KELKAR, V.D., et al. “Chemical Investigation of Mixed Ligand Complexes of Co(II), Ni(II), Cu(II) With Malonic Acid and Pyridine”. Asian Journal of Chemistry, vol. 19, no. 5, Mar. 2010, pp. 3597-00, https://asianpubs.org/index.php/ajchem/article/view/13252.