Additions to C-C multiple bonds are simple and the most efficient methods in organic synthesis in terms of atom utilization due to avoidance of stoichiometric amounts of by-products. The following catalytic C-C, C-O and C-N bond-constructions by direct additions of nucleophiles to alkynes catalyzed by ruthenium complexes have been developed. 1) Dimerization of terminal alkynes to 1, 4-disubstituted butatriens which is an unusual tail-to-tail coupled product. 2) The first example of anti-Markovnikov hydration of terminal alkynes to aldehydes. 3) Intermolecular hydroamination of terminal alkynes with anilines to imines. Some of the reactions proceed in high yields under solvent-free, open-air conditions. Theoretical studies were also performed about transformation of η²-coordinated 1-alkynes to vinylidene complexes which plays important role in regioselectivity of the addition reactions.