A series of hydroborated eta(4)-sigma,pi-alkene-borane complexes have been synthesized from the reaction of ruthenium-bis(sigma)borate complex [Cp*Ru(mu-H)(2)BH(S-CH=S)] (1) and terminal as well as internal alkynes. Likewise, the reactions of manganese-bis(sigma)borate complexes [Mn(CO)(3)(mu-H)(2)BHNCSC6H4(NL)] (L = NCSC6H4 or H) were explored with terminal alkynes that yielded boratabutadiene complexes [Mn(CO)(3){(NCSC6H4)(2)N}-{((RMeC)-Me-1)=B(HC=CHR1)}] [R-1 = phenyl (4a) or p-tolyl (4b)] via triple hydroboration of alkynes. These complexes feature a boratabutadiene ligand that is coordinated to a metal through the eta(4)-CBCC mode. To the best of our knowledge, these are the first examples of eta(4)-E-boratabutadiene-coordinated manganese complexes generated by the trans-hydroboration of alkynes. The steric and electronic effects of the borate ligands have been demonstrated using a less sterically hindered manganese-bis(sigma)borate complex, [Mn(CO)(3)(mu-H)(2)BH(HN2 CSC6H4)], that generated monohydroborated complexes [(CO)(3)Mn(mu-H)(2)(HN2CSC6H4)B((RC)-C-1=CHR2)] (for 6, R-1 = Ph and R-2 = H; for 7, R-1 = p-Tol and R-2 = H; for 8, R-1 = R-2 = Ph). Theoretical studies using density functional theory methods and chemical bonding analyses established the bonding and stability of these species.