RUTHENIUM CARBONYL 1,4-DIAZA-1,3-BUTADIENE (R-DAB) REACTION SEQUENCE .6. REVERSIBLE CO ADDITION TO A TRINUCLEAR SPECIES WITHOUT RUPTURE OF A METAL-METAL BOND - MOLECULAR-STRUCTURE OF [1,4-DICYCLOHEXYL-1,4-DIAZA-1,3-BUTADIENE]NONACARBONYLTRIRUTHENIUM, RU3(CO)9(C-HX-DAB) - A UNIQUE 50E TRINUCLEAR COMPOUND WITH 2 ELONGATED RU-RU BONDSL