A square-pyramidal organochromium(V) compound.

The mononuclear, five-coordinate organochromium(V) compound [NBu(4)][CrO(C(6)F(5))(4)] (1) has been obtained as a dark red solid in moderate yield by treatment of the homoleptic organochromium(III) derivative [NBu(4)](2)[Cr(C(6)F(5))(5)] with NO[BF(4)] in CH(2)Cl(2) solution under an oxygen atmosphere. The Cr(V) centre in the [CrO(C(6)F(5))(4)](-) anion shows a square-pyramidal geometry with the four C(6)F(5) groups in the basal positions and the oxo ligand in the apical one (X-ray). The short Cr-O distance (153.8(2) pm) suggests a high degree of triple bond character for the chromyl unit. The EPR spectrum of 1 in solution shows an isotropic signal with g(iso) = 1.995(1) and a rich hyperfine structure due to coupling with the (53)Cr isotope [a(Cr) = 46.95(4) MHz] as well as with the ortho-F and meta-F substituents of the C(6)F(5) rings [a(F) = 4.20(2) MHz and a'(F) = 2.12(2) MHz] in keeping with the presence of non-interacting, fast tumbling, paramagnetic [CrO(C(6)F(5))(4)](-) units (d(1)). In the solid state, however, both the microscopic (EPR) and macroscopic magnetic properties (isothermal magnetisation and thermal dependence of the magnetic susceptibility) suggest the existence of weak ferromagnetic interactions with T(C) = 0.20(2) K. Such magnetic interactions may probably be favoured by π interactions between C(6)F(5) rings of neighbour [CrO(C(6)F(5))(4)](-) units in the crystal.