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PHOSPHORUS CHEMISTRY: CHEMISTRY

INTRODUCTION: Phosphorus can form bonds with many other elements. Also it can form bonds with varying number of atoms (Coordination Number), which can vary from 1 to 6. Also it can have different valencies, either 3 or 5. Also it has empty d-orbitals which readily accept electrons from any good donors. These properties give an extra edge for the chemistry of phosphorus than the extensive carbon chemistry. Hence, it is very useful to have a phosphorus bond in the place where a reaction needs to be carried out. Phosphorus can extend its number of bonds to take a new group and then get rid of an old group (substitution reaction) much readily. The P-O (Phosphorus-Oxygen) bonds are very strong but very readily replaceable by another P-O bond.

INORGANIC CHEMISTRY: The inorganic chemistry of phosphorus mainly involves the study of variuos phosphates (pyro, cyclic and acyclic polymeric phosphates) and phosphides of variuos elements.

ORGANIC CHEMISTRY: Phosphorus can form bonds readily with oxygen, nitrogen and sulfur; also it can form bonds with carbon much less readily; but the phosphorus-carbon bonds are inert that they do not break readily even though not very strong. These four bonds make it easy to link phosphorus to organic compounds and result in thousands of organophoshporus compounds. All the biological functions of phosphorus comes from organophosphorus compounds except bones and teeth which are inorganic (mostly calcium) phosphates.

BIOCHEMISTRY: Here both the inorganic and organic parts of the phosphorus chemistry are important to understand, nourish and protect the body. The inorganic part is important when the bones and teeth are concerned and for the other biological functions the organic chemistry part is important. Because DNA, RNA and other phosphorus containing enzymes, phospholipids, etc. are organophosphorus compounds.

COORDINATION CHEMISTRY(1): Phosphorus compounds are the most useful in the coordination chemistry of metals. When the donor strenghts are considered, phosphorus is one of the strongest coordinating atoms. Carbon monoxide and cyanide ion are the strongest but they can not be modified and so the next strongest modifiable phosphorus becomes most important. Phosphorus can form strong bonds with soft metals and metals in low oxidation states. This is very useful in the area of catalyst and many phosphorus-metal complexes are used as catalysts in industry to produce chemicals in a better way. In the coordination to metals, the phosphorus acts as an electron donor to the metal and takes back some of the electrons through vacant d-orbitals thereby not leaving too much electron density on the metal which may destabilise the complex. In all these cases the valency and the coordination number of phosphorus are 3 before coordination and becomes 4 after coordination.

COORDINATION CHEMISTRY(2): In the above coordination to metals, the phosphorus acts as an electron donor to the metal. But as recently found out, the phosphorus can also act as electron acceptor with donor atoms such as nitrogen, oxygen or sulfur. These also should be called complexes similar to the above but with the role played by the phosphorus is opposite. These are important in the sense that they serve as models for the reaction intermediates for the organophosphorus compounds. The organophosphorus compounds are very important in the biological systems as we have seen above. These studies have led to new understandings of the reaction mechanisms at phosphorus. The two contradicting nature of phosphorus (both as electron donor and direct acceptor; even in the complex there is an electron acceptance which is quite different) makes it amphoteric (like amphibians). That is, it will either give or take electrons depending on the other atoms (and the groups attached to phosphorus).

STRUCTURAL PROPERTIES: Phosphorus compounds have varying geometries depending on the
Coordination Number (CN) ; CN is the number of atoms to which phosphorus is connected to. The geometries are:

  1. CN = 2; Bent ** (Use PDF File for Printing)
  2. CN = 3; Trigonal Pyramid or Trigonal Planar ** (Use PDF File for Printing)
  3. CN = 4; Tetrahedral (Td) or Trigonal Pyramid ** (Use PDF File for Printing)
  4. CN = 5; Trigonal BiPyramidal (TBP) or Square Pyramid (SP) or Tetragonal Pyramid (TP) (or in between any) ** (Use PDF File for Printing)
  5. CN = 6; Octahedral (Oh) ** (Use PDF File for Printing)
However, additional interactions, as mentioned in Coordination Chemistry(2), can change the structure towards the next higher coordination number's geometry. This is very important in understanding which bonds may undergo cleavage and which bonds will be retained at different conditions. Since in biological systems, specific P-O bonds are cleaved or retained. There is no 'cleave them all' or 'join them all' approach! The bonds are formed and cleaved in a very high specificity.

The phosphates (found in biological systems) have CN = 4. Here one of the bonds is P=O (the phosphorus-oxygen double bond) which is quite different and stronger than the other three bonds. During formation, one of the P-O-H is cleaved and a P-O-C (or P-O-P) bond is introduced. Also in most cases (in nucleic acids, energy carriers and enzymes), one bond is P-O-H and is usually in an anionic form (negative ion) as P-O-. And only two P-O-C bonds or one P-O-C bond and one P-O-P bond are present and only one of them is cleaved usually with high specificity. In the tetraheadral structure with two P-O-C bonds, both these cleavable bonds are equivalent except that they may have different groups on carbon. However, if the structure changes to the next higher or the highest coordination geometry even with weak donor bonds, then the bonds are not similar any more (become non-equivalent) and also one can become weaker and the other stronger which will result in faster and specific cleavage. So there are many basic studies going on to understand the structural properties of compounds which look similar to the reaction intermediates. In our lab, we are presently working on this area.


MECHANISMS

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Created by Dr. A. Chandrasekaran
Department of Chemistry, University of Massachusetts, AMHERST, MA 01003, USA.

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