Phosphorus hybridisation

According to quantum chemical calculations the cations P02 and PS2 should be linear [90,91 ] while in the corresponding 2-phosphallyl cation the central phosphorus atom is reluctant to sp-hybridisation [92] and is bent. Bis-donor adducts 34-36 were reported for a variety of these cations with dimethylaminopyridine (DMAP) (Scheme 21). 

It should be pointed out that sign reversals are likely to occur when the hybridisation of phosphorus, or the nature of the substituents, change in systems such as <1988,62)... 

The interest in considering such couplings in relation to the nature of P-P bonds has been discussed by Cowley in an earlier review.<1965,26) Since then a considerable amount of experimental data has been obtained and sign determination on typical compounds has revealed another interesting fact, namely that sign reversal occurs in this one-bond coupling depending upon the hybridisation of phosphorus and possibly upon the substituents. As mentioned earlier (Section III.A) this has prompted some theoreticians to consider this problem in detail. 

Fig. 10. This feature undoubtedly reflects that there are similar factors effecting these couplings. Another observation which warrants mention is that geminal HPH couplings which have recently been investigated also demonstrate the same sign reversal, without any clear correlation with the H-P-H bond angle, but are very dependent upon the hybridisation of the phosphorus. (1969 41>...

In systems derived from PH3 BH3, changes in the coupling constant, /pH, are found which are similar to those seen in compounds derived from PH3, i.e. compounds with phosphorus in coordination number 3. The observation that the alteration does not correspond to that seen for derivatives of PHi suggests that the very weak donor-acceptor bond in PH3 BH3 does not affect the hybridisation of phosphorus, as compared with that in free PH3, as much as might at first be expected. A small increase in the electron withdrawing action by halogen substitution of the hydrogen atoms bonded to boron causes an increase in the s character of the P-H bonds. 

As far as the data permit, the dependence of bond energies on the hybridisations of the atoms concerned is allowed for. In the case of F—P, Cl—P and Br—P, the bond energies are based upon data for molecules in which the P atom has (approximately) sp3 hybridisation, e.g. PX3, PX2Y, POX3. These values are inappropriate for PX5 species, or for PX3Y2 etc. The mean bond energies found for, e.g., PF5 are considerably lower than for trivalent phosphorus species. 

Of particular interest in the latter case is the use of N-NMR spectroscopy for the determination of J( P, N) data, of which there are few examples for systems having two coordinate sp -hybridised nitrogen linked to phosphorus. " Various acyclic bis(aminophosphines) have also been reported, including the imino-functional system (96), the bis(phosphino)-hydrazines (97) and -ureas (98), and the l,8-bis[bis(dialkylamino)phosphino]naphthalenes (99). There has also been significant activity in the synthesis of cyclic aminophosphines. An improved route to the four-membered ring system (100) is afforded by the reaction of phosphorus trichloride with t-butylamine in a 1 3 mole ratio in THF. Treatment of this with ammonia in THF/triethylamine at — 78°C gives (101),... 

Bipyramidal, spM. A typical bipyramidal molecule is phosphorus penta-chloride, PClg, which is monomeric in the vapour and has the form shown in Fig. 50. The hybridisation in the central plane could be sp, giving the three bonds at 120" with one another. And the third p orbital could be mixed with the appropriate d orbital, since both are symmetrical about the central axis, to give one hybrid directed up and another down as in Fig. 51. In the se circumstances two chlorines would be expected to be... 

The chief differences between nitrogen and phosphorus arise from the availability of d orbitals in the shell with principal quantum number 3. This gives phosphorus valence properties not shared by nitrogen for example, a capacity for 6 co-ordination associated with octahedral hybridisation (p. 105). 

Phosphorus may make use of its 3tf-orbitals, to some extent, to increase its coordination number from five to six, as for instance in the adducts of PF with nitrogen bases (e.g. PF5 pyridine) or in anions of the type PF. The neutral complexes display a distorted octahedral geometry with a very long coordinate bond. Regular octahedral geometry with equivalent bond distances is observed in the PFg anion, for which the hybridisation may be described as sp3d2. 

There are a number of atoms that display optical isomerism, including nitrogen and phosphorus, but the simplest case to consider, and the most commonly encountered in drugs, is that of an sp3 hybridised carbon atom with four different substituents attached to it (Figure 4.2). A carbon like this is said to be chiral and to display the property of chirality. If the four substituents are different, a pair of non-superimposable mirror image forms can be drawn. These two isomers are called enantiomers. A chiral compound always has an enantiomer, whereas an achiral compound has a mirror image that is the same as the original molecule. 

When heated under normal pressures, all allotropic forms of phosphorus will produce a vapour consisting of tetrahedral P4 molecules (Figure 4.3). The interbond angles of 60° represent a highly strained arrangement for which hybridised pd orbitals have been proposed. The tetrahedral P4-type structure is unusual but not quite unique in chemistry. Elements of the same group adopt it, that is, AS4, Sb4 and probably Bi4 (see below). The existence of isoelectronic Si, Ge ", Sn and Pb " anions has been established in recent years. 

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