Using the first-principle approaches, we predict a B-6(BO)(7)(-) cluster with a face-capping mu(3)-BO, which is the boron oxide analogue of closo-B6H7- with a face-capping mu(3)-H. Detailed topological analysis of electron density clearly reveals the existence of three rhombic 4c-2e bonds around the B/H apex in both C-3v, B-6(BO)(7)(-) and C-3v B6H7-, which possesses similar electron densities at their bond and ring critical points. The adaptive natural density partitioning (AdNDP) analysis provides a direct and visual picture of the B-B-B-B/H 4c-2e bonds for the first time. Adiabatic and vertical electron detachment energies of the concerned monoanions are calculated to facilitate their future photoelectron spectroscopy measurements and characterizations. The presence of the B-6(BO)(7)(-) and B6H7- clusters extends the BO/H isolobal analogy to the whole mu(n)-BO/H series (n = 1, 2, and 3) and enriches the chemistry of boronyl.