Vibrational characteristics of CD3CN solution containing Mg(ClO4)(2) have been studied by means of infrared and Raman spectroscopy. A large blue shift of 40 cm(-1) and a relatively small red shift of 3 cm(-1) of the v(2) C equivalent toN stretch are observed as a result of interactions of CD3CN with Mg2+ and ClO4- ions, respectively. The Mg2+ ion is preferentially solvated by residual water over acetonitrile. The primary coordination number of Mg2+ in acetonitrile is determined as 6 from the Raman intensities of the C dropN stretch for free CD3CN and those coordinated to Mg2+. Evidently, formation of contact ion pairs of Mg2+ and ClO4- is more probable at a high concentration of Mg(ClO4)(2). The IR extinction constants of the v(2) and v(4) stretching bands increase dramatically upon coordination. The vibrational characteristics of free CD3CN remain the same in the solution, indicating that the electrophilicity of Mg2+ affects only the CD3CN molecules directly coordinated to Mg2+. DFT calculations have been performed at the BLYP/6-31+G(2d,p) level to examine the structure and vibrational characteristics of CD3CN coordinated to Mg2+. The calculated results are in good agreement with the observed vibrational characteristics.