The reaction of cyclodiborazane [Me2N-BH2](2) with the chloro(dihydrogen) ruthenium complex RuHCl(eta(2)-H-2)((PPr3)-Pr-i)(2) (1) led to the formation of the unsymmetricaly coordinated dimethylaminoborane complex RuHCl(H2BNMe2)((PPr3)-Pr-i)(2) (2). The dimethylaminoborane coordination (H2BNMe2) to the ruthenium center in 2 was carefully studied by combining X-ray, multinuclear NMR, and density functional theory (DFT) techniques, and compared with the recently reported osmium analogue which was originally formulated as sigma-B-H borinium complex [OsH2Cl(HBNMe2)((PPr3)-Pr-i)(2)] (4). All our data are in favor of a bis(sigma-B-H) coordination mode at a very activated stage in the case of the ruthenium complex 2, whereas in the osmium complex 4, full oxidative addition is favored leading to a complex better formulated as an osmium(IV) boryl species with an alpha-agostic B-H interaction. The synthesis and characterization of the symmetrical dihydride complex RuH2(H2BNMe2)((PPr3)-Pr-i)(2) (3) from addition of the lithium dimethylaminoborohydride to 1 is reported for comparison.