The ternary boron compounds TM7TM'B-6(8) (TM = Ta, Nb; TM' = Ru, Rh, Ir) were prepared by high-temperature thermal treatment of mixtures of the elements. An analysis of the chemical bonding by the electron density/electron localizability approach reveals formation of covalently bonded polyanions [B-6] and [TM'B-6(2)]. The cationic part of the structure contains separated TM cations. In agreement with the chemical bonding analysis and band structure calculations, all TM7TM'B-6(8) compounds are metallic pauli-paramagnets (TM' = Ru, Rh) or diamagnets (TM' = Ir).