Distortions from trigonal prismatic geometry for several structurally characterized clathrochelate complexes: Significance of pi-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 degrees) < Ru(12.0 degrees) < Fe(20.3 degrees)), which is different from that predicted by ligand field stabilization energy (LFSE) factors (Co < Fe < Ru). It appears that LFSE considerations in combination with pi-backbonding effects are necessary to interpret the theta values of the three complexes, where the unusually small distortion for the ruthenium complex is a result of strong pi-backbonding, evidenced by the unusually short Ru-N-av distance (1.972(4) Angstrom). 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) Angstrom, beta = 109.95(2)degrees, V = 5831.6(5) Angstrom(3), Z = 8, R = 5.9, N-obs = 3346. Compound FeC26H42B2N6O6: a = 23.046(3), b = 14.920(2), c = 18.064(4) Angstrom, beta = 107.87(2)degrees, V = 5911.6(5) Angstrom(3), Z = 8, R = 5.4, N-obs = 4687. Compound RuC26H42B2N6O6: a = 22.966(5), b = 14.951(2), c = 18.125(6) Angstrom, beta = 107.39(2)degrees, V = 5942(6) Angstrom(3), Z = 8, R = 4.4, N-obs = 4493.