Organo-f-Element Thermochemistry. Absolute Metal-Ligand Bond Disruption Enthalpies in Bis(pentamethylcyclopentadienyl)samarium Hydrocarbyl, Hydride, Dialkylamide, Alkoxide, Halide, Thiolate, and Phosphide Complexes. Implications for Organolanthanide Bonding and Reactivity

Absolute samarium—ligand bond disruption enthalpies in the series Cp'₂Sm-R/X (Cp' = η⁵-(CH₃)₅C₅) have been measured by iodinolytic and alcoholytic isoperibol titration calorimetry of Cp'₂Sm/Cp'₂Sm-I)_n,(Cp'₂Sm-O^tBu)₂/Cp'₂Sm-R/X ensembles in toluene. Derived D(Cp'Sm-R/X) values in toluene solution are as follows (kcal/mol, R/X): 47.0 (1.5), CH(SiMe₃)₂; 45.0 (1.5), η³-C₃H₅; 93.2, CCPh; 54.2 (3.0), H; 48.2 (1.8), NMe₂; 82.4 (3.5), O^tBu; 81.3 (1.0), OCH(^tBu)₂; 97.1 (3.0), Cl; 83.6 (1.5), Br; 69.4 (2.4), I; 7 3.4 (2.4), SⁿPr; 32.6 (2.0), PEt₂. D(Cp'₂Sm-THF) and D(Cp'₂Sm(THF)-THF) values were also determined in toluene and are 7.3 (0.4) and 4.9 (1.0) kcal/mol, respectively, while D(Cp'₂Sm(THF)-I) was found to be 72.7 (2.9) kcal/mol. The observed D(Sm-halogen) parameters are close to the D₁ values of the corresponding samarium trihalides. Important trends in D(Cp'₂Sm-R/X) include a relatively small value of D(Sm-H)-D(Sm-alkyl), a large value of D(Sm-I)-D(Sm-alkyl), and generally strong bonds to group 15 and group 16 ligands. A variety of Sm-centered ligand transposition and oxidative addition/reductive elimination processes are analyzed in light of the present data. The formation of strong Sm-heteroelement bonds makes an important contribution to the driving force. Hydrocarbon functionalization via dinuclear Sm(II)→ Sm(III) oxidative addition processes is only expected to be exothermic in special cases.