To form cluster-assembled materials, the clusters should have low reactivity and be characterized by a closed-shell electronic configuration with a large gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO). Using spin-polarized density functional theory calculations, we investigate the M-substituted Ti8C12 metallocarbohedrynes to search for less reactive clusters as building blocks for cluster-assembled materials (M = Be, Mg, Ca, Sr, Ba and Sc, Y). The selected atoms in the correct stoichiometry would produce a metallocarbohedryne that is isoelectronic with the Ti8C122+, which has a closed-shell electronic configuration and an enhanced HOMO-LUMO gap of 1.735 eV. According to our results, the HOMO-LUMO gaps of the M-substituted Ti8C12 metallocarbohedrynes are in the range of 0.715-0.979 eV for the case of Be, Mg, Ca, Sr and Ba and in the range of 0.865-1.294 eV for the case of Sc and Y. Among all the M-substituted metallocarbohedrynes we consider here, one of the isomers of Ti6Sc2C12 is not only energetically more favorable but also exhibits a larger HOMO-LUMO gap of 1.294 eV. This result indicates that the Ti6Sc2C12(4) metallocarbohedryne should be less reactive than the Ti8C12 metallocarbohedryne which has a narrow HOMO-LUMO gap of 0.146 eV. Moreover, we show that the intercluster interaction between two individual Ti6Sc2C12(4) metallocarbohedrynes is relatively weak compared to the Ti8C12 dimer.