P/ -bond

So to summarise these two-group disconnections we can always disconnect the a, p bond in either of these structures ... 

The IT net charges and p -bond calculated by the PPP tt and cu methods (123) are collected in Table 1-13, and the qualitative variations of both quantities are reported in Table 1-14. 

Carbon-fluorine bonds are quite strong (slightly stronger than C—H bonds) and like polyethylene Teflon is a very stable inert material We are all familiar with the most characteristic property of Teflon its nonstick surface This can be understood by com paring Teflon and polyethylene The high electronegativity of fluorine makes C—P bonds less polarizable than C—H bonds causing the dispersion forces m Teflon to be less than those m polyethylene Thus the surface of Teflon is even less sticky than the already slick surface of polyethylene... 

It is estimated that mote than 25 x 10 different potentially toxic OP esters can be made using Schrader s classic (27) formula for effective phosphorylating agents, (39), where R and are short-chain alkyl, alkoxy, alkylthio, or alkylamino groups, and X is a displaceable moiety with a high energy P-bond such as E or acyl anhydride, and the pentavalent phosphoms atom is bonded to oxygen or sulfur. 

Phosphorus—Hydrogen Bond. A hydrogen bound to phosphoms has Httie acidic or hydric character. Most of the reactions the bond undergoes are those of a reducing agent. P—H bonds are formed by hydrolysis of active metal phosphides or phosphoms haUdes, by the rearrangement of P—O—H or P—S—H linkages, or by the hydrolysis of P—P bonds (6,17). 

B. Cyclohex-2-enones Cleavage of Exocyclic y,d-Bonds and p -Bonding to <3, -Double Bonds... 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

P -Bonding is obviously also the initiating step in the complex photoisomerization sequence of the stereoisomeric 1,5-dien-3-ones (162) and (163) in ethanol. After low conversions of the starting dienones, an isomer containing an analogous chromophoric system [(164) and (165), respectively] was found to build up temporarily in each case. On longer photolysis times, both compound pairs (162)/(164) and (163)/(165), are consumed, and the mixtures of the four diastereomers (166)-(169) were isolated from both runs. According to separate irradiation experiments with each of these products, (166) and (167) on one hand, and (168) and (169) on the other, are... 

H. Hatanaka, Boron Neutron Capture Therapy for Tumours, Nishimura, Niigata, Japan, 1986. R. G. Fairchild, V. P. Bond and A. D. Woodhead (eds.). Clinical Aspects of Neutron Capture Therapy, Plenum, New York, 1989, 370 pp. °M. F. Hawthorne, PureAppl. Chem. 63, 327-34 (1991). 

C), and is much less reactive it is therefore safer and easier to handle, and is essentially non-toxic. The amorphous material can be transformed into various crystalline red modifications by suitable heat treatment, as summarized on the right hand side of Fig. 12.3. It seems likely that all are highly polymeric and contain three-dimensional networks formed by breaking one P-P bond in each P4 tetrahedron and then linking the remaining P4 units into chains or rings of P atoms each of which is pyramidal and 3 coordinate as shown schematically below ... 

Direct reaction with alcohols gives mixed mono-and di-alkyl phosphoric acids by cleavage of the P-O-P bonds ... 

As seen from Fig. 12.15 the structure of a-P4S4 resembles that of AS4S4 (p. 579) rather than N4S4 (p. 723). The 4 P atoms are in tetrahedral array and the 4 S atoms form a slightly distorted square. The 2 P-P bonds are long (as also in P4S3... 

It follows from these structural principles that each P atom is 5-covalent. However, the oxidation state of P is 5 only when it is directly bound to 4 O atoms the oxidation state is reduced by 1 each time a P-OH is replaced by a P-P bond and by 2 each time a P-OH is replaced by... 

There has been much confusion over the structure of these compounds but their diamagnetism has long ruled out a monomeric formulation, H2PO3. In fact, as shown in Table 12.7, isomeric forms are known (a) hypophosphoric acid and hypophosphates in which both P atoms are identical and there is a direct P-P bond (h) isohypophosphoric acid and isohypophos-phates in which 1 P has a direct P-H bond... 

The structure of isohypophosphoric acid and its salts can be deduced from nmr which shows the presence of 2 different 4-coordinate P atoms, the absence of a P-P bond and the presence of a P-H group (also confirmed by Raman spectroscopy). It is made by the careful hydrolysis of PCI3 with the stoichiometric amounts of phosphoric acid and water at 50° ... 

Phosphor compounds have problems when atoms are 3 A apart, producing incorrect geometries. P4O6 for example is predicted to have P-P bonds differing by 0.4 A, although experimentally they are identical. 

Heterocycles in syntheses of natural products with C—P bond 99T12237. 

Syntheses of natural heterocycles with C—P bond in side chain 99T12237. 

It is interesting to determine which bond (the P—O bond marked a or the O—P bond marked b) is cleaved by hydrolysis (reaction with water). 

The axial P-Os-P angle is 166°, so the three atoms are not quite colinear (Os—P 2.311 A) the unique Os-P bond is slightly longer at 2.347 A, showing a frans-effect of hydride. Os-H bonds average 1.663 A [164]. 

In the similar OsH4(PEt2Ph)3, the Os-P bonds are 2.296 A (axial) and 2.339 A (equatorial). MO calculations have been used to explain the... 

There are, however, short Rh—H contacts (2.77-2.84A) to ortho-hydrogens in phenyl groups. The Rh—P bond trans to Cl is some 0.1 A shorter than the others, evidence of the weak trans-influence of chloride [46]. 

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