1. Organometallic Chemistry
JHU Course
Prof. Kenneth D. Karlin Spring, 2009
Kenneth D. Karlin
Department of Chemistry, Johns Hopkins University

2. p. 1
Organometallic Chemistry Prof. Kenneth D. Karlin Spring, 2009 Class Meetings: TTh, 12:00 – 1:15 pm
Textbook – The Organometallic Chemistry of the Transition Metals” 4th Ed., R. H. Crabtree Course Construction: Homeworks, Midterm Exams (1 or 2), Oral Presentations
Rough Syllabus Most or all of these topics
• Introduction, History of the field
• Transition Metals, d-electrons
• Bonding, 18 e– Rule (EAN Rule)
• Ligand Types / Complexes
• Types of Compounds
M-carbonyls, M-alkyls/hydrides
M-olefins/arenes
M-carbenes (alkylidenes alkylidynes)
Other
• Reaction Types
Oxidative Addition
Reductive elimination
Insertion – Elimination
Nucleophilic/electrophilic Rxs.
• Catalysis – Processes
Wacker oxidation
Monsanto acetic acid synthesis
Hydroformylation
Polymerization- Olefin metathesis
Water gas-shift reaction
Fischer-Tropsch reaction

3. p. 2

4. Reaction Examples Oxidative Addition Reductive Elimination
• Carbonyl Migratory Insertion
• Reaction of Coordinated Ligands
Vaska’s complex

5. Reaction Examples - continued
• Wacker Oxidation
C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde)
Pd catalyst, Cu (co-catalyst)
• Monsanto Acetic Acid Synthesis
CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)
• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (a-olefins)
1963 Nobel Prize
Catalyst: Ti compounds and organometalllic Al compound (e.g., (C2H5)3Al )
• Olefin metathesis – variety of metal complexes
2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock
n CH2=CHR –––> –[CH2-CHR]n–

6. Organo-transition Metal Chemistry History-Timeline
p. 5
Organo-transition Metal Chemistry History-Timeline
Main-group Organometallics
Cacodyl – tetramethyldiarsine,
from Co-mineral with arsenic
1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)
1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–
n-Butyl-lithium
Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl Bonding- Dewar-Chatt-Duncanson model

7. Organo-transition Metal Chemistry History-Timeline (cont.)
p. 6
Organo-transition Metal Chemistry History-Timeline (cont.)
st metal-carbonyl, [PtCl2(CO)2]
1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)
1900 – M catalysts; organic hydrogenation (---> food industry, margerine)
1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+
1951 – Ferrocene discovered Sandwich structure proposed
Ferrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer). The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).
The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR spectroscopy and Scanning Tunneling Microscopy > Fluxional behavior. (Note: Fe-C bond distances are 2.04 Å).
Asides:
Oxidation states
US vs. UK
18-electrons ?
(Cp)2Fe
Cp = cyclopentadienyl anion)
(h5-C5H5)2Fe
(pentahapto)
Solid-state
structure

8. Organo-transition Metal Chemistry History-Timeline (cont.)
p. 7
Organo-transition Metal Chemistry History-Timeline (cont.)
Cotton and Wilkinson (of the Text) discover organometallic-complex fluxional behavior (stereochemical non-rigidity)
The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy barrier to which is lower than that allowing for the preparative isolation of the individual isomers at room temperature. It is conventional to assign to the stereochemically non-rigid systems those compounds whose molecules rearrange rapidly enough to influence NMR line shapes at temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The energy barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).
Aside:
Oxidation State
18-electron Rule

9. Fluxional behavior; stereochemical non-rigidity (cont.)
p. 8
Fluxional behavior; stereochemical non-rigidity (cont.)
Butadiene iron-tricarbonyl
Xray- 2 CO’s equiv, one diff., If retained in solution, expect, 2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling causes a change to the 2:1 ratio expected.
Two possible explanations:
Dissociation and re-association or (2) rotation of
the Fe(CO)3 moiety so that CO’s become equiv.
Former seems not right, because for example addition
of PPh3 does NOT result in substitution to give (diene)M(CO)2PPh3.
Note: You can substitute PPh3 for CO, but that requires
either high T or hv. So, the equivalency of the CO groups
is due to rotation without bond rupture, pseudorotation.
13C-NMR spectra
CO region, only

10. p. 9
Berry Pseudorotation
Pseudorotation: Ligands 2 and 3 move from axial to equatorial positions in the trigonal bipyramid whilst ligands 4 and 5 move from equatorial to axial positions. Ligand 1 does not move and acts as a pivot. At the midway point (transition state) ligands 2,3,4,5 are equivalent, forming the base of a square pyramid. The motion is equivalent to a 90° rotation about the M-L1 axis. Molecular examples could be PF5 or Fe(CO)5.

