Reaction-solution calorimetry results for a number of bis(cyclopentadienyl)zirconium(IV) and -mobium(IV) complexes were used to discuss the energetics of Zr-H, Zr-C, Zr-O, Zr-I, Nb-C, Nb-I, and Nb-S bonds. From the enthalpies of reaction of Schwartz's reagent, [Zr(Cp)2(Cl)H]n, with HCl, I2, and alcohols it was concluded that the zirconium-hydrogen bond does not seem to be significantly influenced by the polymeric nature of the substance. The reactions of the same complex with 1-hexene and cyclohexene, on the other hand, gave information on the thermochemistry of alkene insertion into the Zr-H bond and showed that in contrast to the expected order of reactivities (cyclic olefins being less reactive than linear 1-alkenes), the insertion of cyclohexene into the Zr-H bond of Schwartz's reagent is more exothermic than in the case of 1-hexene, although the Zr-C6H13 and Zr-C6H11 bond dissociation enthalpies are comparable. These results were also used to discuss the energetics of non-terminal olefins into the Zr-H bond of Schwartz's reagent. The enthalpies of formation of several complexes of the type Zr(Cp)2(Cl)OR (R = Me, Et, Pr, i-Pr, Bu, t-Bu, Oc, and Ph) were derived from the enthalpies of reaction of Schwartz's reagent with the corresponding alcohols, and the plot of ΔHf° [Zr(Cp)2(Cl)OR,c] against the enthalpies of formation of liquid ROH yielded an excellent unit-slope linear correlation. The derived Zr-OR bond dissociation enthalpies are rather constant for R = alkyl and are close to the average Zr-O bond dissociation enthalpy in the dimer [Zr(Cp)2Cl]2O. The reaction with HCl in two different solvents (toluene and isopropyl ether) was used to obtain the enthalpy of formation of this complex and also to demonstrate that identification of gas phase and solution bond enthalpy data or even the identification of solution bond enthalpy data in different solvents should be made with great caution, particularly when the solvation of one of the reactants is highly exothermic. The thermochemical study of the complexes Zr(Cp)2Me2 and Zr(Cp)2C4Ph4, through their reactions with HCl in toluene, allowed the comparison between the energetics of Zr-C(sp3) and Zr-C(sp2) bonds This comparison indicated a strain-free metallacycle in the tetraphenylbutadiene complex. The bond enthalpy data for M(Cp)2Me2 and M(Cp)2Cl2 (M = Zr, Nb) obtained in this work support the conclusion based on Pauling's electronegativity equation that the difference D̄(M-Cl) - D̄(M-Me) decreases with the increasing electronegativity of M. For M = Nb the enthalpies of reaction of the dimethyl complex with HCl and I2 were both measured, yielding the Nb-I mean bond dissociation enthalpy. Finally, the calorimetric study of the reactions of Nb(Cp)2(SPh)2 and Nb(Cp)2(SC6H4Cl-p)2 with HCl in isopropyl ether afforded the enthalpies of formation of these complexes and their Nb-S bond dissociation enthalpies. These latter values are similar, indicating that the exo or endo conformations of the thiolate ligands and the possible existence of S-S bonding in Nb(Cp)2(SPh)2 are not reflected in the energetics of Nb-S bonds.115727642774