  The copper fluoride Cu$_2$F$_5$ is a compound with 2D-magnetic exchange
interactions between the Cu ions in the $S=1$ and $S=\frac{1}{2}$ spin-states.
Using ab-initio calculations, we predict that the existence of 5% vacancies in
the fluoride sublattice of Cu$_2$F$_5$ results in the drastic transformation of
the spin-state of all copper ions and the final spin-states are $S=\frac{1}{2}$
and $S=0$. Consequently, the anisotropy of magnetic interactions increases, and
the 1D linear chains of the Cu $d^9$, $S=\frac{1}{2}$ ions appear. We also
propose a microscopic mechanism of such exchange interaction transformation via
CuO$_6$ octahedra elongation.
