Volatile Silicon Compounds PDF
Preview Volatile Silicon Compounds
OTHER TITLES IN THE SERIES ON INORGANIC CHEMISTRY Vol. 1. CAGLIOTI (Ed.)—Chemistry of the Co-ordination Compounds Vol. 2. VICKERY—TAé? Chemistry of Yttrium and Scandium Vol. 3. GRADDON—An Introduction to Co-ordination Chemistry VOLATILE SILICON COMPOUNDS by E. A. V. EBSWORTH University Chemical Laboratory Cambridge PERGAMON PRESS OXFORD · LONDON · NEW YORK PARIS 1963 PERGAMON PRESS LTD. Headington Hill Hall, Oxford 4 & 5 Fitzroy Square\ London W.l PERGAMON PRESS INC. 122 East 55th Street, New York 22, NY. GAUTHIER-VILLARS 55 Quai des Grandes-Augustins, Paris 6 PERGAMON PRESS G.m.b.H. Kaiserstrasse 75, Frankfurt am Main Copyright © 1963 PERGAMON PRESS LTD. Library of Congress Card Number 61-18237 Printed in Poland to the order of PV/N—Polish Scientific Publishers by the Scientific-Technical Printing House (DRP), Warsaw PREFACE PREVIOUS accounts of volatile silicon compounds have in the main been written from the organic or the inorganic point of view. Such a division is particularly unfortunate where silicon compounds are concerned, since it leaves compounds like chlorosilane, SiH3Cl, to the inorganic chemist, while trimethylchlorosilane, Me3SiCl, is considered an organic compound· I have tried to present a study of volatile silicon compounds irrespective of whether they contain carbon; anyone who wants a fuller treatment of the organic derivatives is referred to Professor Eaborn's excellent book Organosilicon Compounds. This book is primarily intended for research students, though I hope it may be helpful to undergraduates in their final year. In a subject as ex tensive as this, there are bound to be omissions, many of them inadvertant; the balance of what I have included is a reflection of my own interests. I have combined discussion of molecular structure and chemical pro perties, concentrating on material which has not been reviewed before and summarizing points which are discussed at length elsewhere. The treatment of the compounds of germanium and tin is not intended to be in any way complete; it has been included so that the behaviour of silicon compounds may be considered in the context of the Periodic Table and of the compounds of neighbouring elements. Lead compounds have been perhaps rather arbitrarily excluded, save for some reference to organo- lead hydrides; this is because relatively few lead compounds are strictly analogous to the simple substituted silanes. Some comment should be made about the way in which I have used free energy calculations based only on changes in bond energy in a number of reactions. I know well that such calculations give no absolute measure of the free energy change unless the entropy term is included; none the less, they may be useful in helping to decide why certain reactions do not occur. If, for instance, two compounds might react together in two different ways, each reaction giving rise to the same number of molecules of gaseous product, then the reaction in which the more bond energy is released should be the reaction with the more favourable free energy change. If in practice the other reaction is found to be preferred, the former reaction is likely to have an unfavourable activation energy. 1 2 VOLATILE SILICON COMPOUNDS The use of force constants in studies of molecular structure presents a considerable problem. In principle, the force constant of a bond is as important a property as, say, the bond length; unfortunately, however, the calculation of force constants from the observed vibrational spectra of any but the simplest of molecules is a difficult and complex process, the result is liable to vary with the type of force-field assumed and the precise physical meaning of the parameter obtained is not absolutely clear. On the other hand, there is a clear correlation in simple molecules between bond order and force constant, at least where single, double and triple bonds are compared; moreover, any measurable property which is likely to be of use in discussions of molecular structure is not lightly to be ignored. I have therefore referred to work on force constants even for relatively complicated molecules, though I do not believe that much significance should be attached to small differences. I have only described Siebert's method of predicting the force constants of single bonds, though I know that there are other formulae of this sort; this is not meant to be a monograph on force constants, and I do not think that an elaborate discussion of the different formulae would be justified here. I have tried to avoid using that unfortunate word "stable" without the essential qualification "to (some reagent or set of conditions)". Where the word is used without qualification, it is meant to describe the stability of the compound in question with respect to decomposition, polymeri zation or dissociation given—in other words, with respect to reaction with itself. I have described π-bonds in a way which differs slightly from that most frequently used in published work. I have called an ethylenic π-bond a (p-p)Tc-bond; the π-bond between nitrogen and boron in N-dimethyl- aminoboron dichloride is called a (p-+p) π-bond, the π-bond between nickel and carbon in nickel carbonyl is a (d-*p) π-bond, while that in tri- silylamine is a (p-+d) π-bond. This notation conveys a little more about what the electron-distribution in the bond is believed to be than does the more commonly used method of description. Finally, there is the vexed question of nomenclature. Several systems have been used for naming silicon compounds, and I do not propose to describe