The mono-alkynes Me 3 SiC≡CSiMe 2 (CH 2 CH=CH 2 ) 7 and (CH 2 =CHCH 2 )Me 2 Si-C≡CSiMe 2 (CH 2 CH=CH 2 ) 8, and the di-alkyne (CH 2 =CHCH 2 )Me 2 SiC≡C-C≡CSiMe 2 (CH 2 CH=CH 2 ) 9, have been prepared and treated with Co 2 (CO) 8 and Mo 2 (CO) 4 Cp 2 , respectively, to give the corresponding dimetalla-alkyne tetrahedral complexes. Two of the clusters have been structurally characterised: [Cp 2 Mo 2 (CO) 4 ]Me 3 SiC≡CSiMe 2 (CH 2 CH=CH 2 ), 11, crystallised in the space group C2/c with a = 15.058(3) Å, b = 12.474(2) Å, c = 29.128(6) Å, β = 100.12(3)°, V = 5386(2) Å 3 , Z = 8. [Co 2 (CO) 6 ] 2 ((CH 2 =CHCH 2 )Me 2 SiC≡C-C≡CSiMe 2 (CH 2 CH=CH 2 )), 13, crystallised in the space group C2/c with a = 12.604(3) Å, b = 15.447(3) Å, c = 18.382(4) Å, β = 107.20(3)°, V = 3418(12) Å 3 , Z = 4. Treatment of each of these clusters with HBF 4 yielded the corresponding fluorosilane with concomitant formation of propene. It was shown that for [Co 2 (CO) 6 ] (CH 2 =CHCH 2 )Me 2 SiC≡C-C≡CSiMe 2 (CH 2 CH=CH 2 ) 15, reaction with only one equivalent of HBF 4 yields the fluorosilane with loss of propene at the non-complexed alkynyl terminus. The relevance of these results to the possible intermediacy of a metal-stabilized silylium ion is discussed.