The tight-binding approximation using the CNDO/2 method has been extended to inorganic polymeric chains containing 4th -row elements and the CNDO/2 parameters have been evaluated. This method is used to estimate nuclear quadrupole coupling constants at the halogen and antimony sites in I_3Cl^+_2SbCl^-_6. The NQR parameters obtained from orbital populations have been compared with experimental data. The calculated orientations of the principal axes of the field gradients at the halogen sites are in complete agreement with those determined by Zeeman analysis of NQR, whereas for the coupling constants the quantitative agreement is only fair.