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

🧠 What Stabilises Low Oxidation States?

A low-OS metal centre has many d-electrons. To stabilise this electron-rich state, the metal needs to shed electron density to its ligands. The mechanism:

π-back-donation: filled metal $t_{2g}$ orbitals overlap with empty $\pi^*$ orbitals of the ligand. This requires ligands with good π-acceptor (π-acid) character.

🗺️ Why π-Acceptors Stabilise Low OS

| Ligand type | Behaviour | Effect on metal OS | |---|---|---| | π-acceptor (CO, CN⁻, NO, PR₃, alkenes) | Empty $\pi^*$ accepts metal d-density | Stabilises low OS (M⁰, M⁻¹) | | π-donor (F⁻, OH⁻, O²⁻) | Filled p donates into metal | Stabilises high OS (M⁺⁵, M⁺⁶, M⁺⁷) | | Pure σ-donor (NH₃, en) | Donates only via σ | Neutral effect on OS |

This is why metal carbonyls ($\mathrm{Ni(CO)_4}$, $\mathrm{Fe(CO)_5}$, $\mathrm{Cr(CO)_6}$) all have M(0), and oxoanions ($\mathrm{MnO_4^-}$, $\mathrm{CrO_4^{2-}}$) all have M(VI/VII).

⚡ Synergic Bonding in M–CO

The M–CO bond is the textbook example of synergic bonding:

1. σ-donation: CO's lone pair → empty metal orbital.
2. π-back-donation: filled metal $t_{2g}$ → empty CO $\pi^*$.

Both effects mutually reinforce, making the M–CO bond strong and the M⁰ oxidation state thermodynamically accessible.

⚠️ σ-Donor vs π-Acceptor

CO is both a σ-donor and a π-acceptor. The π-acceptor character is the key to stabilising low OS — pure σ-donors (like NH₃) don't have the same effect.

$\boxed{\text{Answer: (1) have good π-accepting character}}$