Enthalpies dimeric oxides

As was discussed in Section VI for the reactions of enolates with the oxidizing cation Cu(II), Fe(in) also oxidizes enolates to form products associated with formal radical dimerization. The presumed intermediate enoxyl radicals react with FeCls, not by formation of Fe(IV) enolates, but rather from an a-chlorinated ketone product - - FeCl2, thereby providing a thermochemical upper bound for the enthalpy of formation of these haloketones. 

The thermochemical cycle in Scheme 3 was used for this purpose for the Mn, Mo, and Fe dimers. The formal potentials E° (M2/2M ) were derived from equilibrium constant measurements of the redox equilibria, Eqs 26 and 27, by use of suitable reducing agents. When these formal potentials were combined with the anion oxidation potentials, the M-M BDEq could be calculated from Eq. 28 as 117 kJ moE for Mn2(CO)io, 92 kJ mol for Cp2Mo2(CO)e, and 105 kJ moE for Cp2Fe2(CO)4. For the Mn dimer, estimates for the entropy change were available and eventually led to an estimated Mn-Mn BDE of ca 159 kJ moE in enthalpy terms. 

In order to understand the observation that oxygen in the oxidation state +0.5 forms salts that contain the O2" cation with a variety of anions while sulfur in the same oxidation state forms a dimer of the composition S4 [=(S2 )2]> the gas phase dissociation enthalpy of 84 giving two 82 assessed by quantum chemical calculations [20, 21], i.e. [(g)=gaseousj ... 

At that time the radical nature of solutions of "Cr(CO)3Cp was not yet proven. Hydrogenation of solutions of the dimer had been reported by Muetterties and coworkers,42 but the mechanism of oxidative addition was not known. In addition, the complex [Cr(CO)3Cp]2 is known to catalyze hydrogenation of dienes via H-Cr (CO)3Cp that is regenerated under catalytic conditions. Thermochemical data for reaction 10.54 were obtained by indirect thermochemical cycles, however direct measurement of the enthalpy of reaction of H2(g) and "Cr(CO)3Cp allowed the determination of H-Cr(CO)3Cp bond strength as 62.3 1 kcal/mol.74... 

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