Inorganic Hydrides PDF
Preview Inorganic Hydrides
I N O R G A N IC H Y D R I D ES by B. L. SHAW Reader in Inorganic and Structural Chemistry University of Leeds PERGAMON PRESS OXFORD • LONDON • EDINBURGH • NEW YORK TORONTO • SYDNEY • PARIS • BRAUNSCHWEIG Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London W.l Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., 44-01 21st Street, Long Island City, New York 11101 Pergamon of Canada, Ltd., 6 Adelaide Street East, Toronto, Ontario Pergamon Press (Aust.) Pty. Ltd., 20-22 Margaret Street, Sydney, New South Wales Pergamon Press S.A.R.L., 24 rue des ficoles, Paris 5e Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig Copyright © 1967 Pergamon Press Ltd. First edition 1967 Library of Congress Catalog Card No. 66-26874 Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter This book is sold subject to the condition that it shall not, by way of trade, be lent, resold, hired out, or otherwise disposed of without the publisher's consent, in any form of binding or cover other than that in which it is published (3050/67) Preface THERE have been many exciting recent developments in the field of inorganic hydrides. Outstanding perhaps is the work on the boron hydrides, closely followed by that on transition metal hydrides, but there have also been many other developments. Modern physical methods of investigation, particularly nuclear magnetic resonance and infrared spectroscopy, have contributed a great deal to the field. It was felt that there was a need for a small book on inorganic hydrides, suitable for undergraduates, particularly one covering some of the more important recent developments. The chemistry of inorganic hydrides has always formed a substantial part of any undergraduate course and it could well be that in the future it will become even more important. Inorganic hydrides provide many excellent illustrative examples of chemical principles and of the applications of physical methods. After introductory chapters on the classes of hydrides and hydrogen itself the hydrides are discussed, mainly element by element, with frequent comparisons between elements where ap- propriate. The transition metal hydrides are discussed as a whole, however; similarly, the "metallic" hydrides. A chapter on bonding and bond strengths in hydrides follows and finally a chapter on the applications of infrared, Raman and nuclear magnetic reson- ance spectroscopy. Since the general chemistry of water will be familiar to students and its solvent properties dealt with in great detail in physical chemistry courses, there is only a short discussion on it in this book. Similarly, the sections on hydrogen chloride, bromide and iodide are short. A list of recommended books for further reading is included. The author is indebted to Dr. L. D. Pettit, who read the manu- script and made several useful suggestions. vii CHAPTER 1 The Classes of Hydrides HYDRIDES can be classified into four principal categories: (1) ionic; (2) covalent (of non-transition metals); (3) transition metal hydrides, usually containing complexing ligands; and (4) metallic hydrides. In addition hydrides are known which do not fit into this general classification; e.g. hydrides of copper and zinc. These can be described as borderline hydrides and form a transition in type between the covalent hydrides of the later elements of the periodic table and the metallic hydrides of the transition elements. The hydrides of beryllium, magnesium and perhaps aluminium have characteristics of both ionic and covalent hydrides. Ionic Hydrides Given by Li Bet Na Mgt Alt K Ca Rb Sr Cs Ba They are formed by the more electropositive metals, electro- negativities from 0 • 7 (Cs) to 1 • 5 (Be) on the Pauling scale and are salt like with hydrogen present as the hydride ion. They have high melting points and thermal stabilities and, when electrolysed, hydrogen is evolved at the anode. They react with water or acids evolving hydrogen, t On the borderline between ionic and covalent. 1 2 INORGANIC HYDRIDES Covalent Hydrides of Non-transition Elements Given by H Bef B C N O F Mgf Alt Si P S CI Ga Ge As Se Br In Sn Sb Te I Tl Pb? Bi? Po . The electronegativities of elements forming these hydrides vary from 1 -6 (Ga) to 4-0 (F) and the hydrides are, with a few excep- tions, gases or liquids. With the elements of lower electronega- tivity Be (1 -5), Mg (1 -2) and Al (1 -5) the hydrides are involatile solids and on the borderline between ionic and polymeric covalent. The aluminohydride ion (AIH4) would certainly be regarded as covalent, however. The hydrogen may be hydridic (negatively charged) when the element has lower electronegativity than hydrogen (2-1); e.g. diborane-6 reacts with water to give hydrogen (boron has electro- negativity 2-0). When the element has high electronegativity the hydroxonium ion will form with water; e.g. with hydrogen chloride (chlorine, electronegativity 3-0). Some of the hydrides in this class contain bridging hydrogens; e.g. the boron hydrides. Transition Metal Hydrides, Usually Containing Complexing Ligands Formed by V Cr Mn Fe Co Ni Nb Mo Tc Ru Rh Pd Ta W Re Os Ir Pt Transition metals do not usually form simple binary hydrides containing covalent or ionic bonds. By complexing the metal with ligands such as 7r-cyclopentadienyl, carbon monoxide, tertiary f On the borderline between covalent and ionic. THE CLASSES OF HYDRIDES 3 phospines, tertiary arsines or cyanide ion they will form stable molecular hydrides, however. These contain quite strong bonds and are often volatile. Examples include [R3eH(C H ) ], [CoH 5 52 (CO)J, fra2_/w-[PtHCl(PEt )] and [IrH(CN) ]) -. The rhenohydride 32 5 ion ReH is an example of a transition metal hydride where other 9 ligands are not necessary for stability. Metallic Hydrides f transition metals Formed by many of the I lanthanides actinides These often have indefinite compositions and are metallic in appearance. The