The electric field gradient (EFG) at the chlorine site of the hexachlorometallates A2[MCl6] and A'[MCl6], M = Sn, Te, Pb, Pt, and A⊕, A'2⊕ = alkali metal ion, hexahydrated bivalent metal ion, or organic ammonium cation, is investigated and the dependence of the EFG on the size of the cation is discussed. — For the cubic alkali hexachlorometallates the observed trend in the EFG with increasing size of the cation is corroborated by quantum mechanical calculations using the Multiple-Scattering-Xα-method (MS-Xα) and the point charge model. A correlation v(35Cl) ∼ 1/r3 is found, where r is the distance between the ions in the lattice and the chlorine nucleus considered. For hexachlorometallates with organic cations, like pyridine, piperazine, 4-picoline, ethylenediamine, 1.3-propylenediamine, and other aliphatic or aromatic amines, this 1/r3 dependence is found for the averaged 35Cl NQR frequency, too. This is due to the distortion of the [MCl6]2⊕ octahedra, caused by hydrogen bonds N—H…Cl, and to the decrease of the lattice symmetry because of non-sphericity of the cations. — A hydrogen bond has a large influence on the properties of one of the chlorine atoms of the hexachlorometallate complex. The M-Cl distance is increased and therefore the EFG at the nucleus site is decreased. However, the average of the M-Cl bond lengths or the 35Cl NQR frequencies, taken over all six chlorine atoms of a [MCl6]2⊕ complex, is not influenced by the chemical nature of the cation but only by its size or the size of the unit cell, respectively. The increase of 35Cl NQR frequency with increasing size of the cation originates from a decrease of the lattice energy.