[NiCl₄]²⁻ is paramagnetic while [Ni(CO)₄] is diamagnetic though both are tetrahedral. Why?

In [Ni(CO)₄], the oxidation state of Ni is zero whereas in [NiCl₄]²⁻, the oxidation state of Ni is +2. The presence of CO ligand, which is a strong ligand, can pair all electrons in [Ni(CO)₄] and thus it is diamagnetic in nature. But Cl⁻ is a weak ligand and is unable to pair up the unpaired electrons and thus [NiCl₄]²⁻ is paramagnetic in nature.