Organometallic Chemistry

1. Organometallic Chemistry • Involves complexes where the metal is coordinated directly to carbon atoms. • Often deals with transition metals in their lower valent states (note: respective chemistries of high and low valence states are very different). • Here again we want to consider the ligands as Lewis bases and the metals as Lewis acids. • 18 ELECTRON RULE (Does NOT apply to coordination compounds) • most organometallic compounds appear to have either 18 electrons (80%) or 16 electrons (18%) in their valence shells. • can think of the 18 Electron Rule as the total utilization of all of the valence orbitals in the metal: 3d → 10 electrons 4s → 2 electrons 4p → 6 electrons Total = 18 electrons similar to octet rule for main group elements

2. 18 Electron Rule • examples: • Note that the 18 Electron Rule is based on empirical observations; there is no theoretical reason why it holds. Cr(CO)6 6 CO = 12 electrons (each CO is 2 electron donor) Cr = 6 electrons (i.e. Cr0) Total = 18 electrons HFe(CO)4-1 4 CO = 8 electrons H-1 = 2 electrons (note: must be hydride; 2 electron donor) Fe = 8 electrons (i.e. Fe0) Total = 18 electrons

3. 18 Electron Rule • Can predict geometries: • V-1, Cr0, Mn+, Fe+2 are all d6 (metal contributes 6 electrons). Therefore need 12 electrons from ligands (i.e. need 6 ligands). Thus, d6 = Oh in Organometallic Chemistry. • Mn-1, Fe0, Co+ are d8; need 5 ligands, so trigonal bipyramidal or square pyramidal. • Co-1, Ni0, Cu+ are d10; need 4 ligands, so square planar or tetrahedral.

4. 16 Electron Rule • These complexes are generally found for metals at the extremes of the periodic table, but most common for bottom right (Rh, Pd, Ir, Pt). • Geometries: • Rh+, Ir+, Pt+2 are d8; need 4 ligands, and are ALWAYS square planar (most common geometry for 16 electron complexes). • Pd0, Pt0 are d10; need 3 ligands, so are trigonal planar.

5. Counting Ligand Electrons • Two electron donors • :CO, :H-1, :PF3, :Cl-1, etcetera. • a single double bond: >=< • treat NO as :NO+, which is a 2 electron donor • More “exotic” ligands: • allyl anion: H2C=C-CH2-1 ↔ H2C-1-C=CH2 • 4 electron donor • written as: >=< | M .. .. - | M

6. Nomenclature - | M • η3 ≡ “trihapto”; means that the 3 carbons are bound equidistant from the metal; “hapto” from the Greek word for fasten. • cyclobutadiene: η4 or η2 • η4 butadiene • η4 norbornadiene M M M M

7. Aromatic Ligands • cyclopentadiene • η5 ligand; 6 electron donor • can also function as η1 • benzene • η6; 6 electron donor • cycloheptatriene • η6 (not aromatic) • loss of H-1 yields η7-tropylium

8. Bonding Types • Metal-Carbon σ-Bonds. • Anion σ-Donors • simple metal alkys and aryls • Neutral σ-Donors • CO, NO, etc. e.g. Fe(CO)5 • Mode of bonding in σ-complexes is via hybridized orbitals. • These bonds are very highly polarized, so the more electronegative the substituents on the carbon, the more stable the M-C bond. CH3 CH3CH3 Al Al CH3 CH3CH3 -: CR3 stability: M-CF3 > M-CH3

9. Bonding Types π-antibonding MO (empty) • π-Type Bonding • use molecular obitals of H2C=CH2 • synergistic effects • could be represented by either or • however, has been found that there is free rotation about M-olefin bond. dz2 (empty) π-bonding MO (full) dxz (full) ═ C--C | \ / M M

10. Bonding Types • Cyclic π-Type Systems (most common) • “Sandwich” Complexes • e.g. ferrocene • are these ionic or covalent bonds? • ferrocene has a low melting point, a high volatility, and doesn’t produce ions in solution… therefore, covalent. • can also have mixed ring complexes.

