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Hybridization of heavier p block elements

Hybridization of s and p orbitals to form effective sp hybrid orbitals requires that they have comparable radial extent. While 2p orbitals are on average less than 10% larger than 2s, in part attributable to the lack of a radial node in 2p orbitals, 3p orbitals which have one radial node, exceed the 3s orbitals by 20-33%. The difference in extent of s and p orbitals increases further down a group. The hybridization in of atoms in chemical bonds can be analyzed by considering localized molecular orbitals, for example using natural localized molecular orbitals in a natural bond orbital (NBO) scheme. In methane, CH₄, the calculated p/s ratio is approximately 3 consistent with "ideal" sp³ hybridization, whereas for silane, SiH₄, the p/s ratio is closer to 2. A similar trend is seen for the other 2p elements. Substitution of fluorine for hydrogen further decreases the p/s ratio.The 2p elements exhibit near ideal hybridization with orthogonal hybrid orbitals. For heavier p block elements this assumption of orthogonality cannot be justified. These deviations from the ideal hybridization were termed hybridization defects by Kutzelnigg.

TY - JOUR T1 - Orthogonal and non-orthogonal hybrids JO - Journal of Molecular Structure: THEOCHEM VL - 169 IS - 0 SP - 403 EP - 419 PY - 1988/8// T2 - AU - Kutzelnigg, W. SN - 0166-1280 DO - http://dx.doi.org/10.1016/0166-1280(88)80273-2 UR - http://www.sciencedirect.com/science/article/pii/0166128088802732

Kutzelnigg in 1986 highlighted carbon as a unique case which adheres most closely to the  conventional sp hybridization model.

Phosphate Glass Article

Lede

Phosphate glasses have been used in lasers (for example Nd doped glass), alkali metal phosphate glasses are sequestration agents for hard water and dispersants for clay processing as well as pigment manufacture. Biocompatible phosphate glases are used in medical applications. Iron phosphate glasses have been investigated for use as hosts for nuclear waste. The phosphate glasses were first investigated by Schott and coworkers in the early part of last century. 20th century by Schott and coworkers. While the glasses have good optical properties such as high transparency to ultraviolet light they were considered to sensitive to moisture to be generally useful. P2O5 was identified by Zachariesen as one of the prototypical network formers in the 1930's.(Take from Brow, Neel and Salih and Neel Pickup reviews) Glasses formed from multiple network formers have been produced, for example aluminophosphate, silicophosphate as well as molybdophosphate and tungstophosphate glasses formed with molybdenum and tungsten oxide

Structure

Phosphate glasses are based on phosphorus pentoxide, P₂O₅ as the "network former". Tetrahedral {PO4} units are lnked together by bridging oxygen units. This linking of tetrahedral units is shared with the silicate glasses, which are based on silica as the network former. A structural difference between silicates and phosphates is that while silicates can share all four corners in phosphates a maximum of three corners can be shared.. Crystalline compounds can be prepared as well as amorphous glasses. Phosphate compounds can be classified by their P/O ratio as follows:

Group	Ultraphosphate	Metaphosphate	Polyphosphate	Pyrophosphate	Phosphate
O/P	<3	3		3.5	4
Qi	Q2 and Q3	Q2	Q2 and Q1	Q1	Q0
Example compounds	Q2 and Q3	Q2	Q2 and Q1	Q1	Q0
Anions	generally polymeric with a cage anion ??? known	rings and infinite chains	chains	discrete O3P-O-PO3	discrete PO4
Glasses	Q2 and Q3	Q2	Q2 and Q1	Q1	Q0

Binary, Ternary, Quaternary glasses

Simple binary glasess derived from from P2O5 and a metal single metal oxide, e.g. Na2O, CaO. Ternary has two defferent metal oxides and quaternary 3. The addition of other oxides allows modificaion of the properties of the glasses.

Preparation of glasses

Melt quenching- could use P2O5 but too volatile and readily hydrolysed- use a phosphate compound e.g. metaphosphate and a metal carbonate rather than oxide- metaphosphate and carbonate decompose releasing H2O and CO2 respectively which is useful as it aids in homogenizing the melt. .

Sol-gel methods - allow for unusual oxides such as TiO2 to be incorporated. Alkoxides and ???? form sol- wait for gel- calcine to dehydrate.

Mechanism of glass formation

Add ing oxide to P2O5 causes depolymerization. Van wazer suggested that 2Q2 <-> Q3 +Q1 and 2Q1<-> Q2 + Q0m

Invert glasses

These are called because there have a high P/O ratio - contain small anionic units and properties depend on nature of the cations present.

Polyphosphate glasses

Anionic chains of Q2 terahedra terminated by Q1 - metaphosphate with a pyro end-- synthon?? perhaps??

Metaphosphate glasses

First Graham's salt, formed when NaH2PO4 is heated- consists of chains -commercially known as Calgon, an ion exchange water softener. The composition of metaphosphate glasses approximates to M2O.P2O5, (M is a group 1 alkali metal e.g. sodium) Consists almst entirely of Q2 units. Effect of replacing alkali metal oxide with alkline earth metal (eg. substitute MgO for Na2O ) is to increase the durability of the glass.

Ultraphosphate glasses

Sensitive to moisture - like P2O5 - some need to nbe kept in sealed ampoules.

Ternary phosphate glasses

Na2O/CaO/P2O5 glasses much studied as biologically compatible.

