Bond order carbon-phosphorus

While 4bp/-type structures can be considered as resonance structures, 4cp/is also an alternative. The importance of structure 4cp/is supported by the fact that the negative charge at the carbon atoms in the 2 and 5 positions is indeed larger than at the 3 and 4 positions. ° The common feature in all 4ap/-, 4bp7-, and 4cp/-type structures is that the lone pair of the planar tricoordinate phosphorus is fully delocalized, resulting in an enhanced bond order about the tricoordinate planar phosphorus. The d-orbital participation at phosphorus is insignificant in 4, either in the planar or nonplanar form. °... 

The r-delocalisation in the parent phospholide anion I (Fig. 3, R =R =H) can be expressed in the valence bond picture by resonance between the canonical structures lA-IC (and their mirror images). Phosphonio-sub-stituents (R =R =PH3 ) increase the weight of the 1,2-dipolaric canonical structure IB and induce thus, in essence, a partial r-bond localisation and a shift of r-electron density from the phosphorus to the adjacent carbon atoms [16]. Consequences of this effect are the decrease in delocalisation energy for reaction (1) depicted in Fig. 4, and lower C2-C3/C4-C5 and higher C3-C4 bond orders which are reproduced in concomitant variations of computed bond distances [16]. 

Organophosphorus compounds rarely have more than one P atom in the molecule, giving rise to a doublet for any C atom within three bonds (assuming the proton-carbon coupling has been removed) and so these compounds show clearly the connectivity of carbon atoms close to the P atom. The size of the coupling constant is dependent on the number of bonds and the phosphorus oxidation state, such that Vis the largest at about 45-150 Hz, while V and V are of the order of 10-15 Hz. 

Phosphorus atoms in ylides largely retain the tetrahedral ligand geometry of the phosphonium cation precursor, but the bond to the ylene-ylide carbon atom is shortened, indicating an increase in bond order. 

The photoelectron (P.E.) spectra of trimethylarsonium methylide and its phosphorus analogue have been recorded and CNDO/2 calculations were carried out . Results are in accord with a raising of the HOMO levels for the arsenic ylide compared to the phosphorus analogue, thereby lowering the d-population and the ylide bond order, and increasing the charge on the ylidic carbon atom. 

A kinetic rate model for aryl interchange was developed based on the following assumptions. The carbon-phosphorus bonds of the different triarylphosphines cleave with equal ease, and aryl interchange proceeds by a reversible second order reaction. For example, the reaction of a TPP and a TRI molecule always yields one MONO and one DI molecule, and there are nine distinct ways for the forward reaction to occur (any of the three aryls of one molecule can replace or be replaced by any of the three aryls on the other). Of the many possible ways for the molecules to react however, there are three which are unique. These and the respective chemical equilibrium constants are shown in Scheme 3. 

The CoNMR spectra of the tetrahedral [Co(PF3)4] and [Co(CO)4] anions have been recorded (191) and values obtained for the V( Co- P) (1222 25 Hz) and V( Co- C) (287 13 Hz) coupling constants. Approximate values for the cr-bond order were calculated and together with shift data suggested that bonding to the ligand tt orbitals is more important in the carbonylate ion than bonding to the phosphorus 3d orbitals of the tetrakis(trifluorophosphine)cobaltate, and that the cobalt-phosphorus u-bond order apparently is greater than the cobalt-carbon c7-bond order (191). 

All the phosphates can be viewed as sources of phosphoric acid or poly-phosphoric acid and most of the development work in this area has concentrated on identifying the best co-additives to employ. In simple systems such as APP blended with polyhydric compounds like penta-erythritol it has been established that, above 200°C, the components react to form phosphoric ester bonds. Further elimination of water and ammonia leads to a carbon-phosphorus char. Without the pentaerythritol no char is formed. The char structure can be further improved by the addition of melamine which in this case can be viewed as a blowing agent, increasing the char volume. The formation of the char can therefore be seen as a sequence of overlapping reactions which must occur in a particular order for optimum performance [2]. 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

While breaking of the carbon-to-phosphorus bond is a nuisance in catalysis with organometallic complexes, the breaking of carbon-to-nitrogen and -to-sulfur bonds is a desired reaction in the oil industry. Hydrodenitrification (HDN) and hydrodesulfurisation (HDS) are carried out on a large scale in order to remove nitrogen and sulfur from the fuel feedstocks. 

---