REACTIONS OF IRIDIUM AND RHODIUM WITH ETHTLENEDIAMINETETRAACETIC ACID DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By HAROLD DALE McBRIDE, B. A., M. S. The Ohio State University 1958 Approved by 1/Vg-Ar-vq* *" Adviser Department of Chemistry ACKNOWLEDGMENTS The author expresses his sincere gratitude to Dr. William M. MacNevin for his enthusiastic encouragement and wise counsel during the course of this study. Acknowledgment is also made to the Department of Chemistry of the Ohio State University for financial aid in the form of assistantships and to the National Science Foundation for a Fellowship from June, 1957» to the completion of this work. ii DEDICATION Dedicated in memory of iry son, Gary. iii TABLE OF CONTENTS PAGE STATEMENT OF THE PROBLEM .................................... 1 INTRODUCTION............................................... 2 Theory of Coordination Compounds and Conplex Ions ............ 2 Chemistry of EDTA......................................... 7 EDTA, General Information and Properties .................. 7 Preparation of EDTA .............. 10 EDTA as a Chelating A g e n t ................................ 12 Preparation of EDTA Complexes............................. 17 EDTA in Oxidation-Reduction Reactions ................... 21 Analytical Uses of EDTA ............. 23 Chemistry of Platinum Group Metals ................ 26 Oxidation Potentials of Rhodium and I r idium ................ 26 Separations and Analyses of Platinum Group Metals .... 28 BACKGROUND OF THE P R O B L E M .................................. 30 EXPERIMENTAL................................................ 31+ Reactions of Iridium(IV) with E D T A .......................... 31+ Job’s Method Applied to Iridium(IV) and EDTA ............. 31+ Other Preliminary Studies . 35 Attempts to Prepare Iridium(IV)-EDTA Complex ................ 38 Titration of Iridium(IV)-EDTA Product . 1+3 Chromatographic Separation of the Product ................... 1+5 Reactions of Iridium(IV) with Na2H2Y ................... 1+6 iv V PAGE Reduction of Iridium(lV) with EDTA ........ U 8 Postulated Mechanism..................... 56 Additional Evidence for Mechanism ............. $8 D i s c u s s i o n ............................................. 59 Evidence for Rhodium(III)-EDTA Complex . 62 CONCLUSIONS ................................................ 66 BIBLIOGRAPHY................................................ 67 AUTOBIOGRAPHY ............................................. 70 LIST OF TABLES TABLE PAGE I, Directional Characteristics of Covalent Bonds .... 6 II. Solubilities (grams/lOO ml.) of EDTA and Sodium Salts in Water and pH of Aqueous Solutions ...... 9 III. Logarithms of Formation Constants of EDTA Complexes . 1 $ IV. Atomic Ratios of Elements Based on Elemental Analysis of Product . .............. 1*2 V. Atomic Ratios of the Elements Based on Elemental Analysis of Product Prepared from Solutions at pH 3-5 . 1*7 VI. Molar Ratio of Iridium(lV) : EDTA in the Presence of Excess Iridium(IV)................................ 51* vi LIST OF FIGURES FIGURE PAGE 1. Job's Method, for EDTA and Iridium(IV). ...... 36 2. Absorption Curves for I^IrCl^ and H 2lrCl6 Treated with E D T A .............................. 39 3* Absorption Curve for Product Isolated from Reaction of H 2lrCl6 and ED T A ........................ Uli 1*. Absorption Curves for H2lrCl6 and H3lrCl6 Formed by Electrolytic Reduction of H^IrClg ....... 1*9 5. Absorption Curves for IrCl£s Formed by Reduction of ^IrClg with EDTA and Hydroxylamine Hydrochloride . 5l 6. Dependence of Molar Ratio of Reactants on Moles Iridium(IV) : EDTA Reacting . 55 7. Absorption Curves for Rhodium(lIl)-EDTA Complex . 63 vii STATEMENT OF THE PROBLEM In 19$k Kriege'*' reported spectrophotometrie, complexometric, and ■*■0. H. Kriege, Ph.D. dissertation, Ohio State University (195>U). hydrolytic evidence for the formation of a complex of tetravalent irid­ ium with ethylenediaminetetraacetic acid in chloride solution. He found the complex to have the unusual composition of two moles of iridium to one mole of EDTA if EDTA was in large excess in the reaction mixture. There was also evidence for some 3sl complex when EDTA was not in large excess. Further work on these complexes, such as solution studies on the formulas of the complexes, their preparation, and determination of their stability constants, is indicated. Kriege found no evidence for a complex of trivalent rhodium with EDTA in chloride solution. If the chloride complex is too stable for the EDTA complex to form in chloride solution, it can be expected to form in the absence of chloride. It is proposed to prepare the EDTA complex by starting with rhodium in the form of its sulfate, since sul­ fate forms less stable complexes than chloride. 