Late transition states, methane

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

The ALMO-EDA scheme was used to analyze a wide range of methane CH bond cleavage transition states for metal-ligand complexes that have been reported experimentally to promote or catalyze CH activation [57]. This included late transition metals such as Rh(lll), Rh(l), lr(ni), Pt(n), Pd(ll), Ru(II), and Au(III) and early transition metals such as Sc(lll) and W(ll). Ligands attached to these metal centers range from phosphine, N-heterocycle, and halide to 0-donors. Three specific examples of transition states analyzed and their ALMO-EDA interaction energy components are shown in Scheme 10. 

Overall, late transition metals, such as Au(III), Rh(III), Pd(II), and Pt(II), with a variety of N-donor and O-donor ligands were often found to have electrophilic transition states. Ru(II) and Ir(III) with N-donor and O-donor ligands as well as [Cp (PMe3)IrMe] were found to act in an ambiphilic manner towards methane. Despite substitution-type transition states having both a Lewis acid metal and a Lewis base ligand that interacts with the CH bond of methane, these transition states are not always ambiphilic. For example, in the transition state between d° (Cp )2ScMe and methane, the (Cp )2ScMe complex acts as a nucleophile. Other nucleophilic transition states involved methane reaction with W(II), Ir(I), and Rh (I) complexes. 

The photo-excited states of some inorganic complexes are able to abstract an H atom from alkanes. Finally, high-oxidation-state late transition-metal oxo complexes snch as the active Fe =0 or Fe -0 species of cytochrome P450 methane mono-oxygenase enzyme, are also able to abstract an H atom from alkanes, which then leads to their hydroxylation (see Chap. 18). It is also possible, in some cases, to remove an H atom from Hj (see Chap. 15) ... 

The late barrier with the elongated H-H bond should, according to the formalism of Chapter 4, be qualified as a loose transition state, in which the molecule is even allowed to rotate in a plane parallel to the surface. Interestingly, we will also find a loose transition state for the dissociation of methane, to be discussed in Section 6.4.2. 

In the third transition series, the 5d orbital expands and the 6s orbital contracts due to relativistic effects, as we have seen above, and many of the elements of this series display higher oxidation states than those in the second series. The greater participation of the 5d orbital in the late part of the series can change the qualitative bonding picture. An example comes from the activation of methane by Pf ". In nonrelativistic calculations, the PtCHj product has only a single bond between the Pf " d ion and the methylene, and the unpaired electron is located on the methylene carbon. When scalar relativistic effects are introduced, the d s state of Pf " is stabilized, and both electrons of the methylene can bond to the platinum ion, resulting in a double bond. The unpaired electron is now located on the platinum. The Pt-C bond energy more than doubles with scalar relativistic effects, from 200 to 450 kJ/mol (Heinemann et al. 1996). 

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