them all here; an account of some of them is to be found in Pro fessor Eaborn's book. My main concern has been that the names I have used should leave the reader in no doubt as to the structural formulae of the compounds he is reading about. For the rest, I have preferred names for halogen- and organosubstituted mono- and disilanes based on the PREFACE 3 word "silane"; I have not used this system where pseudohalogen substi- tuents are concerned, because I think the name "tetraisothiocyanatosilane" is much clumsier than "silicon tetraisothiocyanate". I have avoided the "silazane" system for amine-derivatives, preferring to call these "silyl- amines"; similarly, I have called the sulphur and selenium derivatives "sulphides" and "selenides" rather than "thianes" and "selenanes", because I believe that the amine-sulphide-selenide system is easier to un derstand for a reader who is not already familiar with organosilicon chem istry. I have capitulated over the oxygen derivatives, calling these^sil- oxanes rather than ethers. The final chapter is in part a summary of material to be found in the rest of the book; in order to avoid repetition, only a few source-references have been included at the end of that chapter, but the source for any un referenced statement should be easily found in the body of the book. Thanks are due to many of my friends and colleagues, at Cambridge and elsewhere, for help in the writing of this monograph. Dr A. D. Buck ingham, Dr L. E. Orgel, Dr A. G. Sharpe, Dr W Sheppard, Dr T. M. Sugden and Dr J. J. Turner have all given me the benefit of their expert opinions; I am most grateful to Mr J. S. Griffith, Dr K. MacKay, Dr D. C. McKean and Mr S. Frankiss for permission to quote observations or ideas as yet unpublished. Mr Frankiss, Dr A. Hass and Mr M. J. Mays read parts of the manuscript and the proofs, and made a number of helpful comments. Most of all, I am indebted to Dr A. G. Maddock, who has always been ready with advice about any difficulty, and has assisted and encouraged me at every stage. E. A. V. EBSWORTH CHAPTER 1 INTRODUCTION: ATOMIC PROPERTIES BEFORE turning to a discussion of particular compounds, it is as well to consider the atomic properties of silicon and the other Group IV elements in relation to one another. Although these atomic properties, which are often determined from a study of the free atoms or of the elements them selves in their standard states, may be profoundly modified by compound formation, they often provide a surprisingly reliable basis for a discussion of the properties of molecules ; moreover, it is sometimes possible to find an explanation for the differences between apparently analogous compounds of two or more elements in terms of differences in atomic properties. This may be a dangerously speculative process, however, and must be regarded with caution. All the elements in question—carbon, silicon, germanium, tin and lead—have the outer electronic configurations in their ground-states 2 2 of (ns np ). The group valency of four is reached by the formal promotion 2 2 1 z of an s-electron to an empty /^-orbital: (ns np ->ns np ). All these elements form compounds derived from the valency-state of four; a valency of two becomes relatively more stable as the atomic weight of the element increases. Carbon and silicon form no compounds derived from the divalent state that are stable at room temperature, if carbon monoxide is excluded; germanium (II) is rather ill-defined, and several of the compounds of formula GeX2 may contain germanium-germanium bonds*, but tin (II) is well- characterized. This increase in the relative stability of the lower oxidation state with atomic number has been attributed both to increasing stabili zation of the ns- relative to the wp-electrons of the valence-shell with in creasing atomic number, and also to the fact that the heavier elements form (la) weaker covalent bonds . All of the compounds with which this mono graph is directly concerned are at least formally derived from the valency state of four, but the stabilization of the lower state has an important effect upon the hydride chemistry of germanium (IV) and tin (IV). The MH bonds are strongly reducing, and compounds such as trichlorogermane * Though Gel, has the Cdl2 structure/*) 4 INTRODUCTION — ATOMIC PROPERTIES 5 are liable to decompose to give germanium (II) chloride and hydrogen (2) chloride : GeHCl3 = GeCl2+HCl The bonds from a saturated carbon atom are usually regarded as formed by ^-hybr id orbitals, with minor changes in hybridization in unsym- metrically-substituted compounds like chloromethane (though it has been (3) suggested that there is some d- and even /-character in the hybrids) . Carbon, having no more orbitals in its valence-shell, has a co valency- maximum of four, but many compounds containing multiply-bonded carbon are of course well known. The position with silicon and the heavier elements is rather different. These all have «d-(and germanium and tin have nf-) orbitals in their valence-shells; these orbitals could in principle be used in forming bonds, but in the neutral atoms they are very much (4) more diffuse and of higher energy than the ns- and np -orbitals . The σ-bonds from silicon (IV), germanium (IV) and tin (IV) are therefore 3 normally regarded, like those from carbon, as formed from 5*p -hybrids. It has, however, been shown that the production at the central atom of a positive charge, which may be quite small, has a strong contracting effect upon the d-orbitals of the valence-shell; such a positive charge can be induced by the presence of electronegative substituents like fluorine 