We futher confirmed these results using CASSCF-SO calculations (see Supporting Information). To compare with the HYSCORE experiment, we approximate solution phase structures by optimising the molecular geometry of each complex in the gasphase using density-functional theory (DFT) methods (see Supporting Information for details), and then use the optimised geometries to perform another set of minimal CASSCF-SO calculations. We then use the Hyperion package to calculate the relativistic hyperfine coupling tensors directly for the nuclei of interest (Tables and); note that the Hyperion method implicitly includes all through space dipolar and Fermi contact terms, as well as relativistic paramagnetic spin-orbit terms. Inspection of the computed hyperfine values show the C(Cp) and H(Cp) atoms couple more strongly than any other Cp tt ligand atoms. Hence, we simulate C and 1 H HYSCORE spectra using five C and three H nuclei from one Cp tt ligand, again using EasySpin; these calculations provide excellent reproductions of the experimental data (Figure ). The calculated Mulliken spin populations (Table ), show that only ~0.1% of the total spin density is transferred to the C1-C5 atoms of the three Cp tt ligands. However, we note that the density does increase slightly with increasing Ln 3+ atomic number (1-Ce < 1-Nd < 1-Sm).