Bond formation free energies

With saturated electrophiles [n-BuBr, Eq. (1.43)] die SET is to a less nucleophilic leaving group [N - + R X (E) - N R + X . The driving force for nucleophile-electrophile electron-transfer reactions is the redox potential for the nucleophile in the solution matrix (HO-/HO-, +0.92 V vs. NHE in MeCN)42 plus the nucleophile-substrate bond-formation free energy (—AGBF, n-Bu—OH 349 kJ mol-1) ... 

TABLE 10.3 Apparent Metal-Ligand Covalent-Bond-Formation Free Energies (— AGBF) for Several Manganese, Iron, and Cobalt Complexes... 

Figures 10.2 and 10.3 illustrate the electrochemistry for several copper(II) and copper complexes in MeCN. The redox potentials for these copper complexes and their ligands are summarized in Table 10.5 (related data for aqueous media are given in Table 10.6).3,8-13 In addition, the shift in redox potential (AE) for the free ligand (L) and when bonded in a complex (CuL ) is tabulated. This quantity is a measure of the apparent copper-ligand covalent-bond-formation free energy (—AGBF) ...

TABLE 13.9 The Bond-Formation Free Energies (-AGbf) for (por)Fera—R and (por)Coni—R Bonds... 

In summary, the electrochemical results indicate that the alkyl-metal bond-formation free energies range from 54 to 146 kJ mol-1 for iron porphyrins and from 84 to 159 kJ mol-1 for cobalt porphyrins. The maximum bond energies are for primary alkyl groups bonded to [(MeO)4TPP]Con and (OEP)Fen porphyrins. The porphyrin dianions [(porT)nFe and (porr)nCo ] facilitate the reduction of C02 to CO via the transient formation of a metal-carbon bond [(por7)M—C(0)0- — AGBF > 50 kJ mol-1 for iron porphyrins]. Thus, iron and cobalt porphyrins are especially effective electrocatalysts for the reduction of C02 ... 

An equivalent approach has been used to evaluate metal-ligand covalent bond-formation free energies (—AGbf) for metal complexes and metal oxygen... 

Table 12 Radical strength of oxygen radicals (Y0-) in terms of their YO-H bond formation free energies, and dissociative bond energies for H-R molecules...

The formation of the same iron-oxygen covalent bonds from either (1) oxidized iron plus oxy anions via electron-transfer (redox) reactions or (2) radical-radical coupling reactions is summarized in Table 3-11. The valence-electron hybridization for the iron center is included as well as the spin state and estimated covalent bond-formation free energy (AGbf)- A similar set of reactions and data for iron-porphyrin compounds is presented in Table 3-12. Section a emphasizes that, just as the combination of a proton with a hydroxide ion yields a covalent H-OH bond (Table 3-11), (1) the combination of protons and porphyrin dianion (Por -) yields covalent porphine (H2Por), and (2) the addition of Lewis acids (Zn2+ or Fe2+) to porphine (H Por) oxidatively displaces protons to give covalent-bonded ZnilPor and Fe iPor. 

Table 7 Redox potentiais for iron compiexes and iigands, and their apparent metai-iigand covaient bond-formation free energies...

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