The M-X distances, the charges on the M and X atoms and the natural electron configurations in the hypothetical MX 4 -molecules, where M is a group 13 element and X = F, Cl, Br, I, have been investigated by quantum calculations. The computed distances are compared with the experimental data, mostly in the Bu 4 N[MX 4 ] salts. We found that: (1) down group 13 both the experimental and calculated M-X distances, the charge on the M atom and the sum of the populations of the M valence orbitals show zigzag variations produced by the filled 3d and 4f shells and by the relativistic effects in the 6s and 6p 1/2 orbitals; (2) the experimental distances are shorter than the calculated values due, at least in part, to libration of the MX 4 -anions in the salts; (3) both the
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