DECAMETHYLPENTASILACYCLOHEPTYNE CENTER-DOT MO-2(CO)(4)(ETA(5)-C5H5)(2) AND CYCLOHEPTYNE CENTER-DOT MO-2(CO)(4)(ETA(5)-C5H5)(2)

A dimolybdenum derivative of decamethylpentasilacycloheptyne (1) was s ynthesized by direct reaction of the heptyne with Mo-2(CO)(4)(eta(5)-C 5H5)(2). 1 crystallized in the space group P (1) over bar, a = 9.386(2 ), b = 9.866(3), c = 20.178(4) Angstrom, alpha = 92.17(2), beta = 97.1 7(2), gamma = 115.71(2)degrees. The acetylenic bond is lengthened from 1.213 Angstrom in the free ligand to 1.359(4) Angstrom and all the Si -Si bond lengths in 1 are significantly lengthened upon complexation. This is due to relaxation of the ring strain as evidenced by the Si-C- C bond angles in 1 of 132.7 and 140.9 degrees compared to 159.2 and 16 2.6 degrees in the uncomplexed ring. Si-29-NMR data exhibit significan t downfield chemical shifts upon complexation for the Si atoms adjacen t to the triple bond, with moderate upheld shifts for the other Si ato ms. The related cycloheptyne.Mo-2(CO)(4)(eta(5)-C5H5)(2) (2) was synth esized by the reaction of cyclohepteno-1,2,3-selenadiazole with Mo-2(C O)(4)(eta(5)-C5H5)(2). 2 crystallized in the space group C2(1)/c, a = 30.396(10), b = 8.9093(3), c = 16.156(4) Angstrom, beta = 115.39(2)deg rees. The acetylenic bond in 2 is 1.345 Angstrom compared with a calcu lated value (ab initio 3-21 G) of 1.190 Angstrom for the free cyclohe ptyne. (C) 1998 Elsevier Science S.A. All rights reserved.