The crystal and electronic structures of Ti5Sb2X (X=Al, Ga, In, Si, Ge, Sn) ternary compounds in the Nb5Sn2Si-type structure (ternary-D8(m)) have been investigated by means of first principles calculations. The calculated structural parameters are in good agreement with the experimental data. The total electronic densities of states as well as the Bader charges of the atoms have been computed. Both electronic and size effects allow to explain the stability of the Ti5Sb2X (X=Al, Ga, In, Si, Ge, Sn) ternary compounds in the ternary D8(m) structure. The enthalpies of formation of the ternary D8(m) compounds have been obtained. The off-stoichiometry domains of D8(m)-Ti5Sb2Ga and D8(m)-Ti5Sb2Si are discussed