Bonding and Metal-Ligand Interactions

Bonding Interactions

• Interactions between atoms or groups classify according to symmetry, forming σ-bonds or π-bonds
• Sigma bonds exhibit directionality and specific nodal planes

Metal-Ligand Interactions

• Metal and ligand orbitals interact, categorized by ligand role: σ-donor, π-donor, π-acceptor

Sigma Donors

• Ligands primarily attach to metals through sigma bonds
• Sigma bonds lack nodes along the M-L vector and sigma framework
• Sigma donor strength is related to the energy (EL) and directionality (SML) of available bonding electrons
• Sigma donor strength correlates with ligand nucleophilicity and basicity

Pi Donors

• Pi-donating ligands have lone pairs or filled orbitals with appropriate symmetry and energy for pi bond formation with the metal
• Common pi-bonding ligands: halides, alkoxides, thiolates, oxides, sulfides, amides
• Halide ions participate via a filled p orbital

Pi Acceptors

• Pi-accepting ligands feature empty orbitals with appropriate symmetry and energy for pi bond formation with the metal
• Empty orbitals are often antibonding orbitals
• Common pi-accepting ligands (also termed π acids): carbonyl, cyanide, isocyanide, nitrosyl, bipyridine, phosphines

σ-Bonded Ligands

• These ligands include alkyls, hydrides, and other X-type ligands
• Transition metal-alkyl complexes are central to organometallic chemistry
• Ta(CH2Ph)5 exemplifies a homoleptic metal alkyl
• Alkyl ligands are vital in catalytic processes, and their interactions with transition metals are the basis for bonding theories

Alkyl Ligand Types

• Common σ-bound ligands from hydrocarbyl fragments include methyl, benzyl, neopentyl, trimethylsilylmethyl, phenyl, vinyl, η1-allyl, alkynyl (acetylide)

Alkyl Ligand Classification

• Ligands from hydrocarbyl fragments (non П-bonded) are X-type ligands viewed as carbanions (anionic) in the donor pair method

Ionic vs Covalent M-C Bonds

• Bonds between transition metals and hydrocarbyl ligands vary in covalency across the series
• Electronegativity differences and metal oxidation states influence this
• Early metals see more ionic M-R bonds, late metals see more covalent bonds

Aqueous Stability of M-C Bonds

• Early metal hydrocarbyl complexes are susceptible to protonolysis because of the ionic M-R bond
• Late metal hydrocarbyl complexes are stable in water

Alkyl Compound Stability

• Thermodynamic stability of metal-carbon bonds in alkyl complexes varies (BDE ~ 25–75 kcal/mol), influenced by steric and electronic factors
• Electron-withdrawing groups stabilize the M-C bond
• Primary alkyl ligands are more stable than branched ones
• Groups stabilizing carbon-centered radicals lead to weaker M-C bonds
• M-C bonds are thermodynamically weaker than M-H bonds for a given metal

Alkyl Compound Kinetic Stability

• Metal-carbon bond kinetic stability in alkyl complexes is impacted to decomposition pathways
• Beta elimination is the main decomposition pathway for alkyls

Beta Elimination

• Occurs when the β carbon of the alkyl has a hydrogen substituent; the M-C-C-H unit assumes a coplanar conformation, bringing the β hydrogen near the metal, and there exists a vacant site on the metal cis to the alkyl
• To have a kinetically stable alkyl, block B-elimination, using alkyls lacking a B-hydrogen and a 18-electron species with ligands that wont dissociate

Steric Factors

• Stereochemistry of hydrocarbyl fragments affects reactivity/stability
• Bulky hydrocarbyl ligands kinetically stabilize and favor low coordination numbers

Preparation of Alkyl Compounds: Transmetallation

• “From one metal to another,” involves main-group organometallic species is the most common means to synthesize transition metal alkyl complexes
• Common transmetallating agents: LiR, RMgCl, MgR2, AlR3, ZnR2, HgR2, RSnBu3
• The choice of transmetallating agent is important because basic organometallic species lead to reduction of metal center and attack ligands

Electrophilic Alkylation

• Metal carbonyl anion reacts with alkyl halides, used less because it requires a nucleophilic metal complex
• Another form is oxidative addition of a C-X bond to a metal center

Migratory Insertion/De-insertion

• Insertion or de-insertion of groups into transition metal-ligand bonds generates new metal-alkyls
• This can occur via the reaction of M-H with unsaturated hydrocarbon, or a the reaction of carbonyl anions with acyl halides

Other Methods to Create Metal Alkyls

• Metal halides can react with non-transition metal alkyls

Aryl Compounds

• M-Ar bonds are more stable than M-R bonds (up to 25 kcal/mol stronger)
• Metal-aryl complexes do not participate in β-H elimination
• Aryl ligands are delivered by transmetallation, oxidative addition of Ar-X bonds synthesizes metal aryl complexes

Alkynyl Compounds (Acetylides)

• Alkynyl compounds feature the strongest M-C bond (greater than vinyl, aryl, alkyl)
• Synthesized by transmetallation (with copper acetylides) or using their pKa

Alkene or Olefins Complexes

• Zeise's salt, [Pt(η2–C2H4)Cl3]–, exemplifies these
• Ethylene (CH2CH2) acts as an η2-type ligand
• Ethylene acts as a sidebound ligand donating electron density to the metal in a σ fashion, and using its π bonding to accept the electron pair

Alkene or Olefins Complexes donation and acceptance

• Sigma donation is the transfer of an L electron of the double bond to an M empty orbital
• Pi acceptance is the transfer to a ligand empty orbital

Pi-Allyl Complexes

• Allyl ligands bind two metals in two fashions

Pi-Allyl Complexes monohapto

• In monohapto (η1) form, it acts as an anionic 1e-donor X type

Pi-Allyl Complexes trihapto

• in a trihapto (η3) form, it acts as an anionic 3e-donor LX type of a ligand
• 2e- from 1, le- from ψ2

Pi-Allyl Complexes MOs

• dπ-orbitals are involved in a covalent X−type bonding
• The Px orbital in the metal donates the electron density to the ligand to the *π-orbitals
• A dative bonding can happen

Ally Complexes bridging

• Some alley groups can function as a bridging ligand

Diene complexes

• Stereoisomers of η4–C4H6 are important for interaction (Cis and trans)
• Most common is cis

1,5-cyclooctadiene (COD)

• It can act as η4–C8H12 ligand, when it bonds to metal atom with the two double bonds
• COD has two double bonds, one or both of which may interact with a metal in a manner similar to ethylene (σ-donation and π-acceptance).

Cyclic Pi systems

• Cp complexes can bond via η1, η3 , the most common η5

Cylic Pi Examples

• 1)η3–Cyclopropenyl; requires negative charge to bond
• 2)η4–Cyclobutadienyl; requires negative charge to bond

Preparation of Olefin, -alkyne and n-cyclic polyene Metal complexes

• All complexes require the transfer of certain types of functional groups
• Metal can be the reagent for a particular synthesis