Model Compounds and Theoretical Calculations

Many compounds have been synthesized, characterized, and studied as models for proposed intermediates in various homogeneous catalytic reactions. Here we discuss two examples. Complex 2.4 is proposed as a model that shows the mode of interaction between an organic hydroperoxide and high-valent metal ions such as Ti4+, V5+, and Mo6+. This type of interaction is considered to be necessary for the oxygen atom transfer from the hydroperoxide to an alkene to give an epoxide (see Chapter 8). 

This diagram illustrates many important points. First of all, it shows the mechanistic complexity that may be anticipated for even an apparently simple reaction, reductive elimination of acetone from 2.5. Second, it shows that RhCl(PPh3)3 is thermodynamically more stable than complex 2.5 by about 40 kJ/mol. However, complex 2.5 does not undergo spontaneous conversion to RhCl(PPh3)3 because it has sufficient kinetic stability ( 92 kJ/mol). Third, the high free energy of activation is associated with a ligand dissociation step that precedes the reductive elimination step. The five-coordinated intermediate, once 

What are the oxidation states and the valence electron counts of the metal ions in the complexes shown in Figs. 2.4, 2.6, and 2.11  

(a) In Fig. 2.2, which phosphine has a larger cone angle, and between R and R which one is more bulky (b) In Fig. 2.6, Reaction 2.3, the six-coordinated rhodium complex may be expected to undergo what further reactions (c) Draw schematic diagrams to show how bonding occurs in Ti4+-alkene and Pt2+-alkene. (d) It is difficult to isolate W(C2H5)6 but not W(CH3)6. Why  

(a) PR, and R. (b) Alkene insertion into Rh-H bond followed by reductive elimination of alkane, (c) No back-donation from Ti4+(d°) but back-donation from Pt2+(d8). (d) Facile /3-elimination from C2H5. 

---

Information about the catalytic cycle and catalytic intermediates is obtained by four methods kinetic studies, spectroscopic investigations, studies on model compounds, and theoretical calculations. Kinetic studies and the macroscopic rate law provide information about the transition state of the rate-determining step. Apart from the rate law, kinetic studies often include effects of isotope substitution and variation of the ligand structure on the rate constants. 

---