RuH2(H-2)(2)(PCy3)(2) (1) and RuH(o-C6H4C(O)Me)(H-2)(PCy3)(2) (2) or, 1 and RuH(o-C6H4C(O)Ph)(H-2)(PCy3)(2) (3) were shown to be efficient catalysts for the ethylene coupling reaction with acetophenone (7a) or benzophenone (9), respectively. This efficiency under such mild conditions is attributed to the facile generation of two vacancies on the ruthenium centre. Such an hypothesis is confirmed by the fact that the corresponding carbonyl complexes RuH(o-C6H4C(O)Me)(CO)(PCy3)(2) (4) and RuH(o-C6H4C(O)Ph)(CO)(PCy3)(2) (5) were found completely inactive for the Murai coupling under the same conditions. Furthermore, we postulate that the bis(chelate) complex Ru(C6H4C(O)CH3)(2)(PCy3)(2) (6), which is the resting state of the catalyst, is responsible for the deactivation of our catalytic system. (C) 2003 Elsevier B.V. All rights reserved.