Hydricities

However, hydride removal creates a vacant coordination site, so AG°H includes the AG of solvation for The interpretation of such data is most straightforward if the coordinatively unsaturated that is generated by abstraction of the hydride does not become solvated—for example, when is a planar 16-electron complex. Many of the results given in Table 3.5 are thus for the removal of H from a five-coordinate cation. 

The thermodynamic hydricity of an M-H bond is related to the ionicity of that bond, which can be calculated from the quadrupole coupling constant (available from the NMR spectrum of the M-D analog). Such data can be compared to rate constants for H transfer, or kinetic hydricities. The rate constants for transfer of the hydride in a series of complexes to trityl cation in CH Cl (Equation 3.130 and Table 3.6), - and from a series of CpRu(P-P)H complexes to the iminium cation in Equation 3.131 (Table 3.7) have been measured. 

With each acceptor the kinetic hydricities cover a range of about 10 The data in Table 3.6 show that many hydride complexes are better H donors than EtjSiH, that second- and third-row hydride complexes are faster H donors than first-row ones, and that electronic factors are more important than steric factors (the substitution of Cp for Cp or PRj for a CO increases the rate). The data in Table 3.7 show that the rate of H transfer from CpRu(P-P)H increases as the size of the chelate ring decreases. (The effect is similar to that of chelate ring size on the thermodynamic hydricity of the five-coordinate hydrides in Table 3.5.) Smaller rings make CpRu(P-P) more pyramidal and raise the energy of its vacant hydride acceptor orbital, making hydride donation from CpRu(P-P)H more favorable.  

The hydride complexes containing metals on the left of the periodic table (i.e., those of Zr, Ti, and Nb) have the most hydridic character, as expected by the electropositive character of these metals, but the difference has proven difficult to quantify. - The relative rates at which such hydrides react with acetone or methanol have been offered as evidence. However, it is possible that acetone and methanol react only after coordination. In this case ttie observed rates may really be the rates of ligand dissociation or coordination.  

The M-H bond behaves as a nucleophile in a number of reactions related to hydride transfer, such as Equations 3.132 and 3.133. However, the range of nucleophilicities observed in these reactions is not very large (about 10 ), and steric effects are much more 

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Acetates. The acetates of monohydric phenols are usually liquids, but those of di and tri-hydric phenols and also of many substituted phenols are frequently crystaUine sohds. They may be prepared with acetic anhydride as detailed under Amines, Section IV,100,7. 

The reaction is particularly facile with di- and tri-hydric phenols. Thus P-resorcyllc acid is readily obtained by passing carbon dioxide through a boiling aqueous solution of the potassium or sodium salt of resorcinol ... 

Glycols, poly-hydric alcohols, polyhydroxy aldehydes and ketones (sugars)... 

When dealing with esters of water-soluble, non steam-volatile, poly-hydric alcohols e.g., ethylene glycol or glycerol), the distillate consists of water only (density 1 00). The water soluble, non-volatile alcohol may be isolated by evaporation of the alkahne solution to a thick syrup on a water bath and extraction of the polyhydric alcohol from the salt with cold ethyl alcohol. 

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). 

The problem of the isolation of quaternary salts, even when fornied, is,-in some cases, an acute one. Water or ethanol is friBquently held very tenaciously and this possibility may be the reason that so many workers still use non-hydrie solvents, such as benzene, despite the fact that reactions in such solvents are usually slow clearly the best solvents are the non-hydric ones of high dielectric constant. 

Note that all the zero-order rate constants are essentially equivalent except those for the poly-hydric alcohols which are exactly half the value of the others. Ingold et al (Ref 49a) interpret this to mean that the rate of attack of nitronium is the same for both OH groups of the glycol molecule. Since there are two such groups the overall rate constant k0 is Vi that for monohydric alcohols. The explanation for the observed k0 for glycerol is more complex. In essence it consists of postulating that the two outside OH s are readily nitrated, ie, the 1-OH is nitrated at the same rate as the 3-OH, but the middle OH is nitrated much more slowly... 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

Aldehydes hydric alcohols. and secondary alkyl amines... 

The synthesis, characterization, and the relative thermodynamic hydricities of hydridoplatinum(II) species-containing bidentate phosphine ligands [PtHL2]+ (L2 = dppe, depe, dmpe, l,3-bis(dimethylphosphino)propane [dmpp]) have been determined, with the best hydride donor found to be [PtH(dmpe)2]+.365... 

Vepraskas MJ, Faulkner SP. Redox chemistry of hydric soils. In Richardson JL, Ve-praskas MJ, editors. Wetland Soils Genesis, Hydrology, Landscapes, and Classification. Boca Raton CRC Press, Taylor Francis Group 2001. pp. 85-105. 

Lespieau, R., Synthesis of Hexitols and Pentitols from Unsaturated Poly-hydric Alcohols, II, 107-118 Levi, Irving, and Purves, Clifford B., The Structure and Configuration of Sucrose (oipta-D-Glucopyranosyl beta-D-Fructofuranoside), IV, 1-35 Liggett, R. W., and Deitz, Victor R., Color and Turbidity of Sugar Products, IX, 247-284... 

Michael addition of di- and tri-hydric phenols to /V-cinnamoylimidazoles followed by a lactonisation offers a route to 4-aryI-3,4-dihydrocoumarins and their [/]-benzologues <00S123>. The lactonisation of the naphthoquinone derivative 66 is sensitive to the acidic cyclising medium and it is possible to obtain the thermodynamically less stable o-quinone derivative exclusively (Scheme 44) <00TL3007>. Some related quinones have been obtained from 1-benzylisoquinolines via an arylnaphthoquinone <00T6O23>. 

Although orbital hybridizations and molecular shapes for hypovalent metal hydrides of the early transition metals and the normal-valent later transition metals are similar, the M—H bonds of the early metals are distinctly more polar. For example, metal-atom natural charges for YH3 (+1.70), HfH4 (+1.75), and TaHs (+1.23) are all significantly more positive than those (ranging from +0.352 to —0.178) for the homoleptic hydrides from groups 6-10. Indeed, the empirical chemistry of early transition-metal hydrides commonly reveals greater hydricity than does that of the later transition-metal hydrides. 

DuBois et al. carried out extensive studies on the thermodynamic hydricity of a series of metal hydrides [13, 15-19]. The determination of thermodynamic hydricity generally requires several measurements (coupled with known thermochemical data) to constitute a complete thermochemical cycle. As with other thermodynamic cycles, obtaining reliable values in an appropriate solvent can be a difficult challenge, and this is sometimes coupled with problems in obtaining reversible electrochemical data. Scheme 7.2 illustrates an example in which the hydricity of cationic monohydrides have been determined. 

Scheme 7.3 Thermodynamic cycle for determination of the hydricity using heterolytic cleavage of hydrogen.

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

HMSO (1987) Methods for the Examination of Waters and Associated Materials (40548). Selenium in Waters 1984 Selenium and arsenic in sludges, soils and related materials, 1985 a note on the use of hydric generator kits, London. 

Pivovarenko VG, Klueva AV, Doroshenko AO, Demchenko AP (2000) Bands separation in fluorescence spectra of ketocyanine dyes evidence for their complex formation with mono-hydric alcohols. Chem Phys Lett 325 389-398... 

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