Organometallic chemistry is the chemistry of compounds that contain M-C ... One
common feature of all organometallic compounds is the metal is in a very low.
Introduction to Organometallic Chemistry . Organometallic chemistry is the chemistry of compounds that contain M-C bonds, where M is either a main group metal or a transition metal. . One common feature of all organometallic compounds is the metal is in a very low oxidation state. . Because of the low oxidation state on the metal, there are many d-electrons in the complex, so π-backbonding between the metal and ligands is very important. One of the most common ligands in organometallic chemistry is carbonyl, CO. . Organometallic reagents are very important in synthesis because of their versatility. For example, metals will often activate the C-H bond, which lowers the activation energy for many organic transformations. . Organometallic catalysts are used to make many important pharmaceuticals and are critical in the petrochemical industry.
Stability of organometallic reagents . There is one problem with many organometallic reagents: because the metal is in a low oxidation state, if the complex is exposed to atmospheric oxygen the metal is readily oxidized. . Therefore, we carry out many organometallic reactions under an inert atmosphere of nitrogen or argon. . If a complex is exceptionally air sensitive, then we use a glove box:
. If we do not need a glove box and can safely carry out the reaction in a hood, we usually use a Schlenck flask to protect the flask contents from atmospheric oxygen:
. We can even transfer liquids between two flasks under an inert atmosphere using cannula tubing:
Formalisms in Organometallic Chemistry . There are four important things to know before being able to predict the reactivity of an organometallic complex: 1. the oxidation state of the metal, 2. the number of d-electrons on the metal, 3. the coordination number of the metal, and 4. the availability (or lack thereof) of vacant coordination sites on the metal. . It is important to realize that these formalisms are merely formal ways of looking at reality; they are not reality, they are not the “truth”, and they may not even be chemically reasonable. However, they allow us to predict trends among a very diverse set of chemical compounds.
General Trends for the Transition Metals Group 8 Metals d3 21
d4 22
d5
d6
d7
d8
27
d 10 28
d 10s 1
23
24
25
Sc Ti
V
Cr
Mn Fe Co Ni Cu
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
39
40
41
42
43
44
45
46
47
Y
Zr
Nb Mo Tc Ru Rh Pd Ag
Yttrium
Zirconium
Niobium
Molybdenum Technetium
Ruthenum
Rhodium
Palladium
Silver
57
72
73
74
76
77
78
79
75
26
d9
29
La Hf Ta W
Re Os Ir
Pt Au
Lanthanum
Rhenium
Platinum
Hafnium
Tantalum
Tungsten
Osmium
Iridium
Gold
Early Transiton Metals
Late Transition Metals
low electronegativities
higher electronegativities
higher oxidation states
lower oxidation states
“harder” metal centers
“softer” metal centers
OXOPHILLIC!!
Hapticity
. Hapticity, η, ‘eta’ is the number of atoms associated with the metal in an organometallic. . If only one atom is attached to the metal, we say the ligand is a monohapto or η 1 ligand. . Cyclopentadiene and the cyclopentadienyl anion, Cp, can bind to a metal through one to five carbon atoms:
Bridging ligand µx “mu-x” is the nomenclature used to indicate the presence of a bridging ligand between two or more metal centers. The x refers to the number of metal centers being bridged by the ligand. Usually most authors omit x = 2 and just use to indicate that the ligand is bridging the simplest case of two metals
Ordering There is no set method of naming or ordering the listing of metal and ligands in a metal/ligand complex that most authors follow. There are IUPAC formalisms, but hardly anyone follows them. There are some qualitative rules that most authors seem to use in American Chemical Society (ACS) publications: 1)
in formulas with Cp (cyclopentadienyl) ligands, the Cp usually comes first, followed by the metal center: Cp2TiCl2
2)
other anionic multi-electron donating ligands are also often listed in front of the metal.
3)
in formulas with hydride ligands, the hydride is sometimes listed first. Rules # 1 & 2, however, take precedence over this rule: HRh(CO)(PPh3)2 and Cp2TiH2
4)
bridging ligands are usually placed next to the metals in question, then followed by the other ligands (note that rules 1 & 2 take precedence): Co2(µ-CO)2(CO)6 , Rh2(µ-Cl)2(CO)4 , Cp2Fe2(µ-CO)2(CO)2
Common Coordination Geometries 6-Coordinate: Octahedral (90° & 180° angles)
L L
L
L
M
L
L
L L
L
L
M
L
M L
L
L
L
5-Coordinate: Trigonal Bypyramidal or Square Pyramidial (90° & 120°) (~100° & 90°)
L L
M
L
axial L
equitorial
L
L
apical
M
L
L
L
L 4-Coordinate: Square Planar or Tetrahedral (90° & 180°)
(109°)
L
L L
M L
M
L L
L
L
basal
Types of Ligand The Carbonyl Ligand, CO . The carbonyl ligand is a σ-donor and very strong π-acceptor. . The CO homo has the appropriate symmetry for overlap with a metal LUMO: . Therefore, there is a σ -interaction between the CO HOMO and the Metal LUMO.
