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Look for the following data for Nickel IE1+IE2 2490 kj/mol IE3+IE4. 8800kj/mol Total. 11290kj/mol

Hence Nickel can show +2 oxidation state but not +4. But according to me Ni in +4 oxidation state can acquire 3d6 and 4s0 which has t2g3 configuration. Why do it has high ionisation energy?

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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.

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