Novel Dinuclear (μ-Hydrido)(μ-alkylideneamido)ruthenium Complexes, [(η5-C5Me5)Ru]2(μ-L)(μ-H)(μ-NCHR) (R = Me, Et; L = MeCN, Arene), as Precursors of Active Species for Bimetallic Activation. Dehydrogenative Coupling of Ethylene in a Dinuclear Ruthenium Complex