The unpaired electrons in Ni(CO)4  are

Ni(CO)4 .  The O. S. of Ni is Zero. Electronic configuration is [Ar] 3d8 4s2 4p0  . In presence of strong ligand CO the paring of electrons take place and electronic configuration will be [Ar]  3d10  4s0  4p0.  Hence unpaired electrons is zero.