Synergic bonding

Synergic bonding refers to a type of interaction in coordination complexes where both the metal and the ligands participate in a two-way electron donation process. This interaction is important in understanding the stability, reactivity, and electronic structure of many metal complexes, particularly those involving transition metals.

1.0Concept of Synergic Bonding

1. σ-Donation: 

The ligand donates electron density to the metal center through a sigma (σ) bond. This is a standard bonding interaction where the ligand, typically through a lone pair of electrons, donates electron density to an empty or partially filled orbital of the metal.

2. π-Back Donation: 

The metal, in turn, donates electron density back to the ligand through a pi (π) bond. This occurs when the metal has filled d-orbitals that can overlap with empty or low-lying π* (antibonding) orbitals of the ligand. This back donation is common in ligands like CO, CN⁻, and phosphines, which have π* orbitals available for interaction.

2.0Mechanism of Synergic Bonding

Basically, synergic bonding involves a mutual exchange of electrons between a metal and a ligand, such as CO in metal carbonyl complexes. In σ-donation, the ligand donates electron density from its lone pair to the metal's orbital, forming a σ-bond. Simultaneously, in π-back donation, the metal donates electron density from its d-orbitals into the ligand’s empty π* orbital. These two processes reinforce each other, creating a stronger, more stable metal-ligand bond.

3.0Examples of Synergic Bonding

1. Metal Carbonyl Complexes (e.g., [Ni(CO)₄​]):

• In metal carbonyl complexes, CO acts as a ligand exhibiting both σ-donation and π-back bonding.
• The carbon atom of CO has a lone pair that donates electrons to the metal, forming a σ-bond.
• Simultaneously, the metal can back-donate electrons from its filled d-orbitals into the π* anti-bonding orbitals of CO. This π-back donation strengthens the metal-CO bond and lowers the energy of the complex.

2. Olefin (Alkene) Complexes (e.g., Zeise's Salt, K[PtCl₃​(C₂​H₄)]):

• In Zeise’s salt, ethylene (C₂​H₄) acts as a ligand.
• Ethylene donates electron density from its π-bond to the metal (Pt) to form a σ-bond.
• The metal (Pt) can then back-donate electron density from its d-orbitals to the empty π* orbital of the ethylene, strengthening the overall bonding interaction.

3. Phosphine Complexes (e.g., [Mo(CO)₃​(PPh₃​)₃​]):

• Phosphines like triphenylphosphine (PPh₃​) donate electrons to the metal through the lone pair on phosphorus, forming a σ-bond.
• Metals like molybdenum (Mo) can back-donate electron density into the anti-bonding orbitals of the phosphine, leading to a synergic bond.

4.0Importance of Synergic Bonding

• Bond Strength: Synergic bonding leads to stronger and more stable metal-ligand bonds because the dual interaction between metal and ligand maximizes bonding efficiency.
• Electronic Effects: The back donation affects the electronic structure of both the metal and the ligand, often leading to changes in the metal’s oxidation state and the ligand's electronic properties (e.g., lowering the stretching frequency of CO in IR spectra due to π-back donation).
• Reactivity: The nature of synergic bonding can influence the reactivity of the complex. For example, strong π-back donation can make ligands like CO less prone to dissociation, affecting the complex’s catalytic properties.
• Spectroscopic Signatures: Synergic bonding is often observable in spectroscopic techniques. For instance, in IR spectroscopy, the stretching frequency of CO in metal carbonyls decreases with increasing