Introduction to d-Metal Organometallic Chemistry

CH5202 (part) Introduction to d-Metal Organometallic Chemistry

Instructor and Resources

  1. Instructor: Dr. Jon Rourke
  2. Key Texts:
       - Primary Resource: Inorganic Chemistry, 7th ed, Chapter 22
       - Undergraduate Texts:
         - Organometallic Chemistry by G.O. Spessard and G.L. Miesler (QD2355.S7)
         - Organotransition Metal Chemistry by A.F. Hill (QD2355.H4)
       - Advanced Level Texts:
         - The Organometallic Chemistry of the Transition Metals, 6th Ed. by R.H. Crabtree (QD2355.C7)
         - Organometallics, 3rd Ed. by C. Elschenbroich (QD2355.E5)
         - Organometallics and Catalysis by M. Bochmann (QD411.B63)

Module Outline

  1. d-Block Organometallics – The 18 Electron Rule (1.5 Lectures)
       - Introduction and History: Overview of organometallic chemistry and the historical development of concepts.
       - Molecular Orbital (MO) Diagrams for octahedral complexes:
         - Bonding involving s orbitals only and s & p orbitals leading to the 18 Valence Electron (VE) Rule being the optimal configuration.
         - Importance of electron counting and distinction between coordination compounds and organometallics.
         - Examination of exceptions to the 18 electron rule, particularly the stability of 16 electron square planar complexes.

  2. Bonding of Ligands to Metal Centres (5.5 Lectures)
       - Carbon Monoxide (CO): Explore s donation, p backbonding, IR spectral effects, and group theoretical analysis of stretching bands.
       - Phosphines: Discuss bonding analogous to CO including the electron donating ability, p-acceptor behavior, and size (Tolman cone angle).
       - Hydrides and Dihydrogen:
         - Bonding dynamics of dihydrogen, backbonding effects, and its transformation into dihydride.
         - Recognition of oxidative addition behavior in hydride bonding.
       - Various Hydrocarbon Ligands:
         - h1-Alkyl, -alkenyl, -alkynyl, and -aryl groups functioning primarily as s donors.
         - h2-Alkene and h2-Alkyne ligands related to DCD model and their p-acceptor capability.
       - Complex Ligands: Discussion of non-conjugated diene and polyene ligands treated as multiple isolated alkenes, including butadiene, cyclobutadiene, and cyclooctatetraene with focus on backbonding, Huckel aromaticity, and bond length analysis.
       - Arenes: Features of benzene and arenes in terms of p donation and d-backbonding.
         - Insight into cyclopentadiene and its structural wonders with references to the MO of ferrocene.
         - Notable discussion of non-18 electron metallocenes and cycloheptatriene.
       - Dinitrogen and Nitric Oxide (NO): N2's role as a versatile ligand; description of NO's characteristics.

  3. Reactions of Organometallic Compounds (2 Lectures)
       - Ligand Substitution: Examination of carbonyl replacements and distinctions between 16e and 18e complexes focusing on associative vs. dissociative substitution pathways.
         - Investigation of masked dissociative pathways.
       - Oxidative Addition and Reductive Elimination: Recap of reactions involving the addition of H2 to produce dihydride and enveloping general X-Y molecules.
       - Migratory Insertion Reactions: Focused examples explaining the mechanisms of migration onto carbonyl ligands and broadly extending to 1,2-insertions and β-hydride elimination.
       - Catalysis: Integration of previously discussed concepts with two example reactions: carbonylation of methanol and hydrogenation of alkenes.

History of d-Block Organometallic Compounds

  1. Early Developments:
       - First organometallic compound (1827): Zeise’s Salt (KPtCl3.EtOH)
         - Initial belief as compound of ethene; verification achieved in 1868, structure elucidated in the early 1950s with modern bonding theories.
       - Carbonyl compounds (containing CO) introduced by Schützenberger in the 1860s.
       - 1890: Discovery of Ni(CO)4 by Mond, Quincke, and Langer; structural knowledge lacking yet commercial processes initiated.
       - 1930s: Progress by Hieber on carbonyl anions and clusters; structural definitions awaited until 1951 with the proof of ferrocene's structure, resulting in a Nobel Prize awarded to Wilkinson and Fischer in 1973.

