Organometallic Structures Flashcards
Electron Counting
- Count Number of anionic ligands
- Determine Oxidation state of Metal
- Count donor electrons (mindful of haptic bonding)
- sum valence electrons and d electrons
18 electron rule exceptions
- Steric exceptions
- high oxidation or early transition
- small radii increases sterics of ligands
- steric energy prevents coordination of more ligands to reach 18 electrons - Electronic exceptions
- late transition or low oxidation
- high atomic number means stable d orbitals
- dz2 no longer bonds
- d8 froms square planar
- d10 froms trigonal planar
Metal Bonding
- Assume complex achieves 18 electrons
- count valence electrons for each metal
- if less tahn 18 add metal metal bonds
one metal bond equals one electron to each metal centre
Molecular orbitals of Geometries
- Octahedral
- ligands approach along axes
- eg bonding sigma
- t2g non-bonding sigma - Square planar
- ligands approach along xy axes
- dx2-y2 bonding
- dz2 interacts
- t2g non bonding - Tetrahedral
- ligands approach in between axes
- t2g bonding sigma
- eg non-bonding sigma
Square Planar or Tetrahedral
Depends on Electronic configuration then sterics. Config with lowest energy is preferred even if higher sterics.
pi acceptors and pi donors
- pi donors
- have filled p orbitals
- donate electron density via pi interactions
- eg amines, halides, oxygen - pi acceptors
- have vacant p orbitals
- accept electron density from metal d orbitals
- eg CO, phosphines, alkenes
pi backbonding
metal donates electron density into π* orbital of ligand.
- weakens ligand bond (decreased frequency)
- increases sigma donation
- increase metal-ligand bond
- strong sigma donors (NHC, PPh3) increase electron density and thus π backbonding
Hard Soft Acid Bases
- Hard
- small radius
- high charge density
- non-polarisable
- eg F, Cl, amines, O - Soft
- large
- low charge density
- polarisable
- eg low valent metals, CO, R-, PPh3
like goes with like
Metal Carbonyl Synthesis
- Direct Ligation of unoxidised metal
- Reductive Carbonylation with CO gas
- Reductive carbonylation with organic carbonyl
Metal Phosphine Synthesis
- Coordination to unsaturated metal
- Coordination to low valent
- ligand substitution
Metal-Alkyl Synthesis
- Direct ligation with alkylating agent
e. g AlMe3, SnMe4, Mao, LiMe
Metal-CP synthesis
- Ligand Exchange and in-situ deprotonation with metal salt
Metal-NHC Synthesis
- Free carbene metallation
- in situ deprotonation and metallation
- silver-NHC transmetallation
Substitution Mechanisms
- Associative
- Dissociative
Associative Sub Increase Rate
- decrease sterics
- trans effect
Dissociative Sub increase rate
- Increase sterics
Oxidative Addition non-polar
- metal coordinates to bond
- donation into π* breaks bond
- coordinates cis
Oxidative Addition Polar
- metal nucleophilic attacks electrophilic atom, breaking bond froming anion
- becomes electrophilic
- anion attacks trans due to sterics
Oxidative Addition Factors
- Electron Density
- donates electron density to ligand - coordinatively unsaturated
Oxidative Addition Rate
- π acceptors reduce electron density and thus rate eg CO, suppress oxidative addition
- sigma donors increase electron density and thus increase rate eg NHC, PPh3
Reductive Elimination Requirements
- 2 ligands cis for orbital overlap
- product makes sense
Reductive elim factors
- locked cis means faster rate
- low valent metals are unstable by reduction
Migration Reactions
- Alkene insertion to form alkyl ligand
- CO insertion to from carbonyl
Hydride Elimination Requirements
- Beta hydrogen present
- M-C-C-H in syn coplanar
- adjacent vacant/psuedo-vacant site
(4. Metal has d electron density to cleave C-H)
Rules for polyenes
- Even before odd
- Open before closed (after even or odd)
(shorter before longer)
(least anionic/most electrophilic carbon/largest orbitals)
Direct CO ligation
CO ligates to unoxidised metal
Reductive carbonylation with CO gas
- coordination to metal salt
- reduction with reducing agent e.g. Al, NaOMe
Reductive Carbonylation with organic carbonyl
- oxidative addition of acyl halide
- reverse migration of alkyl
- reductive elimination of R-X
Metallation of Preformed Carbene
- form carbene by deprotonation
- Metallation
In-situ deprotonation metallation of NHC
requires metal with basic ligands
e.g OAc
Silver-NHC transmetallation
labile Ag-NHC bond due to high d electron density
transfers readily to other metals
Complex geometry factors
Coordination number
electronics
sterics
(metal oxidation state, ligands, sterics)
Dative vs Covalent Ligands
Dative donates 2 electrons (neutral)
covalent donates 1 electron (anionic)
Determining anionic neutral
put both dative bond electrons on ligand
determine electron configuration of ligand
Enhancing insertion rate
- increase steric bulk
insertion decreases coordination
- lewis acid additives/metals e.g. AlCl3
stabilises metal acyl transition
- electrophilic metals
withdraw electron from carbon activating electrophilicity
