E.s.r. and u.v.–visible spectral studies of the complexes [CoX₂(PPhF₂)₃](X = Br or I) indicate a distorted trigonalbipyramidal co-ordination geometry at cobalt with two axial and one equatorial phosphorus ligand. Cobalt-59 hyperfine and ³¹P superhyperfine coupling constants have been obtained by analysis of the e.s.r. spectra of frozen solutions in dichloromethane–paraffin oil at 100 K and interpreted in terms of delocalisation of unpaired electron spin density. The X-ray structure of [CoBr₂(PPhF₂)₃] has been determined in order to substantiate the results of the spectroscopic investigations. Crystals of [CoBr₂(PPhF₂)₃] are monoclinic, P2₁/n, with a= 8.428(4), b= 16.304(8), c= 17.206(9)Å, β= 98.58(6)°, Z= 4. The molecules display a trigonal-bipyramidal geometry with the two bromine atoms equatorially substituted. Principal bond lengths are: Co–P_ax. 2.158(5) and 2.158(5), Co–P_eq. 2.120(5), and Co–Br_eq. 2.383(2) and 2.367(3)Å.