Metal-ligand bond disruption enthalpies (D) have been determined in the series Cp'2MX2, Cp2MX2, Cp'MX3(M = Zr, Hf; Cp' = η5-Me5C5; Cp = η-C5H5; M = Zr, Hf; X = hydrocarbyl, hydride, alkoxide, amide, halide) and in Cp'2Zr(CO)2 by anaerobic isoperibol batch-titration calorimetry. Heats of solution in toluene were measured followed by heats of reaction with HCl, I2, C6F5OH, C6H5OH, CF3CH2OH, or t-BuOH in toluene. Derived D(M-X) values decrease in the order OH ≈Cl > alkoxide ≈ NH2 > phenoxide > I ≈ H > aryl > Me > alkyl, metallacyclopentane > η1:η5-CH2C5Me4 M-C σ bond > CO. By using D(Cl3M-Cl) as a reference point, D(M-X) values are found to be rather large (e.g., for M = Zr(R): 78 (H), 73 (Ph), 67 (Me) kcal/mol) and not highly sensitive to ancillary η5-cyclopentadienyl ligation. D(Hf-X)-D(Zr-X) is estimated to be ca. 4 kcal/mol. Ancillary alkoxide ligands enhance D(Zr-H) in the Cp'2ZrH2/Cp'2Zr(OR)H series by ca. 5 kcal/mol. The metallacycle Cp'2ZrCH2(CHEt)2CH2 exhibits negligible ring strain while that in the zirconaindan Cp'2ZrCH2CH2-o-C6H4 is ca. -10 kcal/mol. A plot of D(Zr-X) vs D(H-X) is not linear but shows very substantial scatter. However, reasonably linear plots are observed within ligand subgroups such as hydrocarbyls, alkoxides, and halides. This behavior can be qualitatively explained on the basis of metal and ligand electronegativities. The quantities D(M-H)-D(M-Me) and D(M-I)-D(M-Me) vary considerably across the transition-metal series and are informative indices of metal-ligand bonding. The former is small for the present group 4 compounds and the latter large. The present data are used to semiquantitatively interpret a number of group 4 centered transformations. Among the conclusions drawn are that β-H elimination processes are usually endothermic; many C-H activating cyclometalation processes are endothermic, hence entropically driven (e.g., Cp'2ZrPh2→Cp'η1:η5-CH2C5Me4) + PhH); Zr(II) →Zr(IV) oxidative additions are highly exothermic; D-(Cp'2Zr-benzyne) ≿120 kcal/mol; early transition metal/lanthanide/actinide M(η1:η5-CH2C5Me4) species are energetically poised to serve as intermediates in a number of addition and elimination processes; and the large magnitude of D(M-OR) is one major driving force for the formation of alkoxide-like end products in group 4 centered CO activation chemistry.
ASJC Scopus subject areas
- Colloid and Surface Chemistry