Cobalt is +3, so it is d6, which can be diamagnetic in the low-spin form, and paramagnetic with 4 u.p.e. in the high-spin form. The implication of this experiment is that water in the aqua ion has sufficient field strength to cause low-spin, but when the water is displaced by the excess fluoride, the lower ligand field strength of fluoride causes ∆o to shrink, and a high-spin state is induced. Water is not particularly high on the spectrochemical series, but evidently high-enough for cobalt +3 to be low-spin.