Introduction to Organometallic Chemistry
Organometallic chemistry is the chemistry of compounds containing metal–carbon bonds. The d-block organometallic chemistry has developed rapidly since the discovery of ferrocene in 1951, spanning new types of reactions, unusual structures, and practical applications in organic synthesis and industrial catalysis.
Historical Milestones
1827 - Zeise's Salt
First organometallic compound: [Pt(C₂H₄)Cl₃]⁻, an ethene complex of platinum(II).
[PtCl₃(C₂H₄)]⁻1890 - Nickel Carbonyl
Mond, Langer, and Quinke synthesized tetracarbonylnickel, [Ni(CO)₄].
[Ni(CO)₄]1951 - Ferrocene
Discovery of the remarkably stable "sandwich" compound [Fe(C₅H₅)₂] revolutionized the field.
1973 - Nobel Prize
Ernst-Otto Fischer and Geoffrey Wilkinson awarded for contributions to organometallic chemistry.
22.1 Bonding and the 18-Electron Rule
Unlike coordination compounds, d-metal organometallic compounds normally have relatively few stable electron configurations and often have a total of 16 or 18 valence electrons around the metal atom. This restriction is due to the strength of the π (and δ) bonding interactions.
18-Electron Rule
Maximum stability achieved when metal has 18 valence electrons (like noble gas configuration)
16-Electron Complexes
Common for d⁸ square planar complexes; reactive towards oxidative addition
Origin of the 18-Electron Rule
In an octahedral complex with strong π-acceptor ligands like CO, six σ-bonding interactions utilize the a₁g, t₁u, and eg orbitals. Additionally, the three t₂g orbitals can form π bonds with ligand acceptor orbitals. This gives nine bonding molecular orbitals that can accommodate 18 electrons.
Molecular Orbital Scheme for Octahedral Carbonyl
22.2-22.3 Electron Counting Methods
Two methods are commonly used to count electrons in organometallic compounds. Both give the same total electron count but differ in how they assign oxidation states.
Neutral-Ligand Method
All ligands treated as neutral; uses L/X notation
- L = 2-electron neutral donor (CO, PR₃)
- X = 1-electron radical (H, Cl, CH₃)
- Cp = L₂X (5 electrons)
Donor-Pair Method
Some ligands treated as anions; uses formal charges
- CO, PR₃ = 2 electrons (neutral)
- H⁻, Cl⁻, CH₃⁻ = 2 electrons (anionic)
- Cp⁻ = 6 electrons (anionic)
Example: Electron Counting for [IrBr₂(CH₃)(CO)(PPh₃)₂]
| Method | Calculation | Total |
|---|---|---|
| Neutral-Ligand | Ir(9) + 2Br(2×1) + CH₃(1) + CO(2) + 2PPh₃(2×2) = 9 + 2 + 1 + 2 + 4 | 18 e⁻ |
| Donor-Pair | Ir(III)(6) + 2Br⁻(2×2) + CH₃⁻(2) + CO(2) + 2PPh₃(2×2) = 6 + 4 + 2 + 2 + 4 | 18 e⁻ |
Common Ligands and Their Electron Counts
| Ligand | Formula | Designation (L/X) | Electrons (neutral) | Electrons (donor-pair) |
|---|---|---|---|---|
| Carbonyl | CO | L | 2 | 2 |
| Phosphine | PR₃ | L | 2 | 2 |
| Hydride | H | X | 1 | 2 (as H⁻) |
| Alkyl | R (CH₃, etc.) | X | 1 | 2 (as R⁻) |
| η²-Alkene | CH₂=CH₂ | L | 2 | 2 |
| η³-Allyl | C₃H₅ | LX | 3 | 4 (as C₃H₅⁻) |
| Butadiene | C₄H₆ | L₂ | 4 | 4 |
| η⁵-Cyclopentadienyl | C₅H₅ | L₂X | 5 | 6 (as Cp⁻) |
| η⁶-Benzene | C₆H₆ | L₃ | 6 | 6 |
22.5-22.17 Ligands in Organometallic Chemistry
Hapticity (η) - Modes of Ligand Binding
Hapticity describes the number of contiguous atoms of a ligand that are coordinated to the metal. It is denoted as ηn, where n is the number of atoms bonded to the metal.
Monohapto
1 atom bonded
CH₃, σ-alkyl
Dihapto
2 atoms bonded
Ethene, alkyne
Trihapto
3 atoms bonded
Allyl
Pentahapto
5 atoms bonded
Cyclopentadienyl
Hexahapto
6 atoms bonded
Benzene
22.5 Carbon Monoxide - The Archetypal Ligand
CO is particularly good at stabilizing very low oxidation states. The bonding involves σ-donation from the carbon lone pair (3σ orbital) and π-backbonding from filled metal d-orbitals to empty π* orbitals on CO.
