Facile Syntheses, Isomeric Structures, and Chemical Reactivity of Diruthenium(I) Complexes [Ru2(CO)4(MeCN)4L2]X2 (L = MeCN, PPh3, PPh2(allyl), PMe3; X- = PF6-, BF4-). A Convenient Route to 1,2-Diamides and 1,2-Dithiolates