Valence Bond Theory of Complexes
Paramagnetism = substances with unpaired electrons attracted into a magnetic field.
Diamagnetism = substance with all electrons paired.
Ligands attach themselves by donating electron pairs to the central metal atom. Two classes of compounds can form when ligands bond to metals; high spin or low spin depending on the nature of the ligands. CFT (Crystal Field theory) can be used to explain the nature of the behavior of the ligands.
The Ni(II) complex has two eg electrons (high spin), whereas the four-coordinate, planar Pd(II) complex, like all planar d8 complexes is diamagnetic (low spin has no eg electrons).
There are multiple effects, but the absence of antibonding electrons and the greater metal-ligand
bond strength are important contributors to the much lower reactivity of the Pd complex.
Spin pairing in the Pd(II) complex is a result of the increased strength of its interaction with the six
ligands, which also contributes to its decreased reactivity.