The molecular geometry and vibrational frequencies of phthalazine (=Ph) in the ground state have been calculated by using density functional methods (B3LYP and BLYP and B3PW91) with 6-31G (d) as the basis set. The optimized geometric parameters obtained by using DFT/6-31G (d) basis set show the best agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of phthalazine and calculated results by density functional methods indicates that these methods give good results for molecular vibrational problems. Complexes of the type Zn (ph)(2) Cl-2, Zn (ph)(2) Br-2, Cd (ph) Br-2 and Co (ph) Cl-2 have been studied 4000-400 cm(-1) region and assignment of all the observed bands were made. The analysis of the infrared spectra indicates that there are some structure-spectra correlations. (c) 2004 Published by Elsevier B.V.