Ionisation energy of Nickel
Metals with a $d^8$ oxidation state are stabilized by a square-planar molecular orbital configuration. In this configuration, the $\ce{d_{yz},d_{xz},d_{z^2}}$ orbitals are stabilized, while the $\ce{d_{xy}}$ orbital is slightly destabilized and the $\ce{d_{x^2-y^2}}$ is severely destabilized (however, in $d^8$ species this orbital is unoccupied). The amount of stability provided by the square planar configuration is far greater than the amount of stability provided by the low-spin $d^6$ configuration.

This is a good online powerpoint that shows the stabilization.