Now the electronic configuration of nickel atom in ground state is 3d8 4s2. But in excited state, the 4s electron of central meal Ni-atom is shitted to 3d-sub shell.
Under this condition, the 3d sub shell occupied 10 electrons. That is, 3d-subshell has no unpaired electrons.
Now one vacant 4s orbital and three 4p orbital of outer shell are mixed to each other and form energetically equivalent four hybridized orbitals, involving sp3 hybridization.
These 4 vacant hybridized orbitals accept four-electron pair from four CO ligand forms L–M co-ordinate bond, resulting in the formation of tetrahedral Ni (CO) 4 molecule.