4 5 6) bound to the central atom in question* » · . If this happens, the ^/-orbitals can be sufficiently contracted to become of energy and spatial extent appropriate for mixing with the bonding s- and ^-orbitals; hence an increase in the maximum number of or-bonds formed is possible, and this explains the well-known acceptor properties of tetrafluorosilane, which readily forms addition-compounds such as SiF4.2NH3 (in which the σ-bonds are almost certainly built from .sp^-hybrids). If the d-orbitals of silicon in tetrafluorosilane can be used to form additional σ-bonds, however, then they could also be used to form the σ-bonds of the parent compound; in other words, the silicon-fluorine σ-bonds of tetrafluorosilane itself are likely to have considerable ^-character. The extent of J-mixing will of course vary with the polarization of the ^/-orbitals involved; since the appropriate wave-functions are unknown, it is difficult even to guess at the extent of such mixing, but the comparison of σ-bonds from carbon and silicon is made even more difficult by the introduction of this unpredictable parameter. Besides this effect on σ-bonds, the J-orbitals of the heavier elements are important in another way. Silicon, germanium and tin do not appear 6 VOLATILE SILICON COMPOUNDS to form (p-p)n-bonds9 analogous to the π-bonds of ethylene*. Symmetrical tetraphenyldichlorodisilane, for example, gives what is probably a dimer (7) when treated with sodium . 2Ph2SiCl.ClSiPh2+4Na -> Ph2Si—SiPh2 I I Ph2Si—SiPh2 The reason for this may be bound up with inner shell repulsions, with the relatively poor overlap between 3p and 2p or 3p and 3/rcr-orbitals (though this could be improved by d-hybridization), and with the re latively greater energies of σ-bonds from silicon (and the heavier elements) to electronegative species when compared with bonds from the same (9) species to carbon ; thus, instead of forming a silicon-oxygen "double" bond, as in the formal silicon analogue of acetone, silicon prefers to form the two σ-bonds that lead to the formation of the polymeric silicones: RN •R / S i = O Si Si R / R/I \ R On the other hand, the heavier elements have empty J-orbitals in their valence shells. Two of these are of π-symmetry relative to the tetrahedral σ-bonds of the saturated element, and so can combine with the π-orbitals of any attached atom or group; if the latter π-orbitals contain electron- pairs, (as in the halogen atoms, or the dimethylamino group), their energy (4) will be lowered by this interaction, and a (p->d) π-bond will result : other y-bond Although this is rather different from (p-p)n-bonding9 it will have an important effect upon the structures and properties of silicon com pounds. The delocalization of the π-electrons of the attached group will lead to a displacement of negative charge towards the silicon atom; since an attached group of this sort is always more electronegative than silicon, the polarity of the system will be reduced. The overall bond between the * From the cracking of tetramethylsilane, a compound has been obtained which was believed to contain a carbon-silicon double bond; it has been given the structure. (ea) têh Me2Si=CHSiMes , but further study > has shown that the molecule has the structure MeaSK >SiMej. X C H / INTRODUCTION — ATOMIC PROPERTIES 7 two species will of course be strengthened by such π-interactions; more important, since overlap is likely to be greatest between a J-orbital and a pure ^-orbital, differences might be expected in the hybridization of an element of groups V or VI bound on the one hand to carbon and on the other hand to silicon, depending upon whether the lone pair or lone pairs were accommodated in hybrid orbitals (as in water or dimethyl ether) or in pure p-orbitals (where more efficient π-bonding would be possible). The extent of (/?->rf)7>bonding is likely to depend on the diffuseness of the J-orbitals concerned and on their principal ^quantum number, since the amount of overlap will depend on both these properties; some d-orbital contraction is probably necessary, so that π-bonding is likely to be most important when silicon is bound to some very electronegative (4) group with π-orbitals containing electron-pairs . A detailed study of how this interaction would be expected to vary with such factors as the prin cipal quantum number of the rf-orbitals has yet to be made. The difference between the π-bonding properties of silicon and carbon can perhaps be made clearer by comparing the radicals triphenylmethyl, PhgC, and triphenylsilyl, Ph3Si.. The former is strongly stabilized by delo- calization of the unpaired electron over the rings; the latter has not been characterized but is almost certainly much less stable (see chapter 4), probably because the 3/?-orbital of silicon does not interact sufficiently with the π-orbitals of the ring to stabilize the system. On the other hand, the radical-ion PI14SÌ" might well be appreciably more stable to oxidation than its carbon analogue, because of (p-*d) delocalization of the unpaired electron. Besides these properties, there are some others which will be of impor tance in the discussions which follow. The atomic radii of the elements are given in Table 1.1; the most surprising thing about the values is the relatively small increase in radius from silicon to germanium. (10) TABLE 1.1.—ATOMIC RADII Element Radius (A) Carbon 0-77 Silicon 117 Germanium 1-22 Tin 140 This can be put down to the interpolation between the two elements of the first transition series, which gives rise to the "scandinide contraction".
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