Das, Pankaj, et al. “Synthesis, Spectroscopic, Thermogravimetric and Electrochemical Characterization of Triphenylphosphineoxide Complexes of the Type [MClx (Ph3PO)6-X] (M = Cu(II), Co(II), X = 2; M = Fe(III), X = 3)”. Asian Journal of Chemistry, vol. 20, no. 8, Mar. 2010, pp. 5958-66, https://asianpubs.org/index.php/ajchem/article/view/13049.