Abstract A series of well accessible cationic ruthenium allenylidene complexes of the general type [(η 6-arene)(R 3 P) RuCl (= C [DOUBLE BOND] C [DOUBLE BOND] CR′ 2)]+ X− is described which constitute a new class of pre-catalysts for ring closing olefin metathesis reactions (RCM) and provide an unprecedented example for the involvement of metal allenylidenes in catalysis. They effect the cyclization of various functionalized ...