Distortions from trigonal prismatic geometry for several structurally characterized clathrochelate complexes: Significance of π-backbonding on clathrochelate coordination geometry

The crystallographic data for several clathrochelate complexes of the form [M(NOX)₃(B-n-Bu)₂] (where M = Co, Fe, Ru; NOX = 1,2-cyclohexanedione dioximato (C₆H₈N₂O₂); n-Bu = n-butyl) are presented. The data display an unexpected trend in distortion from trigonal prismatic geometry (Co(7.0°) <Ru(12.0°) <Fe(20.3°) ), which is different from that predicted by ligand field stabilization energy (LFSE) factors (Co<Fe<Ru). It appears that LFSE considerations in combination with π-backbonding effects are necessary to interpret the θ values of the three complexes, where the unusually small distortion for the ruthenium complex is a result of strong π-backbonding, evidenced by the unusually short Ru-N_av distance (1.972(4) Å). Crystal data: all three compounds are monoclinic, space group C2/c. Compound CoC₂₆H₄₂B₂N₆O₆: a=22.878(6), b=15.026(2), c=18.047(6) Å, β=109.95(2)°, V=5831.6(5) ų, Z=8, R=5.9, N_obs=3346. Compound FeC₂₆H₄₂B₂N₆O₆: a=23.046(3), b=14.920(2), c=18.064(4) Å, β=107.87(2)°, V=5911.6(5) ų, Z=8, R=5.4, N_obs=4681. Compound RuC₂₆H₄₂B₂N₆O₆: a=22.966(5), b=14.951(2), c=18.125(6) Å, β=107.39(2)°, V=5942(6) ų, Z=8, R=4.4, N_obs=4493.