Reversible, stepwise addition of 1,2-diester-substituted alkenes to Ru2[(CH3)2PCH2P(CH3)2]2(CO)5. Crystal and molecular structure of the diruthenacyclobutane complex Ru2[(CH3)2PCH2P(CH3)2]2(CO)4[.mu.-.eta.1:.eta.1-C2(H)2(CO2CH3)2].cntdot.C7H8