  We study a generalized Anderson model that mixes two localized configurations
--one formed by two degenerate doublets and the other by a triplet with
single-ion anisotropy $DS_z^2$-- by means of two degenerate conduction
channels. The model has been derived for a single Ni impurity embedded into an
O-doped Au chain. Using the numerical renormalization group, we find a
topological quantum phase transition, at a finite value $D_c,$ between two
regular Fermi liquid phases of high (low) conductance and topological number $2
I_L/\pi = 0$ (-1) for $D < D_c$ ($D > D_c$), where $I_L$ is the well-known
Luttinger integral. At finite temperature the two phases are separated by a
non-Fermi liquid phase with fractional impurity entropy $\frac{1}{2}{\rm ln}2$
and other properties which remind those of the two-channel Kondo model.