Ultraphosphates (Article)

Ultraphosphates are a group of phosphorus oxyanions that contain a higher proportion of phosphorus than metaphosphates. Lanthanide ultraphosphates, LnP₅O₁₄ have optical properties that may have industrial applications Ultraphosphates in common with other condensed phosphates (metaphosphates and polyphosphates) are made up of corner sharing {PO₄} units. Where ultraphosphates differ is that they contain {PO₄} units that share three corners. The number of shared corners in the {PO₄} tetrahedra is sometimes designated as Qⁱ where i is the number of corners shared. P₂O₅ contains only Q³ units in all of its forms (molecular P₄O₁₀ and polymeric). Ultraphosphate compounds contain both Q³ and Q² units, whereas metaphosphate compounds contain only Q².

Group	Ultraphosphate	Metaphosphate	Polyphosphate	Pyrophosphate	Phosphate
O/P	<3	3		3.5	4
	Q2 and Q3	Q2	Q2 and Q1	Q1	Q0

References

• Brow 2000

TY - JOUR

T1 - Review: the structure of simple phosphate glasses

JO - Journal of Non-Crystalline Solids

VL - 263–264

IS - 0

SP - 1

EP - 28

PY - 2000/3/1/

T2 -

AU - Brow, Richard K

SN - 0022-3093

DO - http://dx.doi.org/10.1016/S0022-3093(99)00620-1

UR - http://www.sciencedirect.com/science/article/pii/S0022309399006201

• Neel Salih 2011

TY - CHAP

AU - Abou Neel, E.A.

AU - Salih, V.

AU - Knowles, J.C.

T1 - 1.117 - Phosphate-Based Glasses

A2 - Ducheyne, Paul

BT - Comprehensive Biomaterials

PB - Elsevier

CY - Oxford

PY - 2011///

SP - 285

EP - 297

SN - 978-0-08-055294-1

DO - http://dx.doi.org/10.1016/B978-0-08-055294-1.00249-X

UR - http://www.sciencedirect.com/science/article/pii/B978008055294100249X

Marie-Thérèse Averbuch-Pouchot, A. Durif Topics in Phosphate Chemistry World Scientific, 1 Jan 1996, 981-02-2634-9

D Corbridge, CHAPTER 3 - Phosphates, Studies in Inorganic Chemistry, Elsevier, 1995, Volume 20, Pages 169-305, ISSN 0169-3158, ISBN 9780444893079, http://dx.doi.org/10.1016/B978-0-444-89307-9.50008-8. (http://www.sciencedirect.com/science/article/pii/B9780444893079500088)

J.M. Cole, M.R. Lees, J.A.K. Howard, R.J. Newport, G.A. Saunders, E. Schönherr, Crystal Structures and Magnetic Properties of Rare-Earth Ultraphosphates, RP5O14 (R=La, Nd, Sm, Eu, Gd), Journal of Solid State Chemistry, Volume 150, Issue 2, March 2000, Pages 377-382, ISSN 0022-4596, http://dx.doi.org/10.1006/jssc.1999.8610. (http://www.sciencedirect.com/science/article/pii/S0022459699986103)

Bioactive functional materials: a perspective on phosphate-based glasses, Abou Neel, Ensanya A., Pickup, David M., Valappil, Sabeel P., Newport, Robert J., Knowles, Jonathan C., 2009,Journal of Materials Chemistry, 19, 6, pages 690 - 701, 10.1039/B810675D, ISSN=0959-9428, url http://pubs.rsc.org/en/content/articlelanding/2009/jm/b810675d#!divAbstract

J.P. Attfield, Phosphates, In Encyclopedia of Materials: Science and Technology (Second Edition), edited by K.H. Jürgen BuschowRobert W. CahnMerton C. FlemingsBernhard IlschnerEdward J. KramerSubhash MahajanPatrick Veyssière, Elsevier, Oxford, 2001, Pages 6896-6901, ISBN 9780080431529, http://dx.doi.org/10.1016/B0-08-043152-6/01222-5. (http://www.sciencedirect.com/science/article/pii/B0080431526012225)

Examples of ultraphosphates

Compounds are known that contain anions with the general formulae of (P₂O₅.(PO₃)_n)^n– where n = 2 - 6, P₄O₁₁^2–, P₅O₁₄^3–, P₆O₁₇^4–, P₇O₂₀^5– and P₈O₂₃^6–. The stoichiometry of the anion is no guide to the structure. For example in the lanthaide ultraphosphates, LnP₅O₁₄, the phosphate anionic framework adopts a number of different forms. In Na3Fe₈O₂₃ the anion ₈O₂₃^6– has a cage "molecular" structure.

P5O143– P6O174- P7O205– P8O236-

In the classification of phosphates as salts of acids with the formula mH₂O.nP₂O₅ ultraphosphates have m/n < 1, and richer in P₂O₅ than the metaphosphates ,which are salts of hypothetical acids formulated as H₂O.P₂O₅ with anions P_nO_3n^n–.

Classification of salts	Ultraphosphate	Metaphosphate	Polyphosphate	Pyrophosphate	Phosphate
Acid		H2O:P2O5	Example	2H2O:P2O5	3H2O:P2O5
(m/n)	(m/n) <1	(m/n) = 1	1 < (m/n) < 2	(m/n) = 2	(m/n) =3
	NdP5O14	Example	Example	Example	Example

An ultraphosphate is a phosphate that is richer in phosphorus pentoxide than the metaphosphates. Examples of some known ultraphosphates showing the proportions of phosphorus pentoxide along with metaphosphate as a comaparison.

	Formula of the hypothetical acid	Proportions of H2O and P2O5	Examples
Ultraphosphate	H2P4O11	H2O.2P2O5 1:2	CuP4O11 FeP4O11 ZnP4O11 CdP4O11
	H3P5O14	3H2O.5P2O5 1:1.67	??
	H4P6O17	2H2O.3P2O5 1:1.5	??
	H5P7O20	5H2O.7P2O5 1:1.4	??
Metaphosphate	H3PO3	H2O:P2O5 1:1

Some known ultraphosphates include

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