The phase diagram of the quasi-binary system (\mathrm{NaCl}-\mathrm{SnCl}{2}) has been derived from DSC and X-ray investigations. In this system a dystectic (\mathrm{NaSn}{2} \mathrm{Cl}{5}) compound phase was found, which was obtained by grinding mixtures of (\mathrm{NaCl}) and (\mathrm{SnCl}{2}) in a closed ball mill and then subsequent tempering at temperatures about (170^{\circ} \mathrm{C}) in evacuated sealed silica ampoules. The single-crystal structure of (\mathrm{NaSn}{2} \mathrm{Cl}{5}) was determined. It crystallizes with a new orthorhombic-type arrangernent (space group (p n n m ; a=) 818.09(5), (b=1203.1(1), c=871.63(8) \mathrm{pm} ; Z=4) J. The structure was solved by Patterson methods with the MolEN program, and refined to a final (R) value of 0.019 for 717 unique reflections. The sodium ions are located in almost octahedral voids of (\mathrm{Cl}) ions. Two kinds of tin ions manifest themselves structurally by a great distortion of their coordination environment, three (\mathrm{Cl}) ions are situated at (<272.77 \mathrm{pm}) from the tin ion, which is a reasonable (\mathrm{Sn}-\mathrm{Cl}) bond length, but the other five or six are repelled by the lone pair to distances of (>306.75 \mathrm{pm}). Different from the structure of (\mathrm{U}{3} \mathrm{Se}{5}), it is another distorted (\mathrm{Rh}{5} \mathrm{Ge}{3}) structure. In this paper the structural relationship between (\mathrm{NH}{4} \mathrm{~Pb}{2} \mathrm{Cl}{5}) and (\mathrm{NaSn}{2} \mathrm{Cl}_{5}) is elucidated systematically by means of the group theoretical representation of group-subgroup relations. 01995 Academic Pres, Inc.