Synthesis, characterisation and magnetic properties of octahedral chromium(iii) compounds with six C-donor ligandsElectronic supplementary information (ESI) available: X-ray crystallographic data (CIF), graphic representations of the bulk magnetic properties of 3 (isothermal magnetisation measurements and thermal dependence of its magnetic susceptibility), as well as graphic comparisons between calculated and experimental EPR spectra. CCDC reference numbers: 777393 (2·CH2Cl2) and 777394 (3·1.75C

The homoleptic, square pyramidal organochromium( iii ) compound [NBu 4 ] 2 [Cr(C 6 F 5 ) 5 ] ( 1 ) reacts with excess organic isocyanides, CNR [R = t Bu, 2,6-dimethylphenyl (Xy)], under dissociation of the apical C 6 F 5 ligand to give the more saturated, singly charged complexes [NBu 4 ][ trans -Cr(C 6 F 5 ) 4 (CNR) 2 ] [R = t Bu ( 2 ), Xy ( 3 )], containing six monodentate C -donor ligands. These compounds exhibit an axially distorted octahedral structure (single-crystal X-ray diffraction) with the four C 6 F 5 groups defining the equatorial positions and the CNR ligands occupying the axial ones. Compounds 2 and 3 both behave as spin quartet species ( S = 3/2) at microscopic level (EPR spectroscopy), their macroscopic magnetic properties depending upon the nature of the terminal R group, as established by magnetisation measurements. When the R substituent is the saturated alkyl group t Bu, the compound ( 2 ) behaves as a simple paramagnet, with no magnetic interaction between individual Cr III centres along the whole temperature range measured (1.8-265 K). By contrast, a weak antiferromagnetic interaction is detected for compound 3 at low temperature with T N = 0.19(1) K. Since the closest intermetallic distances are similar in the crystals of 2 ·CH 2 Cl 2 and 3 ·1.75CH 2 Cl 2 ( ca . 1.1 nm), we conclude that the insaturation of the aromatic Xy group together with the extended intermolecular π-π stacking interactions between Xy rings observed in the crystal lattice of 3 ·1.75CH 2 Cl 2 (centroid-to-centroid distance: 0.35 nm) favour magnetic interaction between the individual magnetic centres. The bulk magnetic properties of the open-shell organochromium( iii ) compounds [NBu 4 ][ trans -Cr(C 6 F 5 ) 4 (CNR) 2 ], in which the paramagnetic Cr III centres (d 3 ) are surrounded by six monodentate C -donor ligands with approximately octahedral geometry, have been found to depend upon the nature of the distal R substituent.