Metallo-carbynes grafted on mesoporous fibrous silica are active towards alkane metathesis. We are comparing the activity of well-defined grafted pre-catalysts (≡Si-O-)W(≡CCMe3)(-CH2CMe3)2, (≡Si-O-)W(≡CH)(-CH3)2 and (≡Si-O-)Mo(≡CCMe3)(-CH2CMe3)2 for Ring Opening Metathesis Polymerization of cyclo-octene and metathesis of cyclo-octane. This comparison is because olefins are assumed to be intermediate products in alkane metathesis. The three pre-catalysts are active in cyclo-octane metathesis, and give the higher, and lower cyclic homologues. However, with cyclo-octene the three catalysts give polyoctenamer by classical ROMP. It is assumed that the metallo-carbynes react differently towards cyclo-alkanes and cyclo-olefins. In one case, with cyclo-alkanes they give a metallo-carbene alkyl and in the other case with cyclo-olefins they give a bis-carbene.