σ Donation
3σ → Metal d orbital
Weak σ donor
σ bond
π backbond
π Backbonding
Metal d → CO π*
Strong π acceptor
CO Stretching Frequencies and Metal Charge
22.6 Phosphines
Phosphine ligands (PR₃) are excellent σ-donors and moderate π-acceptors. Their electronic and steric properties can be finely tuned by changing the R groups.
Tolman Cone Angle (θ)
Measures the steric bulk of phosphine ligands. Larger cone angles mean more steric hindrance.
- PMe₃: 118°
- PPh₃: 145°
- P(tBu)₃: 182°
Electronic Parameter (ν)
Based on CO stretching frequency in [Ni(CO)₃L]. Lower values indicate better σ-donors.
- P(tBu)₃: 2056 cm⁻¹
- PPh₃: 2069 cm⁻¹
- P(OPh)₃: 2085 cm⁻¹
22.15 Carbenes
Fischer Carbenes
Electron-poor at carbon, stabilized by π-donor substituents (OR, NR₂)
Electrophilic, attacked by nucleophiles
Schrock Carbenes
Electron-rich at carbon, with strong M=C backbonding
Nucleophilic, attacked by electrophiles
N-Heterocyclic Carbenes (NHCs)
Strong σ-donors, stabilized by adjacent N atoms
Excellent ancillary ligands in catalysis
22.18 Metal Carbonyls
Binary metal carbonyls contain only metal atoms and CO ligands. They represent the simplest class of organometallic compounds and follow the 18-electron rule.
Simple Binary Carbonyls
Tetrahedral
[Ni(CO)₄]Ni(0): 10 + 8 = 18 e⁻
Colorless, toxic liquid
Trigonal Bipyramidal
[Fe(CO)₅]Fe(0): 8 + 10 = 18 e⁻
Yellow liquid, fluxional
Octahedral
[Cr(CO)₆] [Mo(CO)₆] [W(CO)₆]M(0): 6 + 12 = 18 e⁻
White solids, sublime
Polynuclear Metal Carbonyls
When mononuclear carbonyls cannot achieve 18 electrons, metal-metal bonds form. Each M-M bond contributes 1 electron to each metal.
[Mn₂(CO)₁₀]
Mn-Mn single bond
Each Mn: 7 + 10 + 1 = 18 e⁻
[Co₂(CO)₈]
Co-Co bond + bridging COs
Each Co: 9 + 6-8 + 1 = 18 e⁻
[Fe₂(CO)₉]
Three bridging COs
Fe-Fe bond debated
Bridging vs Terminal CO
| Bonding Mode | Symbol | ν(CO) Range | Description |
|---|---|---|---|
| Terminal | M-CO | 2125-1850 cm⁻¹ | CO bound to one metal |
| Edge-bridging | μ₂-CO | 1850-1750 cm⁻¹ | CO bridges two metals |
| Face-bridging | μ₃-CO | 1730-1620 cm⁻¹ | CO bridges three metals |
22.19 Metallocenes
Metallocenes are "sandwich" compounds with two parallel cyclopentadienyl (Cp) rings bound to a central metal atom. Ferrocene [Fe(Cp)₂] is the archetypal metallocene.
Electron Configurations of Metallocenes
| Complex | Valence e⁻ | Configuration | M-C Bond (pm) | Properties |
|---|---|---|---|---|
| [V(Cp)₂] | 15 | e₂'² a₁'¹ | 228 | Paramagnetic |
| [Cr(Cp)₂] | 16 | e₂'³ a₁'¹ | 217 | Paramagnetic |
| [Fe(Cp)₂] | 18 | e₂'⁴ a₁'² | 206 | Diamagnetic, stable |
| [Co(Cp)₂] | 19 | e₂'⁴ a₁'² e₁''¹ | 212 | Easily oxidized |
| [Ni(Cp)₂] | 20 | e₂'⁴ a₁'² e₁''² | 220 | Paramagnetic |
Bent Metallocenes and Piano-Stool Complexes
Bent Sandwich
Two Cp rings bent away from linear
Important in Ziegler-Natta catalysis
Half-Sandwich (Piano-Stool)
One Cp/arene ring + other ligands
Three "legs" like a piano stool
Triple-Decker
Three Cp rings, two metals
Rare but known
Ring Slipping (Hapticity Change)
Cyclopentadienyl ligands can "slip" from η⁵ to η³ or η¹ coordination, changing the electron count and creating a coordination site for incoming ligands.
22.20 Metal Clusters
Metal clusters are compounds with metal-metal bonds forming triangular or larger cyclic structures. Their electron counts correlate with structure via the Wade-Mingos-Lauher rules.