11. p. 10
The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal molecules, such as PF5, though it can also occur in molecules with a square pyramidal geometry.
The process of pseudorotation occurs when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. This forms a square based pyramid where the base is the four interchanging ligands and the tip is the pivot ligand, which has not moved. The two originally equatorial ligands then open out until they are 180 degrees apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation.

12. Organo-transition Metal Chemistry History-Timeline (cont.)
p. 11
Organo-transition Metal Chemistry History-Timeline (cont.)
1961 – D. Hodgkin, X-ray structure – Coenzyme Vitamin B12 (see other page)
Oldest organometallic complex (because biological) (see other page)
Ziegler/Natta Nobel Prize, polymerization catalysts
Fischer, 1st Metal-carbene complex
1965 – Cyclobutadieneiron tricarbonyl, (C4H4)Fe(CO)3
– theory before experiment
(C4H4) is anti-aromatic (4 p-electrons)
With -Fe(CO)3, C4H4 behaves as aromatic
1965 – Wilkinson hydrogenation catalyst, Rh(PPh3)3Cl
1971 – Monsanto Co. – Rh catalyzed acetic acid synthesis
Methylmalonyl-CoA
––> Succinyl-CoA
(CoA = coenzyme A)
Catalysis
of 1,2-shifts
(mutases) or
Homocysteine
methylation

13. Vitamin B-12 Co-enzyme
p.12
Vitamin B-12 is a water soluble vitamin, one of the eight B vitamins. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. A common synthetic form of the vitamin, cyanocobalamin (R = CN), does not occur in nature, but is used in many pharmaceuticals, supplements and as food additive, due to its stability and lower cost. In the body it is converted to the physiological forms, methylcobalamin (R = CH3) and adenosylcobalamin, leaving behind the cyanide.
5-deoxyadenosyl group

14. Organo-transition Metal Chemistry History-Timeline (cont.)
p. 13
Organo-transition Metal Chemistry History-Timeline (cont.)
1973 – Commercial synthesis of L-Dopa (Parkinson’s drug)
asymmetric catalytic hydrogenation
2001 Nobel Prize – catalytic asymmetric synthesis, W. S. Knowles (Monsanto Co.)
R. Noyori,, (Nagoya, Japan), K. B. Sharpless (Scripps, USA)
1982, 1983 – Saturated hydrocarbon oxidative addition, including methane
1983 – Agostic interactions (structures)

15. AGOSTIC INTERACTIONS:
p. 14
AGOSTIC INTERACTIONS:
Agostic – derived from Greek word for "to hold on to oneself”
C-H bond on a ligand that undergoes an interaction with the metal complex resembles the transition state of an oxidative addition or reductive elimination reaction.
Detected by NMR spectroscopy, X-ray diffraction
Compound above: Mo–H = 2.1 angstroms, IR bands were observed at 2704 and 2664 cm–1 and the agostic proton was observed at –3.8 ppm. The two hydrogens on the agostic methylene are rapidly switching
between terminal and agostic on the NMR time scale.

16. Organometallic Chemistry
p. 15
Organometallic Chemistry
Definition: Definition of an organometallic compound
Anything with M–R bond R = C, H (hydride)
Metal (of course) Periodic Table – down & left
electropositive element (easily loses electrons)
NOT:
• Complex which binds ligands via, N, O, S, other
M-carboxylates, ethylenediamine, water
• M–X where complex has organometallic behavior, reactivity patterns
e.g., low-valent
Oxidation State
Charge left on central metal as the ligands are removed in their ‘usual’ closed shell configuration (examples to follow).
d n for compounds of transition elements
N d < (N+1) s or (N+1) p in compounds
e.g., 3 d < 4 s or 4 p

