  We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$
diamond-like units can host high temperature superconductivity. The layer
appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic
physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co
pairs form a square lattice. The low energy physics near Fermi energy can be
described by an effective two orbital model. Without considering interlayer
couplings, the two orbitals are effectively decoupled. This electronic
structure satisfies the "gene" character proposed for unconventional high
temperature superconductors. We predict that the leading superconducting
pairing instability is driven from an extended $s$-wave (s$^\pm$) to a $d$-wave
by hole doping, for example in Ba$_{1-x}$K$_x$Co$_2$N$_2$. This study provides
a new platform to establish the superconducting mechanism of unconventional
high temperature superconductivity.