11. Bonding Types • Bent Metallocenes • usually (η5-C5H5)2ML2 (note: always have additional ligands) • most common with lanthanides and early transition metals. • why not η6-benzene as ligand? probably steric reasons. • “Open-Face” Sandwiches. • (π-R)MLx

12. Organometallic Reaction Chemistry • Most reactions proceed by elementary steps that involve only 16 or 18 electron complexes (applies to intermediates, too). • Acid-Base Reactions -1 +1 +1 -1 HCo(CO)4 ↔ H+ + Co(CO)4-1 • no change in numbers of electrons • will change oxidation number and coordination number • can occur with either 16 or 18 electron compounds. • Lewis Base Dissociation Pt(PPh3)4 ↔ :PPh3 + Pt(PPh3)3 • removes 2 electrons and changes coordination number (-1) • no change in oxidation number • can only occur with 18 electron compounds (product is 16 electron) • reverse reaction, Lewis Base Association, can only occur for 16 electron complexes.

13. Organometallic Reaction Chemistry • Reductive Elimination/Oxidative Addition (Most Important!) H2IrIIICl(CO)L2 ↔ H2 + IrICl(CO)L2 • changes in valence electrons, oxidation number, and coordination number (ALL -2); so, have elimination coupled with reduction. • only 18 electron complexes undergo reductive elimination. • only 16 electron complexes undergo oxidative addition. • Insertion MeMn(CO)5 ↔ Me-C-Mn(CO)4 • change in valence electrons (-2), and coordination number (-1), no change in oxidation state. • only 18 electron complexes undergo insertion. • only 16 electron complexes undergo deinsertion. = O

14. Detail of Some Reactions • Substitution Mo(CO)6 + R3P → Mo(CO)5PR3 + CO • Goes by LB Dissociation, followed by LB Association. 18e- CO 16e- R3P 18e- Mo(CO)6 → Mo(CO)5 → Mo(CO)5PR3 • 16 electron species use reverse mechanism (LB Association, followed by LB Dissociation). 16 e- species → 18 e- species → 16 e- species • Olefins prefer associative pathways and react slowly when none is available. Room Temp. 6 Days Rh + D2C=CD2 No Reaction ═ ═

15. Detail of Some Reactions • Reactions at Coordinated Ligands • ferrocene is a true aromatic system, so one can do typical aromatic reactions with it. • e.g. electrophilic substitutions • these reactions are typically 106 – 109 x faster than the corresponding benzene reaction. • reason: for electrophilic substitutions, need electron density in π-system and this is gotten from the metal.

16. Detail of Some Reactions • “Insertion” Reactions LnM-X + Z=Y → LnM-Z-Y-X • most common with olefins, but can be run with anything with multiple bonds. • insertion into M-C bonds. ClCl Cp –Rh-║ → Cp-Rh-CH2CH2CH2CH3 CH2CH3 L • insertion into M-H bonds. PEt3PEt3 Cl-Pt-H + CH2=CHCH3 → Cl-Pt-CH2CH2CH3 PEt3PEt3 • insertion over other double bonds, e.g. decarboxylation. CO O (CO)4Mn-CH3 ↔ (CO)4Mn-C-CH3 +L

17. Detail of Some Reactions • Mechanism of “Insertion” reaction. • So, NOT really insertion, but migration. Z=Y association LnM-X + Z=Y LnM X Z=Y migration bond rearrangement LnMLnM-Z-Y-X X

18. Catalysis Using Organometallic Compounds • Advantages: • Olefin coordination makes them more susceptable to nucleophilic attack. • Reactants held close together in specific alignments. • Ziegler-Natta Polymerization • TiCl4 (catalyst) can polymerize ethylenes at 1-atm pressure. • i.e. reacts via olefin migration. Olefin binds in π-fashion, undergoes rearrangement, then “leaves: with open coordination site (repeat). • can get chains ~1 million M.W. CH3 CH2 Ti CH2═CHX X \/ C ║ C /\ CH3 CH2 Ti open X \/ C ║ C /\ CH3 CH2 Ti open Ti -CH2CHXCH2CH3

19. Catalysis Using Organometallic Compounds • Olefin Hydrogenation • Wilkinson’s catalyst (PPh3)3RhCl ≡ L3RhCl at 25oC H3C-CH3 L L Rh L Cl H2 H2C-CH3 L H Rh L H Cl L L H Rh L H Cl +L -L L H Rh L H Cl H2C-CH3 L Rh H L Cl CH2=CH2 L H Rh L H Cl CH2=CH2

20. Catalysis Using Organometallic Compounds - • Monsanto Acetic Acid Process I CO Rh I CO CH3I CH3OH + HI - CH3 C=O I CO Rh I CO I - CH3 I CO Rh I CO I O ║ CH3C-I + H2O O ║ CH3C-OH - CH3 C=O I Rh CO I I CO