The complex can be written as K+[Co(CO)4]−
Since CO is a neutral ligand there its charge is zero.
Let oxidation number of cobalt be 'x'
So the equation is:
oxidation number of Co + 4 charge on CO ligand = charge of complex
x + 4(0) = -1
x = -1
Therefore, the oxidation number of Co in the given complex is −1.