Preparation of four- and five-membered diruthenacycles and an unusual double insertion of carbon monoxide. Crystal and molecular structures of [cyclic] Ru2(Me2PCH2PMe2)2(CO)3[.mu.-C2(CO2Me)2][C(O)C(O)C2(CO2Me)2] and [cyclic] Ru2(Me2PCH2PMe2)2(CO)4[.mu.-C(O)C2(CO2Me)2]