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

Question

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Strategy:
 Understand the phenomenon of synergic bonding (back-bonding) that makes low (or zero) oxidation states stable in coordination chemistry.

Step 1: Identify the bonding mechanism
Low oxidation state metals are very electron-rich.
Standard ligands (Lewis bases) donate electron density to the metal (-donation), which normally increases the electron density on the metal. This makes low oxidation states unstable due to electron repulsion.
However, if a ligand has empty  antibonding orbitals

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