Inorganic Chemistry – Catherine E Housecroft

Oxides of phosphorus Phosphorus(III) oxide, P4O6, is obtained by burning white phosphorus in a restricted supply of O2. It is a colourless, volatile solid (mp 297 K, bp 447 K) with molecular structure 15.56; the PO bond distances (165 pm) are consistent with single bonds, and the angles POP and OPO are 1288 and 998 respectively. The oxide is soluble in diethyl ether or benzene, but reacts with cold water (equation 15.122).

P4O6 þ 6H2O ” 4H3PO3 ð15:122Þ (15.56) Each P atom in P4O6 carries a lone pair of electrons and P4O6 can therefore act as a Lewis base. Adducts with one and two equivalents of BH3 have been reported, but the reaction of P4O6 with one equivalent of Me2S:BH3 followed by slow crystallization from toluene solution at 244 K gives P8O12(BH3)2 (15.57) rather than an adduct of P4O6.

The solid state structure confirms that dimerization of P4O6 has occurred through PO bond cleavage in structure 15.56 and reformation of PO bonds between monomeric units. Free P8O12 has not, to date, been isolated. (15.57) Oxidation of P4O6 with O2 gives P4O10 (see below), while ozone oxidation leads to the formation of P4O18 (equa- tion 15.123) which has been structurally characterized. The square-based pyramidal environment of each P atom is related to that found in phosphite ozonides (RO)3PO3 (see Figure 16.5). In solution, P4O18 decomposes above 238 K with gradual release of O2, but the decomposition of dry P4O18 powder is explosive.

(15.123) The most important oxide of phosphorus is P4O10 (phos- phorus(V) oxide), commonly called phosphorus pentoxide. It can be made directly from P4 (equation 15.8) or by oxidizing P4O6. In the vapour phase, phosphorus(V) oxide contains molecules with structure 15.58; the PObridge and POterminal bond distances are 160 and 140 pm. When the vapour is condensed rapidly, a volatile and extremely hygroscopic solid is obtained which also contains P4O10 molecules. If this solid is heated in a closed vessel for several hours and the melt maintained at a high temperature before being allowed to cool, the solid obtained is macromolecular.

Three polymorphic forms exist at ordinary pressure and temperature, with the basic building block being unit 15.59; only three of the four O atoms are involved in interconnecting the PO4 units via POP bridges. Phosphorus(V) oxide has a great affinity for water (equation 15.124), and is the anhydride of the wide range of oxoacids described in Section 15.11.

. Multiple choice questions to help test your learning . Rotatable three-dimensional structures taken from the book . Interactive periodic table Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsoned.co.uk First published 2001 Second edition 2005 Third edition published 2008 # Pearson Education Limited 2001, 2008 The rights of Catherine E. Housecroft and Alan G. Sharpe to be identified as authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

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All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. ISBN: 978-0-13-175553-6 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library 10 9 8 7 6 5 4 3 2 1 11 10 09 08 07 Typeset in 91 2 /12 pt Times by 73 Printed and bound by Rotolito Lombarda, Italy The publisher’s policy is to use paper manufactured from sustainable forests.

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