Low oxidation state of metals in their complexes are common when ligands

When metal has low oxidation state, it has more electron density in d orbital. So it has more tendency to back donate electrons. Thus compound must have good $\mathrm{\pi }$ acceptor ligand.

 π backbonding, also called π backdonation, is when electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a π-acceptor ligand.It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge. Compounds where π backbonding occurs include $\mathrm{Ni}(\mathrm{CO}{)}_4$ and Zeise's salt.

The metal to ligand bonding creates a synergic effect which strengthens the bond between ligand and the metal