Alkene analogues of bis(N-donor)ketone ligands: ruthenium(II) complexes containing 1-(pyridin-2-yl)-1-(N-methylimidazol-2-yl)ethene and 1,1-bis(N-methylimidazol-2-yl)ethene ((mim)2CCH2), and structural studies of [Ru(bpy)2{(mim)2CCH2}][PF6]2 (bpy=2,2′-bipyridine) and the pyridine-2-carboxylate (pyCO2−) complex [Ru(bpy)2(pyCO2)][PF6]