A new complex of ruthenium bearing the unsaturated N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and an ionic alkylidene moiety was synthesized and fully characterized. The X-ray structure of this catalyst and that of its SIMes (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) analogue were determined. The catalytic performances of these catalysts and their recyclability were investigated for olefin cross-metathesis in a [Bmim][PF₆]/organic solvent biphasic medium. The results show the importance of the proper choice of organic solvent. The solvent effect/catalytic behavior could be explained in terms of clathrate formation.