This invention provides metal complexes being useful as catalyst components in metathesis reactions and in reactions involving the transfer of an atom or group to an ethylenically or acetylenically unsaturated compound or another reactive substrate and, with respect to a sub-class thereof, for the polymerisation of .alpha.-olefins and optionally conjugated dienes, with high activity at moderate tempera-tures. It also provides methods for obtaining polymers with very narrow molecular weight distribution by means of a living reaction. It also provides methods for making said metal complexes and novel intermediates involved in such methods. It further provides derivatives of said metal complexes which are suitable for covalent bonding to a carrier, the product of such covalent bonding being useful as a supported catalyst for heterogeneous catalytic reactions. It also provides a direct one-step synthesis of pyrrole, furan and thiophene compounds from diallyl compounds.