17. d n computation – very important in transition metal chemistry
d n zero oxidation state of M in M-complex has a configuration d n where n is the group #.
Examples: Mo(CO)6 Mo(0) d n = d 6 (CO, neutral)
HCo(CO)4 H is hydride, H–, --> --> Co(I), d n = d 8
Group Group Group 7
V(CO)6– Cr(CO)6 Mn(CO)6+
V(–1) Cr(0) Mn(+1)
d 6 d 6 d 6
Isoelectronic and isostructural compounds (importance of d n)
Effective Atomic # Rule; 18-Electron Rule (Noble gas formalism)
# of electrons in next inert gas =
# Metal valence electrons + s (sigma) electrons from ligands
Rule: For diamagnetic (spin-paired) mononuclear complexes in organotransition metal compounds, one never exceeds the E.A.N.
p. 16

18. Cr(CO)6 Cr ---> d6 6 electrons
p. 17
Cr(CO)6 Cr ---> d6 6 electrons
(CO)6 e– - pairs from 6 ligands 12 electrons
––> to [Ar] configuration electrons
(will see more in M.O. diagram)
Consequence of EAN Rule:
leads to prediction of maximum in coordination #
Max coordination # = (18 – n) / n is from d n
d n
Max Coord #
– Change in 2-electrons results in change of only one in Coord. #
– Any Coord. # less than Max # ---> “coordinatively unsaturated”
Fe(CO)42– Fe(CO)5
18 e– 18 e–
Fe(–2) Fe(0)
d 10 d 8
4-coord 5-coord
both Coord. Saturated

19. A compound not obeying an rules Iron-carbonyl carbide
[ReH9]2– e.g., as Ba2+ salt
Re(VII), (Mn,Tc, Re triad)
d 0, 9 hydride ligands; CN = 9
Geometry: Face capped trigonal prism
A compound not obeying an rules
Fe5(CO)15C
Iron-carbonyl carbide

20. Eighteen-Electron Rule - Examples
Co(NH3) Cr(CO)6
Obey 18-electron rule for different reasons
Carbonyl Compounds in Metal-Metal Bonded Complexes
less straightforward
Fe2(CO) [p-Cp)Cr(CO)3] Co2(CO)8 (2 isomers)

21. d6 Octahedral maximum of 6 coordinate
p. 20
d6 Octahedral maximum of 6 coordinate Δo Δt eg
t2g t2 e M+
Free ion
spherical
six point charges
spherically distributed
four point charges
octahedral
ligand field
tetrahedral

22. Picture of Octahedral Complex Various representations
(ignore “s orbital”
spherical field of 6 charges
dz2, dx2-y2 (e2g)
(destabilized)
dxy, dxz, dyz (t2g)
(stabilized)
10Dq or Δo Oh
lower case letters for orbital

23. p. 22
The five d-orbitals form a set of two bonding molecular orbitals (eg set with the dz2 and the dx2-y2), and a set of three non-bonding orbitals (t2g set with the dxy, dxz, and the dyz orbitals).
eg orbitals point at ligands (antibonding)
appropriate symmetry for s-bonds to ligands
s-bonds will be six d2sp3 hybrids
ndz2, ndx2-y2, (n+1)s, (n+1)px,py,pz
t2g orbital set left as non-bonding

24. p. 23

25. Do small or relatively small, eg* only weakly antibonding
p. 24
Standard MO diagram for
Octahedral ML6 complexes
with s-donor ligands
e.g., [Co(NH3)6]3+ (18 e–)
e.g., W(Me)6 (12 e–)
Case I
Electron-configuration unrelated to 18–-Rule
1st Row-Complexes with “weak ligands”
Do small or relatively small, eg* only weakly antibonding
No restriction on # of d-electrons –– 12 to 22 electrons

26. p. 25

27. Case II Compounds which follow rule insofar as they
never exceed the 18-e– rule
• Metal in high oxidation state
Do is large(r) (for a given ligand)
radius is small –-> ligands approach closely ––> stronger bonding
• 2nd or 3rd Row Metal - 4d, 5d
Do is large(er) (for a given ligand); d-orbitals larger, more diffuse.
Complex d n Total e– Complex d n Total e–
ZrF62– OsCl62– 4 16
ZrF73– W(CN)83– 1 17
Zr(C2O4)44– W(CN)64– 2 18
WCl PtF
WCl6– PtF6– 5 17
WCl62– PtF62– 6 18
TcF62– PtCl42– 8 16
Less than 18 e–, but rarely exceed 18 e–
p. 26

