Ruthenium carbonyl 1,4-diaza-1,3-butadiene (R-DAB) complexes.5. Syntheses, spectroscopic properties and reactivity of Ru2(CO)5(alkyl-DAB), a key intermediate in the Ru3(CO)12-alkyl-DAB reaction. Crystal and molecular structure of (1,4-diisopropyl-1,4-diaza-1,3-butadiene)pentacarbonyldiruthenium, Ru2(CO)5-(i-Pr-DAB)