A Summary of Organometallic Chemistry

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1 A Summary of Organometallic Chemistry Counting valence electrons (v.e.) with the ionic model 1. Look at the total charge of the complex Ph 3 P Cl Rh Ph 3 P PPh 3 OC CO 2 Fe OC CO Co + charge:0 charge: -2 charge: Look at the charge of the ligands (see table in next page) and calculate the formal oxidation state of the metal and therefore the d electrons at the metal center Ph 3 P Cl Rh Ph 3 P PPh 3 OC CO 2 Fe OC CO Co + Rh + Fe 2- Co 3+ d 8 d 10 d 6

2 3. Add the electrons coming from all ligands Ph 3 P Cl Rh Ph 3 P PPh 3 OC CO 2 Fe OC CO Co + 1 x 2e- (Cl) 3 x 2e- (PPh 3 ) TOT 8 e- 4 x 2e- (CO) TOT 8 e- 2 x 6e- (Cp) TOT 12 e-

3 4. Add the electrons of both metal and ligands to get the valence electrons (v.e.) Ph 3 P Cl Rh Ph 3 P PPh 3 OC CO 2 Fe OC CO Co + 16 v.e. 18 v.e. 18 v.e.

4 The 18-electron rule The rule helps us to decide whether a complex is likely to be stable. To have a stable transition metal we need to fill all s, p, and d orbitals. These are a total of 9 orbitals, which means 18 electrons. The rule is similar to the 8-electron rule of main group elements.

5 Exceptions to the 18-electrons rule - Early transition metal complexes and/or complexes with bulky ligands are stable with less than 18 v.e. mainly for steric reasons. - Complexes with many π-donor ligands (see below) are stable even with less than 18 v.e. because the π- donation adds electron density to the metal, stabilizing it. - d 8 complexes with square planar geometry are stable with 16 v.e. because the d x2-y2 orbital is very high in energy, has an anti bonding character and therefore is usually empty.

6 Electron density at the metal center The Pauling s principle of electroneutrality says that molecules arrange themselves so that their net charge falls between a narrow limit close to neutrality. This means that: 1) Electropositive and high oxidation state elements tend to balance their net charge by binding highly electronegative elements. 2) Elements with intermediate electronegativity prefer to bind to each other

7 Trends in electron density - The higher the oxidation state, the more stable the d orbitals. This means that the high oxidation state metal prefers to bind with ligands donating electron density (see also below). - d-orbitals in 2nd and 3rd row transition metals are more available to be donated. Higher electronegativity and higher electron density is observed going down in the periodic table.

8 - The spectrochemical series of ligands below indicates of how ligands have an active role in modifying the electron density at the metal center. π-donor ligands donate more electron density at the metal than pure σ- donor, whereas π-acceptors withdraw electron density from the metal. I > Br > Cl > F > H2O > NH3 > PMe3 > H, PPh3 > CO π-donors σ-donors π-acceptor donate electron density withdraw electron density

9 New Concepts Oxidative Addition / Reductive Elimination Insertion / Elimination Nucleophilic attack onto a Ligand H2-Activation Source: Huheey, Crabtree

10 Oxidative Addition Reductive Elimination

11 Examples Oxidative Addition: Reductive Elimination:

12 Which Factors determine the tendency of a complex to give oxidative additions? Tendency to oxidation (electronic effects) The electron richer the metal, the easier the oxidation: Good donors (e. g., trialkylphosphines) favor oxidative addition. The tendency to give oxidative addition increases upon descending in a triad: Co(I) < Rh(I) < Ir(I) (mainly because the M L-bonding energy increases in this order) Relative stability of the coordination numbers (steric effects) The tendency to higher coordination numbers within a transition period decreases from left to right. Os(0) > Ir(I) > Pt(II) (all d 8 systems): Os(0) has a greater tendency to oxidative addition than Pt(II). Bulky ligands favor low coordination numbers, and therefore disfavor oxidative addition reactions (if they do not dissociate!) Strength of the newly formed M X- and M Y-bonds relative to X Y (see CO-insertion below)

13 Mechanism X Y apolar (H2, O2): Concerted reaction with a three-center transition state: X Y polar, electrophilic molecule (CH3I, HCl): SN2-Mechanism

14 Insertion (Einschiebung) / Elimination Althought the oxidative addition of X Y to M gives a complex that formally results from the insertion of a metal atom into the covalent X Y-bond, the term insertion (Einschiebung) is reserved for reactions in which a molecule (which must possess a multiple bond) is inserted into a metal-ligand bond. The oxidation state of the metal remains unchanged: Both reactions have fundamental significance in homogeneous catalysis!

15 Olefin-Insertion into a M H-Bond: Mechanism:

16 Ligands involved in an insertion reaction are mutually cis. The insertion reaction produces a free coordination site. An elimination from an 18-electron complex can only occur upon dissociation of a ligand. Also elimination reactions from square planar complexes require a free coordination site:

17 CO-Insertion into an M C-Bond Mechanism No 13 C is incorporated into the acetyl group. No 13 CO trans to the acetyl group.

18 Mechanism: insertion (Einschiebung) or migration (Wanderung)?

19 Methyl-Migration, CO-Elimination Unambiguously confirmed by the inverse reaction: 25% 25% 50% 25% 75% Predicted for Me-migration Predicted for elimination observed product distribution not observed (Principle of microscopic reversibility: The direct and inverse reaction follow the same elementary steps)

20 Thermodynamics of CO-Insertion Calculated Reaction Enthalpies: J. A. Connor, M. T. Zafarani-Moattar, J. Bickerton, N. I. El Saied, S. Suradi, R. Carson, G. Al Takhin, H. A. Skinner, Organometallics 1982, 1, 1166.

21 Nucleophilic Attack onto a Ligand A nucleophile (Nu ) can attack either the metal ( substitution) or a coordinated molecule. For instance: The coordination to a metal activates olefins (and related compounds, such as allyls) toward nucleophilic attack (why?). The highest degree of activation occurs in metal complexes that either contain strong π-accepting ligands or are positively charged.

22 Relative Reactivity of Coordinated Unsaturated Ligands Toward Nucleophiles In cationic 18-electron complexes, the reactivity decreases according to: Adapted form: S. G. Davies, M. L. H. Green, D. M. P. Mingos, Tetrahedron 1978, 34, Note that nucleophilic attack onto an allyl complex is accompanied by 2 e -reduction of the metal:

23 H2-Activation Some of the catalytic reactions discussed in this course involve the activation of dihydrogen by transition metal complexes. This is a short overview of the possible activation pathways. Oxidative Addition (catalyst: dihydride complex) Hydrogenolysis (catalyst:monohydride complex) Heterolytic H2-Activation (catalyst: monohydride complex) Homolytic H2-Activation (catalyst: monohydride complex)

Course 201N 1 st Semester Inorganic Chemistry Instructor: Jitendra K. Bera

Course 201N 1 st Semester Inorganic Chemistry Instructor: Jitendra K. Bera andout-9 ourse 201N 1 st Semester 2006-2007 Inorganic hemistry Instructor: Jitendra K. Bera ontents 3. rganometallic hemistry xidative Addition, Reductive Elimination, Migratory Insertion, Elimination