28. Similar Result if ligands are high in Spectrochemical Series
e.g., CN– Do is larger
V(CN)63– d2
Cr(CN)63– d3
Mn(CN)63– d Less than or equal to 6 d-electrons
Fe(CN)63– d5 eg* not occupied
Fe(CN)63– d6
Co(CN)63– d6
however Co(II) d7 ––> Co(CN)53–
Ni(II) d8 ––> Ni(CN)42– and
Ni(CN)53–
Can have less than maximum # of non-bonding (t2g) electrons, because they
are nonbonding. Addition or removal of e– has little effect on complex stability

29. Do can get (or is) very small with p-donor ligands
F– example (could be Cl–, H2O, OH–, etc.)
Filled p-orbitals are the only orbitals capable of p-interactions
1 lone pair used in s-bonding
Other lone pairs p-bond
The filled p-orbitals are lower in energy than the metal t2g set
Bonding Interaction
3 new bonding MO’s filled by Fluorine electrons
3 new antibonding MO’s form t2g* set contain d-electrons
Do is decreased (weak field)
Ligand to metal (L M) p-bonding
Weak field, p-donors: F, Cl, H2O
Favors high spin complexes

30. T1g,T2g T1u,T2u Eg T2g A1g T1u eg (σ) eg (σ*) t2g (π) t2g (π*) 3d 4s
p. 29
T1g,T2g
T1u,T2u Eg
T2g
A1g
T1u
eg (σ)
eg (σ*)
t2g (π)
t2g (π*) 3d 4s 4p
π-orbitals
px, py
σ-orbital pz
Metal
Orbitals
Molecular
Ligand
focus on this part only
both sets of d orbitals are driven ↑ in energy due to lower lying ligand orbitals Δo

31. Have discussed s-donor and p-donor – now p-acceptor
eg (σ*)
t2g (π)
t2g (n.b.)
t2g (π*)
M-L bonding
non-bonding
both are antibonding
π-acceptor
largest separation
between sets of d-orbitals
σ-donor
intermediate separation
π-donor
smallest separation
antibonding

32. Mo(CO)6 CASE III Result: Increase in Do Δo
p. 31
T2g Eg
T1g, T2g
T1u, T2u
A1g
T1u
eg (σ M-L)
eg (σ* M-L)
t2g (π)
t2g (π*) 4d Δo
π* orbitals on CO
(6 x 2 each - orthogonal)
σ orbitals on CO
(6 x 1 each)
Mo(CO)6
Metal Orbitals
(only consider the d orbitals – 4s and 4p orbitals not included in the analysis)
Molecular Orbitals
Ligand Orbitals
CASE III
L high in spectrochemical series:
CO, NO, CN–, PR3, CNR
p-acid ligands – p-acceptors
Can form strong p-bonds
18 e– rule followed rigorously
Orbitals on M used in such p-bonding are just those which are non-bonding
Result: Increase in Do
Imperative to not
Have electrons in eg* orbitals
Want to maximize occupation of t2g because they are stabilizing

33. p. 32

34. p. 33
Implications of 18e– Rule for Complexes with p-accepting ligands
In octahedral geometry almost always have 6 d-electrons
12 electrons from ligands
Other cases: # d-electrons and coordination # complementary
• Coordination # exactly determined by electron-configuration and vice-versa
(see previous notes)
BrMn(CO)5 (d ?) I2Fe(CO)4 (d ?) Fe(CO)5 (d ?) Ni(PF3)4 (d ?)
All 18-electron
When M has odd electron ––––> metal-metal bond (often bridging CO’s)
Mn2(CO) Co2(CO)8
Some 17 electron species known: V(CO)6 d Mo(CO)2(diphos)2]+ d 5
See MO diagram: Want to fill stable MO’s’ there is a large gap to LUMO

35. Major Exception: d 8 square-planar complexes
As one goes across periodic table, d and p orbital energy
Level splitting gets larger – hard to use p orbitals for s-bonding
Common to have 4-coordinate SP complexes – dsp2 hybridization eg
t2g
dxz
dyz
dxy
dz2
dx2-y2 Δo
(degenerate )
ML6
ML4
Which d-orbitals?
Common for:
Rh(I), Ir(I)
Pd(II), Pt(II)
Rationalize d-orbital splittings
look at d-orbital pictures/axes

