Carbon Dioxide Insertion into M-H Bonds

Aresta et al.. Reaction Mechanisms in Carbon Dioxide Conversion, DOI 10.1007/978-3-662-46831-9 4 

There are a number of early kinetic investigations on such reactions that reveal their reversibility [6-8] and irreversibility [9] and the reaction mechanism [6,7,9]. More recent studies of the reaction of CO2 with monohydrido complexes such as [Re(bpy)(CO)3H] [10] and [Os(bpy)2(CO)H] [10] have clearly shown that the insertion is a second-order reaction (4.1) influenced by the solvent. 

The first order in both the metal hydride and CO2 confirms that 5 and 7 are possible transition states (TSs). The reaction shows an inverse isotope dependence (kii/ d = 0.55) [8]. The activation parameters are reported to be AH = 53.5 kJ/mol and A5 = —138 J/mol K. If the ancillary ligands are changed, one can observe an effect on the reaction rate. This effect can be explained by considering that ancillary ligands may 

Structure 4.1 Possible transition state (7) for normal CO2 insertirai into a M-H bond 

The tendency of a transition metal hydride to transfer H to a substrate is called hydricity [ 12]. It is possible to determine the Gibbs free energy of the splitting of the covalent polar M-H bond to afford a metal cation and the hydride ion in solution. The hydricity is not parallel to the polarity of the M-H IxMid, nor can it be predicted on the basis of the electronic structure of the metal atom. It is a complex property that can be modeled for transition metal hydrides using multiparameter approaches. The hydricity concept applies to the interaction of M-H bonds with CO2 as well [13]. The reactivity of M-H bonds toward CO2 is linked to reactions that may have industrial interest, such as the hydrogenation of CO2 to afford formic acid (4.2) and the electrochemical reduction of CO2 to other Cl or C1+ molecules (4.3). 

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