Decamethylpentasilacycloheptyne·Mo2(CO)4(η5-C5H5)2 and cycloheptyne·Mo2(CO)4(η5-C5H5)2

A dimolybdenum derivative of decamethylpentasilacycloheptyne (1) was synthesized by direct reaction of the heptyne with Mo2(CO)4(η5-C5H5)2. 1 crystallized in the space group P 1 , a=9.386(2), b=9.866(3), c=20.178(4) A, α=92.17(2), β=97.17(2), γ=115.71(2)°. The acetylenic bond is lengthened from 1.213 A in the free ligand to 1.359(4) A 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° compared to 159.2 and 162.6° in the uncomplexed ring. 29Si-NMR data exhibit significant downfield chemical shifts upon complexation for the Si atoms adjacent to the triple bond, with moderate upfield shifts for the other Si atoms. The related cycloheptyne·Mo2(CO)4(η5-C5H5)2 (2) was synthesized by the reaction of cyclohepteno-1,2,3-selenadiazole with Mo2(CO)4(η5-C5H5)2. 2 crystallized in the space group C21/c, a=30.396(10), b=8.9093(3), c=16.156(4) A, β=115.39(2)°. The acetylenic bond in 2 is 1.345 A, compared with a calculated value (ab initio 3-21 G*) of 1.190 A for the free cycloheptyne.