The Lanthanide Contraction Is a Variable

Upon examination of the bond distances of the recently reported series of [Ln(SST)₃(THF)₂] [Ln = lanthanides, SST = tris(trimethylsilyl)siloxide (OSi(SiMe₃)₃), and THF = tetrahydrofuran] compounds, it was found that over the Ln-series (La through Lu), the Ln-O(THF) bond changed by 0.257 Å, whereas the Ln-O(SST) bond varied by 0.164 Å. Examination of all similarly ligated Ln-O(THF) (Ln = La vs Lu) structures available in the Cambridge Structural Database (CSD) revealed that this previously unreported, increased Ln-contraction is pervasive. Further evaluations showed that this enhanced Ln-contraction also occurs for pyridine (py) in the [Ln(SST)₃(py)₂] family as well as the average Ln-N(py) (La vs Lu) structure distances recovered from the CSD. Additional ligands, such as halides (Cl and I) were found to display this enhanced Ln-contraction, while other species (i.e., cyclopentadienide, alkoxide, SST, and dimethyl sulfoxide) yielded a "normal" Ln-contraction (La-L vs Lu-L). Gas-phase electronic structure density functional theory calculations were carried out to evaluate the molecular orbital influence on the Ln-contraction between Ln-O(SST) and Ln-O(THF). The calculated [Ln(SST)₃(THF)₂] structures were found to demonstrate the same capricious Ln-contraction. Based on these studies, one can say that the variability of the Ln-contraction noted in the [Ln(SST)₃(THF)₂] experimental data is due to the different bonding types, ion-ion for the Ln-SST bond versus ion-dipole for the Ln-THF bond.