Organometallic Chemistry: Ligands & Metal Carbonyls

Organometallic Chemistry Basics

• Organometallic chemistry is a field focused on compounds with metal-carbon bonds.
• Examples include CH₃Li and RMgBr.

Ligands

• Metal carbonyl (M-CO) ligands, where M=C, function as CO-π-acidic ligands.
• These are sigma donors and pi acceptors
• Cyano ligands (M-CN) have triple bonds, exhibiting unique characteristics
• CN- ligands are strong sigma donors but poor pi acceptors.

Metal Carbonyls

• Metal carbonyls involve transitional metal complexes with carbon monoxide ligands.
• V(CO)₆] is an example of a metal carbonyl.
• Ni(0) has a 3d¹⁰ electron configuration, while Ni(I) has 3d⁸.
• Fe(0), Fe(II), and Fe(III) have 3d⁸, 3d⁶, and 3d⁵ configurations, respectively.
• [Ni(CO)₄] results in Ni(0) with a 3d¹⁰ configuration.
• [Zn(CO)₄]²⁺ has Zn(II) with a 3d¹⁰ configuration.

Synergic Effect and MO Diagram

• The synergic effect involves a cooperation effect between the metal and CO ligand.
• Molecular Orbital diagrams illustrate the interactions between metal and CO orbitals, including sigma and pi bonding/antibonding.
• Carbon monoxide has a HOMO (Highest Occupied Molecular Orbital) that is carbon-centric and a LUMO (Lowest Unoccupied Molecular Orbital) that is also carbon-centric.

18-Electron Rule

• The 18-electron rule states that complexes with 18 valence electrons are more stable
• It is also known as the EAN (Effective Atomic Number) rule.
• Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, and [V(CO)₆]⁻ all follow the are examples of this Rule (18 electrons)

Ligational Modes of CO

• CO can attach in different ways such as terminal or bridging modes.
• Terminal CO ligands directly attach to a single metal center, involving 4 attachments
• Bridging CO ligands connect two or more metal centers.
• CO₂[CO]₈ serves as an example.

Bonding in Terminal and Bridging CO

• Terminal CO involves sigma donation and pi backbonding between the metal and CO.
• Bridging CO uses sp² hybridization
• This results in overlapping interactions.

Classification of Carbonyls

• Metal carbonyls are classified into non-bridged, bridged, mononuclear, and polynuclear types.
• Non-bridged carbonyls include terminal CO ligands and M-M bonds.
• Examples are Ni(CO)₆ and Mn₂(CO)₁₀.
• Bridged carbonyls involve CO ligands bridging multiple metal centers, such as Co₂(CO)₈ and Fe(CO)₉.
• Mononuclear carbonyls contain a single metal atom, e.g., [Ni(CO)₄] and [Fe(CO)₅].
• Polynuclear carbonyls contain multiple metal atoms, like [Mn(CO)₁₀] and [MnRe(CO)₁₀]

Chemical Properties: Substitution Reactions

• Substitution reactions include:
• Fe(CO)₅ + 2PF₃ → [Fe(CO)₃(PF₃)₂]
• Ni(CO)₄ + 2PF₃ → [Ni(CO)₂(PF₃)₂]
• Cr(CO)₆ + C₆H₆ → [Cr(C₆H₆)(CO)₃]

Formation of Anionic Carbonyls

• Anionic carbonyls are formed through reactions with reducing agents.
• Reactions include: -Fe(CO)₅ + Na (liq. NH₃) → Na₂[Fe(CO)₄] -Fe(CO)₅ + NaOH → Na₂[Fe(CO)₄]

Reduction of Metal Carbonyls

• Metal carbonyls are reduced using various reagents.
• [Cr(CO)₆] + Na/liq. NH₃ → Na₂[Cr(CO)₅]
• [Cr(CO)₆] + NaBH₄ → Na[HCr₂(CO)₁₀]
• [Co(CO)₈] + Na/Hg → Na[Co(CO)₄]

Formation of Cationic Carbonyl Complexes

• Cationic carbonyl complexes form through reactions with acids.
• Fe(CO)₅ + HCl/AlCl₃ → [Fe(H)(CO)₄] BCl₄

Formation of Carbonyl Halides

• Carbonyl halides form through reactions with halogens.
• [Fe(CO)₅] + I₂ → [Fe(CO)₄I₂]
• [M(CO)₆] + Cl₂ → [M(CO)₄Cl₂] (M = metal)

Carbonyl Hydrides

• Carbonyl hydrides form through reactions with hydrogen.
• [Mn₂(CO)₁₀] + H₂ → [HMn(CO)₅]

Stability of Carbonyl Complexes

• [Fe(CO)₅] is known, while [Fe(CO)₆] does not readily form. The reasons behind their differences in stability relate to their electronic structure.

