Hybrid delocalized coordinates

The chemistry of trivalent phosphorus compounds in which phosphorus is one or two coordinate is rapidly developing. These systems contain sp or sp2 hybridized phosphorus and multiple TT-bonds between P and other elements and until 1964 it was believed that bond formation involving ptr-pir overlap was unfavourable. Subsequently certain -P=C systems resulted from the use of charged ( 1, 2, 3) and or delocalized systems (4J but it is only in the past few years that successful syntheses of phospha-alkenes, R2C=PR , and phospha-alkynes, RC=P, have been reported. These novel compounds are the subject of this paper. 

In all the following systems, the polymerization process arises from the twofold coordination of the F atoms which coimect the elementary polyhedra in a 3-D maimer. The F ions sitnated between two highly charged M"+ cations appear to be strongly sp hybridized, with a strong delocalization of the electrons located in the sp, orbitals towards the empty orbitals of the M"+ cation. The formation of long polymerized chains, F - MF - F - M , contribntesto the increase in the viscosity in the melt. 

The Hiickel method is a simple semi-empirical method for determining approximate LCAO molecular orbitals to represent delocalized bonding in planar molecules. It treats only pi electrons and assumes that the framework of sigma bonds has been treated separately. As an example we consider the allyl radical, CH2 = CH - CH2.. If the plane of the molecule is the xy plane, each carbon atom has an unhybridized 2pz orbital that is not involved in the sigma bonds, which are made from the Isp hybrids in the xy plane with the appropriate rotation of the coordinate system at each atom to provide maximum overlap. We construct linear combinations from the three orbitals, as in Eq. (21.6-2). 

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