Because of the great biological significance of the truns-diaquabis ( f ) hydrogen malato and trance transition metal ions [l, 21, the title compound has been studied. In this work, the complex of Ni[OOC-C,H&OOH],
. 4H20 was synthesized from ( f )malic acid and nickel carbonate. In this system, the ( f )malic acid is transformed into truns-maleic acid as the solution is heated. It then coordinates with the Ni(I1) ion forming the title compound. The electronic absorption spectrum of the product was determined at room temperature in the 6000-30,OOOcm-' region. The photoacoustic spectra of this complex as powder and crystal were also recorded in the UV-vis. region. Recently, we have suggested a scaling radial theory and proposed double-c radial wave functions for complexed transition metal ions [3-51. Therefore, the EA and PA spectra of the title compound are explained quantitatively and theoretically with this ligand field model. The crystal field parameters and the d-d transition energy levels of the complex were calculated and are in good agreement with the experimental values. The ligand field properties of the complexes [Ni(HrO)$+ and [Pb -Ni(HEDTA)(H,O)]Cl are also discussed. In addition, the sensitivity of EA and PA spectroscopy and fluorescent properties of the title complex are also discussed.