Terminal vs. Bridged CO Differentiation

• Terminal and bridged CO ligands can be differentiated using X-ray diffraction studies and IR spectroscopy.

IR Spectroscopy

• IR spectroscopy is useful in determining the structure and bonding characteristics of metal carbonyls.
• Normal free CO has a bond length of 112.8 pm, while in a real complex, it is around 115 pm.
• The vibrational frequency (ν) is related to the force constant (k) and reduced mass (μ) via the equation: ν = (1/2π) * √(k/μ).

Properties of Metal Carbonyls

• Metal carbonyls are crystalline solids and covalent in nature
• They tend to be soluble in organic solvents (non-polar).
• Most metal carbonyls are diamagnetic.
• An exception is [V(CO)₆], which is paramagnetic
• This exception highlights the influence of electronic configuration on magnetic properties.
• Metals in carbonyls usually exhibit low oxidation states (zero).

Preparation of Metal Carbonyls

• Metal carbonyls are prepared using various methods, including the use of Lewis acids and reducing agents.
• CrCl₃ + 6CO + Al(Et)₃ → Cr(CO)₆
• VCl₃ + 6CO + Na → [V(CO)₆]

Thermolysis and Photolysis

• Thermal and photochemical methods are used to form higher nuclearity carbonyls.
• 2 [Fe(CO)₅] → [Fe₂(CO)₉] + CO
• 2 [Co₂(CO)₈] → [Co₄(CO)₁₂]
• 3 [Os(CO)₅] → [Os₃(CO)₁₂]

Substitution Reactions (Carbonyls)

• 3 [Fe(CO)₅] + WCl₆ → [W(CO)₆] + 3FeCl₂ + 3CO

Structure of Metal Carbonyls

• The structure of metal carbonyls can vary, with CO ligands in terminal or bridging positions.
• Examples include:
• Co₂(CO)₈
• [Fe₂(CO)₉]
• [Fe₃(CO)₁₂]
• [Ru₃(CO)₁₂]
• [Co₄(CO)₁₂]
• [Ir₄(CO)₁₂]
• [Rh₄(CO)₁₂]

Cluster Compounds (Higher Carbonyls)

• Cluster compounds are higher nuclearity carbonyls, often symmetrical.
• Examples include: -[Os₆(CO)₁₈]²⁻
• [Rh₆(CO)₁₆]
• Terminal COs have higher stretching frequencies compared to bridged COs.
• M-C bond strength is inversely proportional to CO bond strength.

Ligand Effects

• The electronic properties of ligands affect the behavior of carbonyl complexes.
• Factors such as charge, electron density, and backbonding influence the stability and reactivity of the complex.

Nitrosyl Complexes

• Nitrosyl (NO) complexes are analogous to carbonyl complexes
• They exhibit unique bonding characteristics and reactivity.
• [Cr(CO)₆] + 4NO → [Cr(NO)₄] + 6CO
• Nitrosyl complexes are generally more stable than carbonyl complexes due to stronger M-N bonds.
• Cation favors > neutral > anion.

Bonding in Nitrosyl Complexes

• NO can bind in linear or bent configurations.
• Linear NO+ donates 3 electrons, while bent NO donates 1 electron.

Vibrational Frequencies

• IR stretching frequencies are useful for characterizing NO complexes.
• Terminal NO+ has higher frequencies (1650-1900 cm⁻¹) compared to bridging NO.

Organometallic Compounds: Heptacity

• Heptacity (η) refers to the number of atoms in a ligand that are bonded to the metal center.
• Examples include:
• Monohepto (η¹): M-R (alkyl group)
• Dihepto (η²): 2 carbon atoms attached
• Trihepto (η³): 3 carbon atoms attached
• Tetrahepto (η⁴): Cyclobutadiene
• Pentahepto (η⁵): Cyclopentadiene
• Hexahepto (η⁶): Hexaheptochromium complex
• Heptahepto (η⁷): 7-membered ring
• Octahepto (η⁸): Uranium complex

Composition of matter

• Organometallic compounds can be simple or mixed.
• Simple compounds include (CH₃)₂Cd and (CH₃)₄Pb.
• Position of Metal Periodic Table: Main group, d-block, and f-block.
• Nature of Metal Carbon Bond: Characterized by polarity and reactivity.

Ferrocene Characterization

• Ferrocene (Cp₂Fe) is a classic example of a metallocene complex.
• It exhibits aromaticity and undergoes electrophilic substitution reactions.
• Ferrocene is more stable in its neutral form and can be oxidized/reduced reversibly.
• Reactions of ferrocene include:
• Mannich reaction
• Nitration
• Acylation
• Ferrocene derivatives also exist, such as biferrocene.