The solid state structures of 1-Ln were determined by single crystal XRD (see Figure for depictions of 1.08 Å). These geometries were quantified by Shape2.0 (see Supporting Information Tables ), where the lowest value shape indices of 1-Sm (3.728) and 1-Eu (3.718) correspond to a square-based pyramid and that of 1-Yb (3.311) is a tetrahedron. The distortion of the coordination spheres in five-coordinate complexes can be gauged by the geometric parameter τ5 = (β -α)/60, where β and α are the largest and second-largest angles in the coordination sphere, respectively. The τ5 parameter quantifies the degree of trigonality within the structural continuum between square-based pyramidal (τ5 = 0) and trigonal bipyramidal (τ5 = 1), and the τ5 values of 1-Sm (0.8535) and 1-Eu (0.8560) are closer to the latter (see Supporting Information Table ). Similarly, four-coordinate systems can be defined by the geometric parameter τ4 = [360 -(α + β)]/141, which quantifies the degree of tetrahedrality between square planar (τ4 = 0) and tetrahedral (τ4 = 1). For 1-Yb, the complex is distorted from an ideal tetrahedral geometry with a τ4 value of 0.7976 (see Supporting Information Table ).