Coordination Compounds (Metal Carbonyls And Applications)

Bonding In Metal Carbonyls

Metal Carbonyls: Metal carbonyls are coordination compounds in which carbon monoxide ($CO$) acts as a ligand bonded to a transition metal atom.

General Formula: Typically $[M(CO)_x]$, where $M$ is a transition metal and $CO$ is the carbon monoxide ligand.

Examples: $Ni(CO)_4$, $Fe(CO)_5$, $Cr(CO)_6$, $[Mn(CO)_5]^-$, $[Co(CO)_4]^-$.

Bonding in Metal Carbonyls: The bonding in metal carbonyls is unique and involves a synergistic effect between sigma donation and pi back-bonding.

1. Sigma ($\sigma$) Donation:

• The carbon atom of the $CO$ ligand has a lone pair of electrons in a sigma ($ \sigma $) orbital.
• This lone pair is donated to an empty hybrid orbital of the central metal atom, forming a coordinate covalent bond.
• This donation increases the electron density on the metal atom.

2. Pi ($\pi$) Back-Bonding:

• The $CO$ ligand has empty antibonding $\pi^*$ orbitals (specifically, $\pi^*_{C-O}$ orbitals).
• The metal atom, having donated electron density to the ligands (or having excess electron density), can donate its own filled $d$ electrons back into these empty $\pi^*$ orbitals of the $CO$ ligand.
• This back-donation of electron density from the metal to the ligand strengthens the $M-C$ bond and weakens the $C-O$ bond in the carbon monoxide ligand.

Synergistic Effect: The $\sigma$ donation from $CO$ to the metal and the $\pi$ back-donation from the metal to $CO$ reinforce each other, leading to a strong metal-carbon bond and increased stability of the carbonyl complex.

• The $\sigma$ donation increases electron density on the metal, making it more likely to back-donate.
• The $\pi$ back-donation decreases electron density on the $CO$ ligand, making it a better $\sigma$ donor and also weakening the $C-O$ bond, causing its stretching frequency to shift to lower wavenumbers in IR spectroscopy.

Evidence for $\pi$ Back-Bonding:

• IR Spectroscopy: The stretching frequency of the $C-O$ bond in metal carbonyls is lower than that in free $CO$ (which is around $2143 \text{ cm}^{-1}$). This shift to lower frequencies indicates weakening of the $C-O$ bond due to back-donation into the $CO$ $\pi^*$ orbitals.
• Bond Lengths: The $M-C$ bond length is shorter than expected for a single bond, and the $C-O$ bond length is longer than in free $CO$, indicating a partial double bond character in the $M-C$ bond and weakening of the $C-O$ bond.

18-Electron Rule: Many stable neutral metal carbonyls obey the 18-electron rule, where the total number of valence electrons around the central metal atom (from the metal and the ligands) is 18. This rule helps predict the formulas and stability of carbonyl complexes.

Examples of Bonding:

• $Ni(CO)_4$ (Tetrahedral): Ni is in the zero oxidation state. Ni: $3d^8 4s^2$. Hybridization: $sp^3$. Tetrahedral geometry.
• $Fe(CO)_5$ (Trigonal Bipyramidal): Fe: $3d^6 4s^2$. Hybridization: $sp^3d$. Trigonal bipyramidal geometry.
• $Cr(CO)_6$ (Octahedral): Cr is in the zero oxidation state. Cr: $3d^5 4s^1$. Hybridization: $d^2sp^3$. Octahedral geometry.

Importance And Applications Of Coordination Compounds

Coordination compounds have diverse and vital applications in various fields of chemistry, biology, industry, and technology.

Importance And Applications

1. Analytical Chemistry:

• Qualitative Analysis: Many metal ions form characteristic colors with specific ligands, aiding in their detection (e.g., thiocyanate complex of $Fe^{3+}$ is blood red, $[Ni(dmg)_2]$ is a red precipitate used to detect nickel).
• Quantitative Analysis: Complexometric titrations using EDTA are widely used for determining the concentration of metal ions.
• Gravimetric Analysis: Formation of insoluble precipitates like $[Ni(dmg)_2]$ or $Ag[Co(NH_3)_6]Cl_3$ for quantitative determination.

2. Biological Systems: Coordination compounds are central to many biological processes.

• Hemoglobin: Contains the iron-porphyrin complex (heme) which transports oxygen in the blood. The central metal ion is $Fe^{2+}$.
• Chlorophyll: The green pigment in plants essential for photosynthesis, containing magnesium ($Mg^{2+}$) in the center of a porphyrin ring.
• Vitamin $B_{12}$ (Cobalamin): Contains cobalt ($Co^{3+}$) as the central metal ion, crucial for enzyme activity and DNA synthesis.
• Enzymes: Many enzymes require metal ions as cofactors (e.g., $Zn^{2+}$ in carbonic anhydrase, $Mg^{2+}$ in many enzymes utilizing ATP).

3. Industrial Applications:

• Electroplating: Used to deposit thin layers of metals like chromium, nickel, silver, and gold onto surfaces for protection, decoration, or improved conductivity. The metal ion is dissolved in a complex cyanide or ammonia solution to control the deposition rate.
• Catalysis: Many coordination compounds act as catalysts in various industrial processes.
• Ziegler-Natta catalysts (containing Ti and Al complexes) are used for polymerization of alkenes.
• Wilkinson's catalyst ($RhCl(PPh_3)_3$) is used for hydrogenation.
• Metal carbonyls ($Ni(CO)_4$, $Fe(CO)_5$) are used in carbonylylation reactions.
• Water Treatment: Complexing agents can be used to remove metal ions from water.
• Hydrometallurgy: Used in the extraction and purification of metals (e.g., leaching of gold with cyanide solutions forms $[Au(CN)_2]^-$ complex).

4. Medicinal Applications:

• Anti-cancer drugs: Cisplatin ($[Pt(NH_3)_2Cl_2]$) is a coordination compound used in chemotherapy.
• Diagnostic Agents: Gadolinium complexes ($Gd^{3+}$) are used as contrast agents in MRI scans.
• Detoxification: The coordination compound BAL (British Anti-Lewisite, dimercaprol) is used to treat arsenic poisoning.

5. Other Applications:

• Pigments: Many coordination compounds have intense colors and are used as pigments (e.g., Prussian blue, $[Fe_4(Fe(CN)_6)_3]$).
• Photography: Silver complexes like $[Ag(NH_3)_2]^+$ are used in photographic processes.
• Electroplating: Metal plating for decorative or protective purposes.