This dissertation presents an overview of synthesis and chemical reactivity of a novel diruthenium complex (eta5-C5H 3)2(SiMe2)2Ru2(CO) 4 (1) and its derivatives. Complex 1H +, [(eta5-C5H3)2(SiMe 2)2Ru2(CO)4(mu-H)]+, with a protonated Ru-Ru bond, is deprotonated only very slowly (hours or days) by basic amines and phosphines although the proton in 1H + is acidic thermodynamically. This remarkable kinetic inertness of the bridging proton allows primary and secondary amines to react with 1H+ by attacking the CO ligand to give a formamide and the CO-substituted ruthenium complex (eta5-C5H 3)2(SiMe2)2Ru2(NHR 1R2)(CO)3. We have demonstrated that a CO ligand in the protonated complex (eta5-C5H3) 2(SiMe2)2Ru2(CO)4(mu-H)] + (1H+-) is the site of reaction with amines because of the high electrophilicity of the CO ligands and the low kinetic acidity of the bridging hydride---a unique combination of kinetic properties that are not found in the Fe and Os analogs. On the other hand, I-, RS- and phosphines add at one of the Ru centers in 1H+, resulting in cleavage of the Ru-H-Ru bond. The final type of addition to 1H + is that exhibited by MeO- and F- , which results in cleavage of Si-C(cyclopentadienyl) bonds. Except for the reaction with F-, all of these types of reactions depend on the presence of the proton on the Ru-Ru bond. The unprotonated (eta 5-C5H3)2(SiMe2)2 Ru2(CO)4 (1) undergoes no reactions with these nucleophiles (except F-) under the mild room temperature conditions of these studies. The protonated diruthenium alkene complexes (eta5-C5H3)2 (SiMe2)2Ru2(CO)3(eta 2-CH2=CH-R)(mu-H)]+ react with a variety of nucleophiles to give hydrofunctionalized alkenes with predominantly Markovnikov regioselectivity. The dinuclear ruthenium complex (eta5-C 5H3)2(SiMe2)2Ru2(CO) 4 (1) is a precursor for the synthesis of a variety of new dinuclear ruthenium complexes, in which the bridging (eta5-C 5H3)2(SiMe2)2 ligand controls the geometry of the final product. Thus, complex 1 reacts with diphenylacetylene to form a series of the unusual ruthenacarbocyclic complexes as major products.