36. p. 35

37. dn C.N. Coord. Geom. Example(s)
Again, examples of complexes:
dn C.N. Coord. Geom. Example(s)
d Td Ni(CO)4, Cu(py)41+
d Trig.planar Pt(PPh3)3
d Linear (PPh3)AuX, Cu(py)2+
d8 5 TBP Fe(PF3)5
d8 4 (square) planar Rh(PPh3)2(CO)Cl (trans)
d4 7 capped octahedral Mo(CO)5X2
d sq. antiprism ReH5(PMePh2)3, Mo(CN)84–
d0 9 D3h symmetry [ReH9]2–
tricapped trig. prism

38. LIGANDS in Organometallic Chemistry:
p. 37
LIGANDS in Organometallic Chemistry:
Ligands, charge, coordination # (i.e., denticity)
X SnCl3 H (hydride) CH3 (alkyl, perfluoroalkyl)
Ar RC(O) (acyl) R3E (E = P, As, Sb, N) R2P
CO RNC (isonitrile, isocyanide) R2C (cabenoid, carbene)
R2N N2 C2H4 (olefin, alkene) R2C2 (acetylene)
C4H4 (cyclobutadiene) C3H5– (p-allyl) CH=CH-CH2– (s-allyl)
benzene (arenes) p-C5H5 (p-Cp) p-C7H7 (tropylium)
p-C3H3 (cyclopropenium, +) O (O-atom; oxide) NO (nitrosyl)
ArN2+ (diazonium)

39. Carbon Monoxide – exceedingly important ligand
CO-derivatives known for all transition metals
Structurally interesting, important industrially, catalytic Rxs
Source of pure metal:
Ni (Mond); Fe contaminated with Cu, purify via Fe(CO)5
Fe & Ni only metals that directly react with CO
Source of oxygen in organics: RC(O)H, RC(O)OH, esters
Processes: hydroformylation, MeOH ––> acetic acid
double insertion into olefins, hydroquinone synthesis (acetylene + CO; Ru catalyst), acrylic acid synthesis (acetylene, CO, Ni catalyst)
Fischer Tropsch Rx: CO + H2 ––> ––> CnH2n+2 + H2O
Most of these involve CO “insertion”

40. Metal-Carbonyl Synthesis:
p. 39
Metal-Carbonyl Synthesis:
Reduction of available (in our O2-environment) metal salts, e.g., MX2, M’X3, other (e.g., carbonates)
M-carbonyls generally in low-valent oxidation states
––––> “Reductive Carbonylation”
Reductants: CO itself ( ––> CO2), H2, Na-dithionite
Some Reactions: WMe xs CO –––> W(CO) Me2CO
NiO + H2 (400 °C) + CO ––> Ni(CO)4
Re2O7 + xs CO ––> (OC)5Re–Re(CO) CO2
RhCl3 + CO + pressure + (Cu, Ag, Cd, Zn)
–––> Rh4(CO)12 or Rh6(CO)16
Structures Possible: X-ray diffraction, Infrared spectroscopy
Ni(CO)4 Fe(CO)5 M(CO)6
Td D3h Oh
2058 cm– , 2034 cm– cm–1
Cl– acceptor/reductant

41. 13C NMR spectroscopy of M-CO fragments: 180 – 250 ppm
H3B–CO = cm–1
no backbonding possible
13C NMR spectroscopy of M-CO fragments: 180 – 250 ppm
Useful to use 13C enriched carbon monoxide
Can be useful to observed “coupling” to other spin active nuclei,
e.g., 103Rh or 13P

42. Metal-Carbonyl Structures (cont.):
p. 41
Metal-Carbonyl Structures (cont.):
Polynuclear Metal-Carbonyls

43. p. 42

44. p. 43

45. p. 44

46. Valence Bond formalism:
p. 45
The backbonding between the metal and the CO ligand, where the metal donates electron density to the CO ligand forms a dynamic synergism between the metal and ligand, which gives unusual stability to these compounds.
Valence Bond formalism:

47. C–O stretching frequencies, n(C-O)
p. 46
C–O stretching frequencies, n(C-O)
Put more electron density on metal
– by charge
– by ligands which cannot p-accept
Remaining CO’s have to take up the charge (e–-density) on the metal
See effects on n(C-O).
Ni(CO)4 [Co(CO)4]– Fe(CO)42–
2057 cm– cm– cm–1
–––––––> –––––––> more –ve charge
Mn(dien)(CO) ~ 2020, 1900 cm–1
Cr(dien)(CO)3 ~1900, 1760 cm–1 (dien not p-acceptor)