σ -interaction . In addition, the metal HOMO can donate electron density into the CO LUMO since these orbitals have the appropriate symmetry for overlap: . Therefore, there is a π -interaction with the metal donating electron density into the π * LUMO of CO. . As electron density is donated from the metal into the antibonding π * LUMO of CO, there is a decrease in CO bond order when CO binds to a metal. . Therefore, the CO bond is weaker when bonded to a metal than in the gas phase.
π -interaction . There are four different CO bonding modes.
The Hydride Ligand, H. When coordinated to a metal, H is formally negative and is, therefore, called the hydride ligand. . Metal hydrides often behave as proton donors: HCo(CO)4 + H2O −−> [Co(CO)4]- + H3O+ . There are four different binding modes for hydrides:
Phosphines and Phosphites . Phosphines are ubiquitous in catalysis because their stereoelectronic properties are easily manipulated. . Phosphines are σ -donors and π -acceptors. . The more electronegative the substituents on P, the weaker the σ -donor ability and the stronger the π -acceptor ability. . For example, PF3 is a weak σ -donor but as strong a π -acceptor as CO. . Conversely, P(tBu)3 is a strong σ -donor (because of the inductive electron release ability of the tBu groups) but a weak π -acceptor. . The π -accepting abilities of phosphines increase as: PF3 > P(OPh)3 > P(OMe)3 > PMe3 > P(tBu)3 . Steric effects in organometallic chemistry were first defined by Tolman in 1970. . Bidentate phosphines are extremely important in catalysis. . Bidentate phosphines have two P-donor atoms per molecule. . Some examples of important bidentate phosphines, including BINAP, which was responsible for Nyori’s and Knowles’ Noble Prize in 2001, are shown below:
π -Bonded Organic Ligands . In addition to being able to bind hydrogen, metals can also bind unsaturated organic molecules. . When Rh binds ethylene, for example, the C=C bond lengthens from 1.339 Å for H2C=CH2 to 1.445 Å in the Rh-( η2-H2C=CH2) complex. . This increase in C=C bond length upon coordination to a metal indicates that the metal donates electrons into the π* LUMO of the ethylene. . Consequently, the C=C bond weakens, which makes the C=C bond more susceptible to attack by an incoming nucleophile. . In other words, coordination of the olefin to a metal results in activation of the organic molecule. . There are two extremes to consider when an olefin binds to a metal: the one is pure 2 coordination, the other involves the formation of a metallocyclopropane:
Other Ligands . Dinitrogen often forms an η 1-bond with a metal, analogous to CO: . Dihydrogen, on the other hand, forms an η 2-bond with a metal.
M-N2.
The 18 Electron Rule
. The effective atomic number (EAN) rule states that most stable organometallic complexes have 18 electrons surrounding the metal. This is also known as the 18-electron rule. . Metals that contain 18 electrons around them are called coordinatively saturated. . Metals that contain less than 18 electrons around them are called coordinatively unsaturated and have vacant coordination sites. . To determine whether or not a metal is coordinatively saturated, we need to know how many electrons the ligand donates. . There are two ways to count electrons: ionic and covalent. Both give the same answer. . Ionic counting assumes the metal is in a formal oxidation state and the ligands are charged. . Covalent counting assumes the metal is in the zero oxidation state and the ligands are neutral.
The covalent counting method Classes of Ligands
. We classify ligands by the number of electrons they donate to the metal: One Electron Donors Alkyls, R Aryls, Ar Bent nitrosyl, NO Hydride, H η 1-ligands Halides
Two Electron Donors Phosphines, PR3, PAr3 Phosphites, P(OR)3 Carbonyl, CO η 2-olefins, R2C=CR2 η 2-alkynes, RCαCR Amines, R3N-M Nitriles, RCN-M Isonitriles, RNC-M Carbenes, M=CR2
Three Electron Donors Linear nitrosyl, NO η 3-allyls, R2C-CR=CR2 η 3-Cp
. Notice that the hapticity of the ligand is generally equal to its electron count. . Certain ligands, like allyl and cyclopentadienyl, can bind in more than one manner:
. When an η 3-allyl complex forms, the ligand bonds through all three carbon atoms and is considered a 3-electron donor. However, the allyl only occupies two coordination sites on the metal. . We are now in a position to consider any transition metal organometallic complex, assign the oxidation state at the metal, count the total number of electrons involved in bonding, and decide whether or not the complex is coordinatively saturated.
