To probe the role of the rotaxane framework in determining D, we optimized the simplified model of the axle and macrocycle fragment common to the three molecules (Figure ). The D term obtained for this moiety is -93.9 cm -1 , a large negative value close to the experimental value for [Co(1-3)](ClO4)2. We further simplified the model to reflect a purely conformational constraint: the rotaxane framework was replaced by NH3 ligands (and one molecule of H2O in the case of 3), with the ligand-metal distances and related angles constrained to those obtained by SCXRD (SCXRD-derived models). Unsurprisingly, given the equivalent symmetry of their primary coordination sphere (Figure ), all three complexes exhibit similar d-orbital splittings (Figure ). Comparable splitting diagrams have been reported for other distorted sbpy and elongated octahedral Co II complexes that display large and negative D values. The calculated zero-field splittings for these SCXRD-derived models are in excellent agreement with the experimental data (Table ), reproducing both the trend in |D| ([Co(3)](ClO4)2 > [Co(1)](ClO4)2 > [Co(2)](ClO4)2) and giving values in close to quantitative agreement with the experimental data for the complexes derived from 2 and 3. Remarkably, even the de novo models gave predicted values of D in good qualitative agreement with those observed (Table ). These calculations show that the rotaxane framework itself is not of direct relevance in determining the value of D, but rather serves to impose a geometrically constrained environment on the metal ion analagous to that found in proteins said to exist in an entatic state.