Cluster Valence Electron (CVE) Counts
Triangle
3 metals, 3 M-M bonds
Tetrahedron
4 metals, 6 M-M bonds
Trigonal Bipyramid
5 metals, 9 M-M bonds
Octahedron
6 metals, 12 M-M bonds
Isolobal Analogy
Fragments that have frontier orbitals of the same symmetry, similar energies, and same electron occupancy are called isolobal (symbol: ↔).
| Fragment | Isolobal With | Electrons in Frontier Orbital |
|---|---|---|
| CH₃ | [Mn(CO)₅], [Co(CO)₄] | 1 |
| CH₂ | [Fe(CO)₄], [Mn(CO)₄]⁻ | 2 |
| CH | [Co(CO)₃], [Fe(CO)₃]⁻ | 3 |
This allows prediction of structures: if CH₃-CH₃ (ethane) exists, then [(CO)₅Mn-Mn(CO)₅] should also be stable.
22.21 Ligand Substitution Reactions
Substitution in organometallic complexes follows similar principles to coordination chemistry, with the additional constraint that the electron count should not exceed 18.
16-Electron Complexes: Often undergo associatively activated substitution via an 18-electron intermediate.
Thermal and Photochemical Substitution
Thermal Substitution
Requires heating; goes through solvated intermediate [Cr(CO)₅(solv)]
Photochemical Substitution
UV light promotes CO loss; faster and milder conditions
Effect of Ligand Properties
Better σ-donor ligands (like phosphines) increase electron density on the metal, strengthening M-CO bonds through enhanced π-backbonding. This decreases the rate of further CO substitution.
Dissociation Constants for [Ni(PR₃)₄] Complexes
| Phosphine (L) | Cone Angle (θ) | Kd |
|---|---|---|
| PMe₃ | 118° | <10⁻⁹ |
| PEt₃ | 137° | 1.2 × 10⁻⁵ |
| PMePh₂ | 136° | 5.0 × 10⁻² |
| PPh₃ | 145° | Large |
| P(tBu)₃ | 182° | Large |
Larger cone angles lead to greater dissociation (steric crowding).
22.22 Oxidative Addition and Reductive Elimination
These reactions are fundamental to organometallic catalysis. They involve changes in both coordination number and oxidation state by 2.
Oxidative Addition
- Coordination number: +2
- Oxidation state: +2
- Electron count: +2
- X-Y bond broken
Common for 16e⁻ d⁸ complexes (Ir(I), Rh(I), Pt(0), Pd(0))
Reductive Elimination
- Coordination number: -2
- Oxidation state: -2
- Electron count: -2
- X-Y bond formed
Requires X and Y to be cis; reverse of oxidative addition
Examples of Oxidative Addition
H₂ Addition (Concerted)
16e⁻ Ir(I) → 18e⁻ Ir(III)
Products are cis-dihydrides
Alkyl Halide (SN2-like)
Stereochemistry at C inverted
CH₃ and I may be trans
22.23 σ-Bond Metathesis
When oxidative addition cannot occur (e.g., early d-metals with d⁰ configuration), a concerted σ-bond metathesis can exchange ligands through a four-membered transition state.
σ-Bond Metathesis
No change in oxidation state; concerted 4-center transition state
22.24-22.26 Insertion and Elimination Reactions
1,1-Migratory Insertion
An X group migrates to an adjacent ligand (like CO), reducing the electron count by 2. Despite the name "insertion," the mechanism involves migration of X to the CO.
CO Insertion (Migratory Insertion)
The methyl group migrates to an adjacent CO; incoming ligand occupies the vacated site. Stereochemistry at carbon is preserved.
1,2-Insertion and β-Hydride Elimination
1,2-Insertion involves migration of X to an η²-ligand (like an alkene), creating an η¹-alkyl. The reverse, β-hydride elimination, is a common decomposition pathway.
1,2-Insertion
Hydride migrates to coordinated alkene
Key step in alkene hydrogenation and polymerization
β-Hydride Elimination
Reverse of 1,2-insertion (requires β-H and vacant site)
Common decomposition pathway for metal alkyls
- A β-hydrogen on the alkyl group
- A vacant coordination site on the metal (or ability to create one)
- Syn-coplanar arrangement of M-C-C-H
Alkene Isomerization via β-H Elimination
Sequential 1,2-insertion and β-elimination allows metal-catalyzed alkene isomerization
Chapter Summary
Key Concepts
- 18-electron rule dominates stability
- π-backbonding crucial for CO, alkenes
- Hapticity describes ligand binding modes
- Oxidation state changes drive reactivity
Important Ligands
- CO: σ-donor, strong π-acceptor
- Phosphines: tunable σ-donors
- Cp: versatile, stabilizing, aromatic
- Carbenes: Fischer vs Schrock types
Key Reactions
- Ligand substitution (D or A activated)
- Oxidative addition / Reductive elimination
- Migratory insertion (1,1 and 1,2)
- β-Hydride elimination
Applications
- Catalytic hydrogenation
- Alkene polymerization
- Cross-coupling reactions
- Hydroformylation