THE STRUCTURE AND STABILITY OF SIMPLE TRI -IODIDES by ANTHONY JOHN THOMPSON FINNEY B.Sc.(Hons.) submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy UNIVERSITY OF TASMANIA HOBART OCTOBER, 1973 . r " • f (i) Frontispiece (reproduced as Plate 6 - 1, Chapter 1) - two views of a large single crystal of KI 3 .H20. The dimensions of this specimen were approximately 3.0 cm x 1.5 cm x 0.5 cm. • - - . ;or • - This thesis contains no material which has been accepted for the award of any other degree or diploma in any University, and to the best of my knowledge and belief, this thesis contains no copy or paraphrase of material previously published or written by another person, except where reference is made in the text of this thesis. Anthony John Finney Contents page Abstract (iv) Acknowledgements (vii) Chapter 1 - The Structure and Stability of Simple 1 Tri-iodides Chapter 2 - The Theoretical Basis of X-Ray Structural 32 Analysis Chapter 3 - The Crystallographic Program Suite 50 Chapter 4 - The Refinement of the Structure of NH I 94 4 3 Chapter 5 - The Solution of the Structure of RbI 115 3 Chapter 6 - The Solution of the Structure of KI 3 .1120 135 Chapter 7 Discussion of the Inter-relation of 201 Structure and Stability Bibliography 255 Appendix A - Programming Details 267 Appendix B - Density Determinations 286 Appendix C - Derivation of the Unit Cell Constants of 292 KI .H 0 3 2 Appendix D - I -3 force constant Calculation 299 Appendix E - Publications 311 ( iv) THE STRUCTURE AND STABILITY OF SIMPLE TRI-IODIDES Abstract In this work the simple tri-iodides are regarded as being those in which the crystal lattice contains only cations, tri-iodide anions and possibly solvate molecules. The alkali metal tri-iodides CsI 3 and KI 3 .H20 exemplify compounds of this type; polyiodides like the (CH3)4NI5 H ) I in which it is penta-iodide or the hepta-iodide (C 2 5 4 7' ' known that the anionic group is made up of I -3 units accompanied by iodine molecules, are not encompassed by this study. In the solid state the simple tri-iodides exhibit a wide range of stability, and the probable causes underlying this range in behaviour are reviewed against existing knowledge concerning the crystalline structure and current models of the anion bonding of these compounds. The presence of solvate molecules in the lattice has a profound effect upon the stability of the simple tri-iodides; also among the unsolvated species there is a range in stability which can be attributed to differences in the electrostatic environment experienced by the anion. This sensitivity to electrostatic environment is reflected in small differences in the geometric configuration of the anion among the unsolvated tri-iodides as revealed by crystal structure analysis. As the number of accurately known simple tri-iodide structures is was refined and the structure of RbI small, the structure of NH4I3 3 and the hydrated species, KI 3 .H20, were determined by the X-ray cryst- • allographic method. A preliminary to this work, made necessary by the unusual features . of the only available computing facilities, was the development of a complete integrated suite of crystallographic programs. Using the single crystal X-ray diffraction technique in conjunction with - group this program suite the accuracy with which the geometry of the 1 3 (v ) in is known was significantly improved. The structural analysis NH4I3 of RbI 3 proved that this compound was isostructural with CsI 3 , as had been anticipated on the basis of the known isomorphism of these two tri-iodides. In the case of KI 3 .H20, the structure differs from that of the unsolvated simple tri-iodides (which all crystallize with unit cells having centrosymmetric orthorhombic symmetry) in that this compound possesses a noncentrosymmetric monoclinic cell. However, both K1 •H 0 and the unsolvated tri-iodides share the same planar 3 2 structural motif defined by the relative arrangement of cations and anions in the cell. On the basis of the extended body of structural data made available by the examination of these compounds, it was shown that the stability of the simple tri-iodides can be qualitatively and quantitatively related to two effects. The first of these is the stabilizing influence of electronic interactions between neighbouring 1 groups 3 within the planar structure motif. The second is the destabilizing influence of any asymmetry in the electrostatic field experienced by the terminal atoms of the anion. The stability of the unsolvated species is determined by the balance struck between these two effects. It is suggested that in the case of the solvated simple tri-iodides, the presence of solvate molecules leads to a reduction in the de- stabilizing effect due to