  We study theoretically the fluoro-nickelate series $A$NiF$_2$ ($A=$ Li, Na,
K, Rb, Cs) in the tetragonal $P4/mmm$ infinite-layer structure. We use density
functional theory to determine the structural parameters and the electronic
band structure of these unprecedented compounds. Thus, we predict these
materials as model $d^9$ systems where the Ni$^{1+}$ oxidation is realized and
the low-energy physics is completely determined by the Ni-3$d$ bands only.
Fluoro-nickelates of this class thus offer an ideal platform for the study of
intriguing physics that emerges out of the special $d^9$ electronic
configuration, notably high-temperature unconventional superconductivity.
