Phosphines bond enthalpies

In order to assess steric and electronic effects on the bond enthalpies of reaction of phosphine and phosphite ligands to platinum(II), the enthalpies (AH) of reaction of these ligands L on reaction with [PtMe(PPhMe2)2(THF)]PF6 have been measured (equation 412). These authors measured the enthalpies of 34 ligands, and correlated the measured enthalpy with the ligand cone angles.1342,1343 Representative data are shown in Table 8. 

IR spectra, 427 reactions, 427 halides, 488 heterobimetallic, 433 hexafluoro-2-butyne, 415 homometallic, 433 hydrazones, 438 imidazole, 433 isocyanides, 380 2,6-lutidine, 430 1,8-naphthyridine, 431 1,10-phenanthroline, 430 phosphido-bridged, 456 phosphines, 445 bond enthalpies, 447 NMR, 448 structure, 447 synthesis, 445 phosphites, 446 phthalazine, 432 phthalocyanine, 434 planar... 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

At this point it is fair to ask. what is the driving force for carbonyl insertion These reactions involve breaking a metal-carbon bond and formation of a carbon-carbon bond. In addition, a bond is formed between the metal and the incoming I. ewis base (CO in the foregoing examples, but frequently a phosphine or an aminel. The enthalpy change, AH. for the reaction... 

An extensive theoretical investigation does not exist for the siloles, but PM3 calculations of formation enthalpies of 2 and its tautomers have indicated that the l//-silole is the most thermodynamically stable species200. The activation barrier for 11 — 2 isomerization was calculated to be 96 kJ moC1, comparable to that for cyclopentadiene2d 116. The (1H + 1H) dimer 1019 is isolated rather than the (2H + 1H) dimer as in the case of phosphole. This directly confirms the thermodynamic stability and the Diels-Alder kinetic instability of 2. The marked difference in the stability of the parent silole and phosphole was explained3 by the relative stabilities of the a bonds in silanes and phosphines (Si > P) and of the ji bonds in silenes and phosphenes (P > Si)117. 

Calculated (B3LYP) enthalpies of addition and the effects of phosphine methylation (substitution of PMca for PH3) in reactions of some iridium complexes with C-H compounds (as well as with molecular hydrogen) are summarized in Table VI. 3 [46], It can be seen that the addition of aryl and especially acetylene C-H bonds is thermodynamically more favorable than the addition of simple alkyl C-H bonds. Addition of an aryl C-H bond has been found to be at least 16 kcal mol less exothermic than H2 addition. However, on the basis of the Bryndza-Bercaw relationship [47]... 

Coordinatively unsaturated transition metal complexes are known to bind to a variety of donor molecules. Qualitatively, the enthalpies of these interactions range from very strong dative bonds with ligands such as phosphines and CO to very weak interactions with saturated hydrocarbons. Despite the importance of these interactions in determining structure and reactivity, limited quantitative data exist for metal-ligand bond strengdis. 

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