1,1,2,2-Tetramethyl-1,2-disilacycloocta-3,7-diyne - structure and bonding properties of a highly strained cyclic diyne

1,1,2,2-Tetramethyl-1,2-disilacycloocta-3,7-diyne (4) could be prepared by reaction of 1,8-dibromo-4,4,5,5-tetramethyl-4,5-disilaocta-2,6-diyne (9) wi th lithium in the presence of catalytic amounts of biphenyl. X-Ray investig ations on single crystals of 4 revealed an almost planar structure adopting C-2 nu symmetry The transannular distances between the pairs of carbon ato ms in the triple bonds are 2.672 and 3.153 Angstrom, respectively. The anal ysis of the He(I) photoelectron spectrum of 4 reveals a strong interaction between the in-plane pi-orbitals of the triple bonds and the Si-Si sigma-bo nd.