Which complex is likely to exhibit strong paramagnetism?

The complex [CoCl4]²- is likely to exhibit strong paramagnetism due to the presence of unpaired electrons in its electronic configuration. Cobalt in its +2 oxidation state has the electron configuration of [Ar] 3d⁷. In an octahedral field, such as what would be encountered in complexes, this configuration can result in partially filled d-orbitals, which is critical for paramagnetism since unpaired electrons can align with an external magnetic field.

In the case of [CoCl4]²-, the tetrahedral geometry leads to a different splitting of the d-orbitals compared to an octahedral field, favoring occupancy of the higher energy d-orbitals. This leads to more unpaired electrons, thus enhancing its paramagnetic properties. The presence of four chloride ions (which are weak field ligands) does not significantly pair the electrons and promotes a high-spin configuration where unpaired electrons are maintained.

On the other hand, the other options typically show either fully paired electrons or configurations that result in fewer unpaired electrons, leading to lesser or no paramagnetism. In complex ions with strong field ligands or fully filled orbitals, such as those found in the other complexes listed,