. Our calculations show that the transition from Q0 to Q1 represents the largest contribution to D for all complexes. With relatively similar SOC average matrix elements between Q1 and Q0 of 325, 305 and 295 cm -1 and considerably different Q1-Q0 energy gaps (obtained with the NEVPT2) of 790, 1169 and 415 cm -1 for [Co(1-3)](ClO4)2, respectively, it is evident that the latter largely determines D. Further analysis illustrates how the geometrical restrictions enforced by the rotaxane scaffolds influence the electronic properties of the metal ion and so modulate the value of D. For the three complexes, Q0 is highly multiconfigurational (see ESI), the electronic configuration shown in Figure (dxz) 2 (dyz) 2 (dxy) 1 (dx2-y2) 1 (dz2) 1 , contributes only 50% to Q0 for [Co(1-2)](ClO4)2. In contrast for [Co(3)](ClO4)2, the electronic configuration (dxz) 2 (dyz) 1 (dxy) 2 (dx2-y2) 1 (dz2) 1 represents 52 % of Q0. Q1 is dominated by the electronic configuration (dxz) 1 (dyz) 2 (dxy) 2 (dx2-y2) 1 (dz2) with contributions of 69%, 76% and 60% for [Co(1-3)](ClO4)2). Because of this, the dxy orbital has a significant population in the excited state and mainly controls the Q1-Q0 gap. The stability of the dxy orbitals is determined by the intramolecular interactions in the xy plane (Figure ). In the case of the complexes [Co(1-2)](ClO4)2, one of the d lobes is projected along a Co-N bond, increasing the energy of the dxy orbital. Both complexes display similar angles and the energies of their dyz orbitals are approximately the same (2.5 eV). The smaller Q1-Q0 gap in [Co(1)](ClO4)2 in comparison with [Co(2)](ClO4)2 is due to a relatively large contribution of 20% from the lowest energy configuration to Q1 for the former in contrast with only 7% for the latter. For [Co(3)](ClO4)2, the N-Co-N angles are closer to 90° and the dxy orbital does not significantly overlap with any of the Co-N bonds (Figure ). Consequently, the dxy orbital is more stable in [Co(3)](ClO4)2 resulting in a significantly smaller Q1-Q0 gap and a more negative D term.