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

We need to identify which ligand property stabilizes low oxidation states of metals in complexes.

In low oxidation state complexes (e.g., metal carbonyls like $$Ni(CO)_4$$, $$Fe(CO)_5$$), the metal has zero or low positive oxidation state. This means there is excess electron density on the metal.

For the complex to be stable in a low oxidation state, the ligands must be able to accept electron density from the metal through back-bonding (also called $$\pi$$-back donation). This involves:

- The metal donates electron density from its filled $$d$$-orbitals into the empty $$\pi^*$$ (antibonding) orbitals of the ligand.

- This stabilizes the complex by delocalizing the excess electron density away from the metal center.

Common $$\pi$$-acceptor ligands include $$CO$$, $$CN^-$$, $$NO^+$$, phosphines, etc. These ligands have empty orbitals of suitable symmetry ($$\pi^*$$ or $$d$$-orbitals) to accept electrons from the metal.

Therefore, low oxidation states are stabilized when ligands have good $$\pi$$-accepting character.

Hence, the correct answer is Option A.