⋮CC 6 H 5 ) ( 5 ) was obtained by hydroalumination of Me 2 AlC⋮CC 6 H 5 by Me 2 AlH at 60 °C. Its structure is similar to that of 3, but the bridging hydrido ligand has been replaced by an ethynyl group, which is coordinated to four Al atoms and shows a quite unusual C⋮C stretching vibration of 1956 cm - 1 . The first carbaalane possessing only aliphatic substituents, (AlMe) 7 (CCH 2 CH 3 ) 4 H 2 ( 6 ), was obtained by the reaction of Me 2 AlC⋮CCH 3 with Me 2 AlH. Its structure contains an Al 7 C 4 cluster, which is quite similar to that of the closo -borate [B 11 H 11 ] 2 - . An intense emission of a cluster vibration characteristic for carbaalanes was assigned at about 325 cm - 1 in the Raman spectra. The structure and bonding of arachno -type cluster compounds related to 4 were investigated by quantum chemical calculations. A delocalized multicenter bonding situation was verified with only weak direct Al−Al interactions.