arrangement of the metal atoms in the hydride may be quite different from that in the free metal. In many ways they are similar to metal carbides or nitrides. Some lanthanide hydrides have characteristics of ionic hydrides. CHAPTER 2 Hydrogen and the Hydrogen Molecule Hydrogen Atoms Hydrogen has three isotopes, protium JH, deuterium *H or D, and tritium or T, which is radioactive. The proportions by wei-4ght of deuteriu18m and tritium in natural hydrogen are 1 -6 x 10 and ca. 10~ re1sp2ectively. The atomic weights of the three isotopes (based on C = 12-000) are 1 -008, 2-015 and 3-017. Some important properties of hydrogen atoms and ions are the following: Ionization 13-59 eV or This is higher than potential 312 kcal/g-atom the first ionization potential of most elements and muc h higher than those of the alkali metals Electron 0-72 eV or Much lower than for affinity 16-5 kcal/g-atom the halogens Bohr radius 0-53 A 5 Radius of ~l-5 X 10" A proton Radius of ' 1-4 A for The hydride ion is hydride ion co-ordination very polarizable number six and its radius varies. 1 - 4 A is a good average value for the alkali metal hydrides, see Table 3.1 4 HYDROGEN AND THE HYDROGEN MOLECULE 5 Covalent ca. 0-30 A radius Van der Waals 1-2 A radius Deuterium. The existence of a heavy isotope of hydrogen was first suspected because of a slight discrepancy between the chemical atomic weight and that determined by mass spectro- metry. Examination of the optical spectrum of the residue ob- tained by evaporating a large volume of liquid hydrogen confirmed the existence of this isotope and was soon followed by the pre- paration of nearly pure heavy water (D O) by electrolysis—when a ordinary water is electrolysed the residue becomes richer in heavy water. Heavy water is now manufactured in ton quantities by electrolysis of ordinary water, by careful fractional distillation (D 0 has b.p. 101 -4°C) and also by making use of the isotope 2 effect for the following reaction HOD + HSH % HOH + HSD. At 100°C the equilibrium is in favour of the forward reaction, at 25°C the reverse reaction is favoured; hence by carrying out the exchange at 100°C and then leading the sulphide mixture into water at 25°C a slight enrichment of deuterium in the water is achieved. The enriched water is then further concentrated by dis- tillation and electrolysis; 99-8% pure heavy water is now readily available. Heavy water is used to slow down fast fission neutrons in nuclear reactors. It is also the starting material for preparing most deuterated compounds, which are of enormous importance in chemistry for studying the mechanisms and kinetics of reactions. Tritium. Tritium was discovered by bombarding deuterium compounds with deuterons 6 INORGANIC HYDRIDES but is now made mainly by bombarding lithium with slow neutrons *He + iH and is commercially available. Tritium is a weak f$ emitter with a half-life of 12-5 years and is very useful for labelling hydrogen containing compounds in reaction mechanism and other studies. Tri1tiu4m is formed in the upper atmosphere by the reaction of N with neutrons from outer space. , 2 i! C + JH. Molecular Hydrogen Hydrogen is manufactured on a large scale industrially by methods such as (1) the electrolysis of water; (2) by the water gas reactions: H 0 + C^CO + H (i.e. steam reacting with red hot coke) 2 2 H 0 + CO->H + C0 (using an iron catalyst) 2 2 2 and (3) by thermal cracking of hydrocarbons. It is used for making methanol CO + 2H • CH OH 2 3 for making the higher primary alcohols by the OXO-process RCH:CH + CO + 2H > RCH CH -CH OH 2 2 2 2 2 (cobalt catalyst) in the Haber process for ammonia N + 3H -> 2NH (iron catalyst) 2 2 3 and in the catalytic hydrogenation of vegetable oils to margarine. In addition hydrogen chloride is made by direct combination of the elements. Hydrogen is also used in the production of im- portant metal hydrides such as sodium borohydride and lithium aluminium hydride. HYDROGEN AND THE HYDROGEN MOLECULE 7 Physical Properties. Some physical properties of hydrogen (natural isotopic mixture) are: m.p. -259-14°C b.p. -252-78°C Critical temperature -239 • 9°C Density of solid at freezing point 0-076 g/ml Density of liquid 0 • 0899 g/ml Since natural hydrogen is almost entirely the JH isotope these are also the properties of H—H. Deuterium (D—D) has a higher melting point (-254-55°C) and boiling point (-249-7°C). Ortho and Para Hydrogen. Protium (}H) atoms have a nuclear spin of one half and hydrogen molecules exist in two forms— ortho hydrogen, in which the two nuclei are spinning in the same direction and para hydrogen, in which they are spinning in op- posite directions. Ortho hydrogen has three quantum states, para hydrogen has only one and at room temperature the equilibrium ratio ortho:para is 3:1. Para hydrogen has a slightly lower internal energy, however, and at low temperatures is the preferred form; e.g. at the boiling point the equilibrium ratio is 99% para, 1 % ortho. Except at high temperatures and in the absence of catalysts the interconversion of ortho hydrogen to para hydrogen is slow. If ordinary hydrogen is liquefied, because the para isomer has the lower internal energy and because the latent heat of evaporation of hydrogen is so small, the conversion 25 % para to 99 % para, although slow in the absence of catalysts, eventually releases enough energy to evaporate over half of the liquid hy- drogen. Many substances will catalyse the ortho-para conversion and hydrated ferrous oxide is used to convert hydrogen almost entirely into the para isomer during liquefaction and therefore make storage easier. Ortho and para hydrogen have been separated recently by gas chromatography. Para hydrogen has a much greater thermal conductivity and specific heat than ortho hydrogen and thermal conductivity is often used to determine the propor- tions of ortho and para hydrogen in a mixture.
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