crystal field asymmetry, and that for this class of compounds their stability is largely conditioned by the strength of cation-solvate and solvate-solvate interactions. The crystal chemistry of this class of compounds can be system- atized by regarding the common simple tri-iodide structural motif as a variant of the structure of solid iodine on the one hand, and the penta- iodide structure on the other. Further, for those very stable tri- iodides which are solvated by organic molecules of moderate size so that (vi) the charged species interpenetrate a continuous hydrogen-bonded organic substructure, the dominance of their behaviour by the solvate-solvate interaction appears to be accompanied by the severe modification or loss of the structural motif. (vii) Acknowledgements It is a pleasure to acknowledge my indebtedness to my supervisor, Dr. G.H. Cheesman; firstly, for introducing me to the problems of polyhalogen chemistry, and secondly for his sustained encouragement, advice and valued criticism throughout the extended period occupied by this work. Thanks are also due to my colleague, Mr. W.F. Donovan, for many useful discussions concerning the development of the crystallographic program suite in particular, and the art of X-ray structural analysis in general. At the same time I would like to acknowledge helpful discussions held with past and present members of the staff of the Chemistry Department of the University of Tasmania, and also to thank Professor H. Bloom for making it possible for this project to be undertaken in this Department. I am also indebted to the staff of the Hydro-University Computing Centre; in particular to its Director, Mr. J. Boothroyd, for his deep interest in the special problems of crystallographic computer programming. In this connection I would like to acknowledge the hospitality and assistance given to me by Dr. B. Gatehouse and the members of the crystallographic group at the Department of Chemistry, Monash University. These acknowledgements would be incomplete without reference to the help given at various stages during this project by the members of the workshop staff of the Chemistry Department. I would also like to record my indebtedness to Mrs. H. Hen for her assistance with the preparation of the graphic material included in this work, and to Mrs. B. Dix who has cheerfully undertaken the demanding task of typing the text. Lastly, I gratefully acknowledge the understanding support given throughout this project by my wife and children; this work is • respectfully dedicated to them in recognition of a display of tolerance beyond the call of family duty. A.J. Finney, October 1973. 1 Chapter 1 - The Structure and Stability of Simple Tri-iodides page 1 - 1 Introduction and Historical Perspective 2 1 - 2 Polyiodide Stability 1 - 3 Stability and Solvation 8 1 - 4 Polyiodide Structural Data 15 1 - 4 - 1 CsI 15 3 1 - 4 - 2 NH I 15 4 3 1 - 4 - 3 (C H ) NI 15 2 5 4 3 1 - 4 - 4 (C H ) AsI 16 6 5 4 3 1 - 4 - 5 (C H ) FeI 17 5 5 2 3 1 - 4 - 6 CsI 17 4 1 - 4 - 7 (CH ) NI 18 3 4 5 1 - 4 - 8 (C H ) NI 18 2 5 4 7 1 - 4 - 9 (CH ) NI 20 3 4 9 1 - 4 - 10 K I .6(CH CONHCH ) 22 2 4 3 3 1 - 4 11 HI .2(C H CONH ) 22 3 6 5 2 1 - 4 - 12 HI .2(C H 0 N) 23 5 10 13 2 1 - 5 Polyiodide Bonding in relation to Stability and 24 Structure 1 - 6 The Aim of this Work 28 1 - 1 Introduction and Historical Perspective Observations which suggested that halogen or interhalogen molecules would interact with halides to form a complex species were made from the beginnings of modern chemistry. In 1814 Gay-Lussac made the following comment - "Att 06 the hydADiodates (iodides in modern terminology) have The ptopeAty o6 dissotving iodine abundantty, thekeby becoming At/tong/4 cotouted teddizh-btown. But they onty netain the iodine with yeAy ieebte 6o)tce; 6o/t. they giye it up on boiting ot evaponation in ait; moteovet, the iodine doeo not change the neuttatity o6 the hydtoiodates (iodides) and the teddi.sh-bitown cotomation o the tiquid, simitak to that o6 othet 401 .&0n4 06 iodine, a new p.'wo o the Oebteness o the combination' (Gay-Lussac [18141). In 1819 Pelletier and Caventou [1819] described a new compound, which they named 'hydroiodure iodure', formed by the addition of iodine to strychnine. This compound was in all probability strychnine polyiodide, C 2 02302N2 I 3 . Their work, published in Annates de chimie et de physique, has some significance as it is the first report of a polyhalogen complex in the literature. The first account of a polyhalogen complex of an inorganic cation was published in 1839 in a series of four papers by Filhol [1839a, 1839b, 1839c, 1839d] in which he described the preparation and reactions of the compound potassium tetrachloroiodate, KIC1 4 .