The DFT PBE0/SDD method was used to calculate bond lengths and bond indices in As20, Ni12As20, [As@Ni12As20]3–, As@C60 and As@C70 clusters. The degrees of oxidation and reduction of the endo-atom and the shell are expressed in terms of the populations of one-electron states localized in these components of the complexes. Each As atom in clusters has a entirely localized lone electron pair. The arsenic atom inside fullerenes retains the electronic configuration and spin of the ground state of the free As atom. Inside the [Ni12As20]6– shell, it has an oxidation state of 3+. There is no covalent bond between the endo-atom and the shell in clusters. The bond indices refute the opinion about the “onion” structure of [As@Ni12@As20]3–: the nickel atoms are not bonded to each other, the As-As bond indices are three times lower than in As20.