Coordination and Dissociation

T.W. Root, G. Fischer, and F.D. Schmidt, Electron energy loss characterization ofNO onRh(lll). I. NO coordination and dissociation, J. Chem. Phys. 85(8), 4679-4686 (1986). 

Figure 27. MP2/II free-energy profile (in kilocalories per mole) for the entire catalytic cycle including solvation by ethylene, corrected by the thermal contribution of -1-10 and —10 kcal/mol upon each coordination and dissociation, respectively [77],...

The most fundamental step is the simple coordination and dissociation of ligands (Scheme 1.3). This is important because a stable complex cannot coordinate the substrate, but must first dissociate a ligand. Although some ligands are sufficiently labile to dissociate under mild conditions, in other cases it is necessary to use heat or light to achieve this. Often, reaction conditions are dictated by this initial dissociation. 

An efficient catalytic cycle requires a facile entry and exit of the ligand. Both coordination and dissociation of hgands must occur with low-activation free energy. Labile metal complexes are therefore essential in catalytic cycles. Coordinatively unsaturated complexes containing an open or weakly coordinated site are labile. 

A variety of reaction chemistry has already been eovered, sueh as Lewis base coordination and dissociation, and redox chemistry. In those specifie eases, one isolates new products that retain pentadienyl ligands. This section will therefore focus on reactions in which pentadienyl ligands are altered, as may oeeur via a coupling reaction, and on applications of metal pentadienyl eompounds in materials and catalytic processes. 

Variational RRKM theory is particularly important for imimolecular dissociation reactions, in which vibrational modes of the reactant molecule become translations and rotations in the products [22]. For CH —> CHg+H dissociation there are tlnee vibrational modes of this type, i.e. the C—H stretch which is the reaction coordinate and the two degenerate H—CH bends, which first transfomi from high-frequency to low-frequency vibrations and then hindered rotors as the H—C bond ruptures. These latter two degrees of freedom are called transitional modes [24,25]. C2Hg 2CH3 dissociation has five transitional modes, i.e. two pairs of degenerate CH rocking/rotational motions and the CH torsion. 

Coordination results in a lengthening of the S-N bond involving the coordinated nitrogen atom by ca. 0.1 A this N atom is also displaced out of the PN2S2 plane by 0.63 A. The Pt-N bond in 13.6 is weak and dissociation of the adduct occurs in solution. 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

The THF ligand of (Ti5-Mc4C5-SiMe2-N Bu)Crni(THF)Ph is only weakly bound, and dissociates in solution. Indeed, with sterically more demant g alkyl groups base-ftce five-coordinate alkyl con lexes, e.g. (n -Me4C5-SiMe2-N Bu)CrCH2SiMe3, could be pr ared. 

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