Example: Which of the following is coordinatively unsaturated: CpFe(CO)2(propyl) or Rh(PPh3)3Cl?
The ionic counting method To determine the electron count for a metal complex: 1) Determine the oxidation state of the transition metal center(s) and the metal centers resulting d-electron count. To do this one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal center (ionic ligand method) c) know the number of electrons being donated to the metal center from each ligand (ionic ligand method) 2) Add up the electron counts for the metal center and ligands the charges and donor # for the common ligands: Cationic 2e- donor: NO (nitrosyl) Neutral 2e- donors: PR3 (phosphines), CO (carbonyl), R2C=CR2 (alkenes), RC CR (alkynes, can also donate 4 e-), N CR (nitriles) Anionic 2e- donors: Cl (chloride), Br (bromide), I (iodide), CH3 (methyl), CR3 (alkyl), Ph (phenyl), H (hydride) The following can also donate 4 e- if needed, but initially count them as 2e- donors (unless they are acting as bridging ligands): OR (alkoxide), SR (thiolate), NR2 (inorganic amide), PR2 (phosphide) Anionic 4e- donors: C3H5 (allyl), O2 (oxide), S2 (sulfide), NR2 (imide), CR22 (alkylidene) and from the previous list: OR (alkoxide), SR (thiolate), NR2 (inorganic amide), PR2 Anionic 6e- donors: Cp (cyclopentadienyl), O2 (oxide) Please note that we are using the Ionic Method of electron-counting. inorganic/organometallic chemists use the ionic method.
95% of
Example:
CH3 R 3P
Re CO
CO PR3
1)
There is no overall charge on the complex
2)
There is one anionic ligand (CH3, methyl group)
3)
Since there is no overall charge on the complex (it is neutral), and since we have one anionic ligand present, the Re metal atom must have a +1 charge to compensate for the one negatively charged ligand. The +1 charge on the metal is also its oxidation state. So the Re is the in the +1 oxidation state. We denote this in two different ways: Re(+1), Re(I), or ReI. I prefer the Re(+1) nomenclature because it is clearer. Most chemistry journals, however, prefer the Roman numeral notation in parenthesis after the element.
Now we can do our electron counting: Re(+1) 2 PR3 2 CO CH3 CH2=CH2 Total:
d6 4e4e2e2e18e-
16-Electron Species Most stable organometallic compounds obey the 18-electron rule. However, stable complexes do exist with electron counts other than 18, as factors such as crystal field stabilisation energy and the nature of the bonding between the metal and the ligand affect the stability of the compound. The most widely encountered exceptions to the rule are 16-electron complexes of the transition metals on the right hand side of the d block, particularly Groups 9 and 10. These 16-electron, square-planar complexes commonly have d8 electron configurations, for example Rh(I), Ir(I), Ni(II), and Pd(II).
Exceptions to the 18e “Rule” Group 8 Metals d3 21
d4 22
d5
d6
d7
d8
27
d 10 28
d 10s 1
23
24
25
Sc Ti
V
Cr
Mn Fe Co Ni Cu
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
39
40
41
42
43
44
45
46
47
Y
Zr
Nb Mo Tc Ru Rh Pd Ag
Yttrium
Zirconium
Niobium
Molybdenum Technetium
Ruthenum
Rhodium
Palladium
Silver
57
72
73
74
76
77
78
79
75
26
d9
29
La Hf Ta W
Re Os Ir
Pt Au
Lanthanum
Rhenium
Platinum
Hafnium
Tantalum
Early Transition Metals 16e and sub-16e configurations are common Coordination geometries higher than 6
Tungsten
Osmium
Iridium
Gold
Middle Transition Metals
Late Transition Metals
18e configurations are common
16e and sub-16e configurations are common
Coordination geometries of 6 are common
Coordination geometries of 5 or lower
Fundamental Reactions of Transition Metal Complex 1. 2. 3. 4. 5. 6. 7.
