The coordination chemistry of the f-block elements is dominated by complexes with hard donor atom ligands formed through ionic bonds. Molecular species with covalent interactions were historically considered unlikely because the radial distributions of the valence orbitals are too close to the nucleus to participate in bonding. [1] However, this early assumption has slowly been eroded, beginning with the discovery of complexes such as [U(cot) 2 ] [2] (cot = cyclooctatetraene), in which it was demonstrated that the 5f orbitals overlap with ligand-based orbitals to form a covalent interaction. Complexes of this type have prompted fundamental and intensely debated questions in f-element chemistry: To what extent and generality is covalent bonding important? What are the limits of "hard-soft" interactions? And, crucial to understanding trivalent An/Ln separations, how do these interactions differ between An III and Ln III ions of identical ionic radii (i.e. how do relativistic effects affect coordination chemistry)? [3] Our approach to addressing these questions, in part, is to synthesize and characterize comparable, isostructural, homoleptic actinide and lanthanide complexes with increasingly "soft" heavier chalcogen donor ligands. Preparing Te-donor complexes is particularly challenging because there is no precedent for molecular actinide tellurides and few suitable