A compound of the formula II (I) in which X is substituted pyrrolide with the general structure of (II) in which R.sup.a-R.sup.d are independently selected from H, C1-C4 alkyl, C1-C4 alkoxy, aryl, aryloxy, dialkylamino, diarylamino, halogen, trifluoromethyl, cyano, nitro, sulfonyl and sulfinyl. Y is C1-C6 alkoxy, C1-C10 aryloxy, optionally substituted; R.sup.1 is selected from H, C1-C12 alkyl and 5- to 18-membered aryl, optionally substituted; R.sup.2 is selected from C1-C12-alkyl, 5- to 18-membered aryl, optionally substituted; R.sup.3 is selected from C1-C12 alkyl, 5- to 18-membered aryl, optionally substituted; and 15 12.sup.4-R11 are independently selected from H, C1-C4 alkyl and halogen. The compounds are particularly effective precursors of metathesis catalysts for the polymerisation of dicyclopentadiene.

##STR00001##