Development in d-Block Bonding Theories

  1. Modern Bonding Theories: Following the structural proof of ferrocene, a surge of interest and research into the bonding of organometallic and similar compounds surged.
       - Development of spectroscopic techniques allowed comprehension of bonding theories.
       - D-block organometallic compounds reveal both strong valence electron count preference for 18 electrons while coordination complexes reveal variable electron count.

  2. Metal Atomic Orbitals:
       - Consideration of d, s, and p atomic orbitals of metals alongside ligand σ-donor orbitals, contributing to understanding electron configurations.

Molecular Orbital Approach to d-Block Organometallics

  1. Electron Configuration: Predicts 18 electrons as the best configuration for octahedral complexes, exemplifying stability with complexes such as M(CO)6 (where M represents transition metals like Cr, Mo, or W).
       - Predicts colorless nature of such complexes; discusses general applicability of 18-electron rule across different geometries.
  2. Square Planar Configurations: An alternative stable electron configuration involving 16 electrons in square planar complexes.
       - Commonly relates to specific transition metals and emphasizes steric preference compared with tetrahedral arrangements.

Electron Counting Methods in Organometallic Chemistry

  1. Two Methods of Electron Counting:
       - Neutral Ligand Method:
         - Assigning electrons based on type classification of ligands (L = 2e donor, X = 1e donor).
         - Metal contributes its group number electrons while adjustments for charge are made.
       - Donor Pair Method:
         - Treat each ligand as donating pairs of electrons and ascertain the oxidation state of the metal. This approach is widely used despite the tenuous definition of oxidation state in organometallics.

  2. Examples of Electron Donating Ligands (per Method):
       - Neutral Ligand Method:
         - CO: 2e donor
         - PR3: 2e donor
         - Cl: 1e donor
         - H: 1e donor
         - H2: 2e donor
       - Donor-Pair Method: Similar classifications exist with noted variations in the specifications of ligands.

  3. Ligand Nomenclature: Defines bridging (μ), point of attachment (κ), and number of atoms involved in bonding (η).

Carbon Monoxide Ligand Characteristics in Organometallic Chemistry

  1. Carbon Monoxide as a Ligand:
       - Noted for its role in stabilizing low oxidation state complex formations within transition metals.
       - CO exhibits a weak σ donor but significantly contributes to backbonding via its lone pair and facilitates electronic interactions with filled metal d orbitals.
       - The synergistic nature of σ donation and p backbonding strengthens the overall M–CO interaction.

  2. Spectroscopic Evidence:
       - IR stretching frequency observed around 2143 cm⁻¹ is characteristic of CO bonding in organometallic complexes. Notably, the strength of the M–C bond inversely correlates to the CO bond strength in the carbonyl linkage.

  3. Bridging Carbonyls: The existence of bridging carbonyls results in structurally informed complexes with lower stretching frequencies due to altered steric and electronic interactions owing to both s and p bonding dynamics.

Group Theory Application to Carbonyl Stretching Modes

  1. Carbonyl Stretching Analysis: Utilizing group theory to assess the effects of point group symmetry on carbonyl stretching frequencies in various molecular configurations (cis, trans) leads to full characterization of IR activity based on the transformation modes of carbonyl stretches into irreducible representations.

  2. Modal Analysis and IR/Raman Activity:
       - Illustrates how the dipole moment changes characterize the IR active modes and their relationship to polarizability affecting Raman activity.

Synthesis and Reactions of Metal Carbonyls and Ligands

  1. Synthesis: Discusses methods for creating metal carbonyls through direct combination, reductive carbonylation, and ligand replacement strategies.

  2. Phosphine Ligands: Different phosphines' behaviors and structures allow tunable properties in metal coordination environments, capturing how different phosphines' Tolman cone angles influence reactivity and complex stability.
       - Phosphine compounds are characterized by varying donor abilities, affecting bond strength with metals based on electronic environment.

  3. Hydrides and Dihydrogen: Examines the characteristics and bonding dynamics pertaining to hydride complexes, outlining bonding interactions, IR signatures, and synthesis pathways, including oxidative additions and protonation processes.

  4. Alkyl, Alkenyl, and Aryl Complexes: Focus iventory on bonding and stability across various hydrocarbons detailing the significant bonding interactions where back-bonding is minimal.

  5. Reactions Involving Carbonyl Complexes: Focused discussions about substitution reactions, oxidative additions, and their accompanying pathways, integrating established principles with specific reactions to provide a structural understanding of behavior across various ligand systems in organometallic chemistry.