Ligands Substitution Oxidative Addition Insertion Transmetallation Reductive Elimination Dehydrometallation (Elimination of Hydrogen) Nucleophilic Attack on Ligands Coordinated to Transition Metals
Figure 2.10 A hypothetical catalytic cycle with the precatalyst MLn_1 and four catalytic intermediates.
Ligands Substitution
MLn + xP
MLn-xPx + xL
The mechanism of this substitution will almost always depend on whether the parent MLn complex is coordinatively saturated or not! Substitutions reactions occur by a combination of ligand addition and ligand dissociation reactions. O C OC
Mo
OC
O C
-CO
CO
OC
OC
CO
Mo
CO
O C
+PMe3
CO
OC
Mo
OC
C O
CO
CO PMe3
18e- saturated complex
16e- unsaturated complex
18e- saturated complex
Ligand Addition (association): this is when an incoming ligand coordinates to a metal center that has one or more empty orbitals available. Ph3P
Ph3P
Rh
Cl
+ CO
OC
Ph3P
Rh
Cl
PPh3
8
This Rh(+1) complex is d and only 14e-. Adding a ligand takes one to the more stable 16e- square-planar complex.
Ligand Dissociation: this is when a ligand coordinated to a metal dissociates (falls off). The probability of a specific ligand dissociating depends on how strongly or weakly it is coordinated to the metal center and steric effects. Ligand Dissociation Ph3P
Ph3P
Rh
Cl
PPh3
Ph3P
Rh
Ph3P
Cl
+ PPh3
The steric hindrence of the three bulky PPh3 ligands favors dissociation of one to form the 14e- RhCl(PPh3)2 complex. The moderate electron-donating ability of the PPh3 ligand (not a strongly coordinating ligand) makes this fairly facile.
Steric Factors Bulky (large) ligands occupy more space around a metal center and can block incoming ligands trying to access vacant coordination sites on a metal. Due to steric hindrance, however, they are also more often to dissociate to relieve the steric strain. Consider, for example, the following equilibrium: Ni(PR3)4
KD 25ºC
Ni(PR3)3
+
PR3
Ligand:
P(OEt)3
P(O-p-tolyl)3
P(O-i-Pr)3
P(O-o-tolyl)3
PPh3
Cone angle:
109º
128º
130º
141º
145º
KD :
< 1010
6 x 1010
2.7 x 105
4 x 102
> 1000
Solvent Effects Cl
Ph3P
Pt
PPh3
Cl
-Cl−
Cl
Ph3P
Pt
+ solvent
PPh3
- solvent
Cl
Ph3P
Pt
PPh3 solvent
+Br− Cl
Ph3P
Pt
PPh3
Br
The 14e- three coordinate intermediate is actually almost immediately coordinated by a solvent molecule to produce the solvated 16e- complex shown to the far right. The solvent is usually weakly coordinated and readily dissociates to constantly produce the 14ereactive intermediate. Few organometallic chemists formally write solvated metal complexes down in their mechanisms, but they certainly are formed. The coordinating ability of the solvent, therefore, can often affect reactions. The presence of lone pairs and electron-rich donor atoms on the solvent usually makes it a better ligand. The polarity of the solvent can also have a definite impact on a reaction. Polar solvents
are usually quite good for reactions, such as that shown above, involving charged species. A non-polar hydrocarbon solvent (like toluene, for example) would probably inhibit the chloride dissociation mechanism. Instead, the dissociation of the neutral, less polar phosphine ligand would probably be favored. Some Common Coordinating Solvents: acetone, THF, DMSO, water, alcohols, DME, DMF
Trans Effect The trans effect concerns the electronic effect of one ligand on another ligand when they are trans to one another. The classical trans effect involves two σ-donating ligands trans to one another. The stronger σ-donor ligand preferentially weakens the bond of the weaker σ-donor ligand trans to it, making it easier to dissociate and do a ligand substitution reaction.
Cl
Et3P
Pt
PEt3
L
-Cl
−
Et3P
Pt
N
PEt3
N
L
Et3P
Pt
PEt3
L
Relative rate of substitution based on trans ligand L : Cl− = 1, Ph− = 100, CH3− = 103, H− = 104
There is a cis effect, but it is much weaker and basically ignored:
Allyl
The allyl anion has a similar facile ability to switch between η3 and η1 coordination modes that can promote ligand additions and/or substitutions. O C Mn C O
η3
O C CO
CO
Mn C O
CO
CO
η1 usually not observed experimentally
O C
+L
Mn
L C O
CO
CO
η1 can be stable and isolated
- CO
O C Mn
L
η3
CO
CO
