Aminoboranes, H2BNRR', represent the monomeric building blocks from which novel polymeric materials can be constructed via metal-mediated processes. The fundamental capabilities of these compounds to interact with metal centers have been probed through the coordination of H2BNCy2 at 16-electron [CpRu(PR3)(2)](+) fragments. In contrast to the side-on binding of isoelectronic alkene donors, an alternative mono(sigma-BH) mode of aminoborane ligation is established for H2BNCy2, with binding energies only similar to 8 kcal mol(-1) greater than those for analogous dinitrogen complexes. Variations in ground-state structure and exchange dynamics as a function of the phosphine ancillary ligand set are consistent with chemically significant back-bonding into an orbital of B-H sigma* character.