1 INTRODUCTION Theory of Coordination Compounds and Complex Ions In general, a coordination compound may be defined as the addition product of the combination of an electron donor and an electron accep­ tor, each capable of independent existence. The electron acceptor or the donor may be an atom, an ion, or a molecule. Compounds resulting from the combination are referred to as complex confounds or coordina­ tion confounds, and the resulting ions are complex ions. However, the above definition, which is based on the formation of a coordinate bond according to the theory of Sidgwick2 and Lowry,^ includes only a part 2N. V. Sidgwick, J. Chem. Soc., 123, 72$ (1923). •^T. M. Lowry, J. Soc. Chem. Ind., U2, 316 (1923). of the types generally included as complexes. Although chemists do not agree on a simple definition of complexes, the addition compounds includ­ ed as complexes have a wide range of bond types from electronic to cova­ lent, and they possess a wide variety of properties. Complexes were classified by Biltz^ according to stability. He ^W. Biltz, Z. anorg. Chem., l6U, 3U$ (1927). called those that undergo reversible dissociation normal complexes, and those that exhibit little or no reversible dissociation he called pene­ tration complexes. Whereas the term normal complex is associated mainly 2 3 with complexes whose bonding is of a loose nature possessing a high degree of ionic character* penetration complexes are those whose bond­ ing is essentially covalent* The classification is one of convenience rather than strict division* The modern theory of complexes had its beginning in 1893 when Alfred Werner^ published his theory explaining how it is possible for 5a . Werner, Z_. anorg* Chem,* 3 , 267 (1893)* species capable of existence as entities to combine to form molecular complexes* His theory involved the following ideas* Metals not only have principal, or primary, ionizable valencies, but also auxiliary, or secondary, valencies* The secondary valencies, which are non-ionizable, combine with a maximum fixed number of atoms in a first sphere* This coordination number is usually k or 6 and sometimes 2 or 8* Since they are directional, six secondary bonds would form a regular octahedron and four secondary bonds would form either a plane or a tetrahedron* Therefore, isomerism is possible* Whereas secondary valencies may be satisfied by anions or molecules, primary valencies are satisfied only by anions less firmly attached in a second sphere* In 1923 Sidgwick^ and Lowry? gave an electronic interpretation to V. Sidgwick, J. Chem* Soc.* 123 , 725 (1923). 7T. M. Lowry, J. Soc. Chem* Ind., 1*2, 316 (1923). k the Werner theory* They considered Werner's primary valencies to be the formation of electrovalent bonds by the transfer of electrons* His secondary valencies consisted of the formation of electron-pair bonds by the donation of electrons to the metal atom by the coordinating group. These are coordinate bonds which are indistinguishable from covalent bonds after they are formed, since the source of electrons in the electron-pair bond has no influence on the strength or direction of the bond* The idea of accumulating electrons on an electropositive metal atom required in the fomation of coordinate bonds is an inherent weak­ ness in their theory when it is applied to coordination compounds and complex ions* The hybridization of bond orbitals as set forth by Pauling® explains in a very satisfying way how these strong covalent, ®L. Pauling, "The Nature of the Chemical Bond," 2nd. ed., Cornell University Press, Ithaca, 19k0, Ch. III. directional bonds are fomed* If the three 2p orbitals and the s orbi­ tal of the tetravalent carbon atom were used individually to form bonds, there would be three mutually perpendicular bonds and another weaker one without any particular direction* However according to the quantum- mechanical treatment, if there is a linear combination of the s orbital with the three 2p orbitals, there could be formed four strong bonds of equal strength and directed toward the corners of a regular tetrahedron* This is consistent with the experimental facts* Also, d orbitals may be used in hybrid bonds to give a variety of arrangements according to Table I«? Whereas some of the possible bond types and corresponding % . Moeller, "Inorganic Chemistry," John Wiley & Sons, Inc*, Hew York, N. Y., 1952, p. 203. spatial arrangements are rare among complexes, others are quite common. It is seen, for example, that metals with a coordination number of 6 may form d^sp3 bonds arranged octahedrally about the metal atom* It is important that not all the bonds in complexes are covalent as the theory of hybridization of orbitals