It is shown that tubular structures arise in the solid state through close chalcogen-chalcogen (X...X) contacts. As examples a variety of cyclic systems containing sulfur and selenium centers is presented. Common to all of them are close contacts between the chalcogen centers of neighboring stacks giving rise to a zigzag or ladder-type arrangement. In the case of cyclic systems of ring size 24-33 the resulting tubes are able to include hydrocarbons as guest molecules. For 2,7-ditelluraocta-3,5-diyne (17) the close contacts between the tellurium centers of the acyclic C(2) symmetric building blocks generate a helical arrangement in which n-hexane was included. The favored X...X contacts can be traced back to np(X)-sigma*(X[bond]C) interactions.

Close contacts between chalcogen atoms in cyclic dienes and diynes are used to create columnar structures in the solid state. Rigid and fairly planar cycles with chalcogen centers are the preconditions for forming those columnar structures.

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The bis(hexacarbonyldicobalt) complexes of nine ten-to fourteen-membered cyclic diynes (1-3, 5, 6, 8 and 10-12) have been isolated. The structural parameters of the complexes have been examined by means of X-ray investigations on single crystals. It was found that the chair like conformation of the uncomplexed rings is changed into a twisted one upon complexation. In addition to the pure hydrocarbons the syntheses of the bis(hexacarbonyldicobalt) complexes of eight heterosubstituted cyclic diynes (4, 7, 9, 13, 15-17) and of a mono(hexacarbonyldicobalt) complex 18 are reported and their structural parameters are presented. DALTON

The bis(hexacarbonyldicobalt) complexes of cyclic diynes with sila-, thia-and selena-substituted acetylene units have been isolated. The structural features of these complexes have been elucidated by means of X-ray diffraction analyses on single crystals. In three cases (24Ј, 25Ј and 29Ј) the X-ray investigations show bis(pentacarbonyldicobalt) complexes instead of bis(hexacarbonyldicobalt) ones. Their empty coor-