Activation by Second-Row Transition-Metal Atoms

3 Methane activation by second-row transition-metal atoms 

None of the neutral second-row transition metal atoms react with methane at 300 K, but some of them (Rh and Pd) react with ethane and larger alkanes [15]. Ab initio calculations [15] indicate that Y, Zr, Nb, Ru and Rh atoms produce very stable insertion products H-M-CH3 in the reaction (1) with an activation barrier ( 0 4-20 kcal/mol), which is much lower than the C-H bond dissociation energy (103 kcal/mol). Some metals (Nb, Ru, Rh) have to change spin multiplicity [15] in the course of the reaction (1). 

In addition to this diabatic state the triplet excited covalent state localized on the activated C-H bond has to be taken into account [43]. Three electrons in three orbitals can be represented by the second doublet state wave function, Eq. (3). The coefficients in both expansions, Eq. (2)andEq. (3), are obtained from the requirement S2 f = 5(5+l) k, where the total spin quantum number, S=0.5, corresponds to the doublet state [44]. 

At the beginning of the quantum chemistry era London proposed an approximate VB solution of the three-center, three-electron problem in the form [28]  

We consider now the insertion reaction, Eq. 1, for M=Y with the optimized transition state, where the Y-H distance is much longer than the C-H separation both are shorter than the Y-C distance. The following ratios are introduced  

---

Table 6 PCl-80 Energies (kcal mol" ) of Minima and C-H Insertion Saddle Points on Potential Energy Surfaces for Ethylene Activation by Second Row Transition Metal Atoms ...

---