Introduction to Organometallic Chemistry. Stability of organometallic reagents

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1 Introduction to rganometallic hemistry. rganometallic chemistry is the chemistry of compounds that contain M- bonds, where M is either a main group metal or a transition metal.. ne 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. ne of the most common ligands in organometallic chemistry is carbonyl,.. rganometallic reagents are very important in synthesis because of their versatility. For example, metals will often activate the -H bond, which lowers the activation energy for many organic transformations.. rganometallic 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:

2 . We can even transfer liquids between two flasks under an inert atmosphere using cannula tubing:

3 Formalisms in rganometallic hemistry. 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.

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5 General Trends for the Transition Metals Group 8 Metals 21 d 3 d 4 d 5 d 6 d 7 d 8 d 9 d 10 d 10 s 1 Sc Scandium 39 Y Yttrium 57 a anthanum 22 Ti Titanium 40 Zr Zirconium 72 Hf Hafnium 23 V Vanadium 41 Nb Niobium 73 Ta Tantalum 24 r hromium 42 Mo Molybdenum 74 W Tungsten 25 Mn Manganese 43 Tc Technetium 75 Re Rhenium 26 Fe Iron 44 Ru Ruthenum 76 s smium 27 o obalt 45 Rh Rhodium 77 Ir Iridium 28 Ni Nickel 46 Pd Palladium 78 Pt Platinum 29 u opper 47 Ag Silver 79 Au Early Transiton Metals ate Transition Metals Gold

6 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.. yclopentadiene and the cyclopentadienyl anion, p, can bind to a metal through one to five carbon atoms: Bridging ligand µ x 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 rdering 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 IUPA formalisms, but hardly anyone follows them. There are some qualitative rules that most authors seem to use in American hemical Society (AS) publications: 1) in formulas with p (cyclopentadienyl) ligands, the p usually comes first, followed by the metal center: p 2 Til 2 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()(PPh 3 ) 2 and p 2 TiH 2 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): o 2 (µ-) 2 () 6, Rh 2 (µ-l) 2 () 4, p 2 Fe 2 (µ-) 2 () 2

7 ommon oordination Geometries 6-oordinate: ctahedral (90 & 180 angles) M M M 5-oordinate: Trigonal Bypyramidal or Square Pyramidial (90 & 120 ) (~100 & 90 ) axial apical M equitorial M basal 4-oordinate: Square Planar or Tetrahedral (90 & 180 ) (109 ) M M

8 Types of igand The arbonyl igand,. The carbonyl ligand is a σ-donor and very strong π-acceptor.. The homo has the appropriate symmetry for overlap with a metal UM:. Therefore, there is a σ -interaction between the HM and the Metal UM. σ -interaction. In addition, the metal HM can donate electron density into the UM since these orbitals have the appropriate symmetry for overlap:. Therefore, there is a π -interaction with the metal donating electron density into the π * UM of.. As electron density is donated from the metal into the antibonding π * UM of, there is a decrease in bond order when binds to a metal.. Therefore, the bond is weaker when bonded to a metal than in the gas phase. π -interaction. There are four different bonding modes.

9 The Hydride igand, H-. When coordinated to a metal, H is formally negative and is, therefore, called the hydride ligand.. Metal hydrides often behave as proton donors: Ho()4 + H2 > [o()4] - + H3 +. There are four different binding modes for hydrides:

10 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.. onversely, 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(Ph)3 > P(Me)3 > PMe3 > P(tBu)3. Steric effects in organometallic chemistry were first defined by Tolman in 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:

11 π -Bonded rganic igands. In addition to being able to bind hydrogen, metals can also bind unsaturated organic molecules.. When Rh binds ethylene, for example, the = bond lengthens from Å for H2=H2 to Å in the Rh-( η2-h2=h2) complex.. This increase in = bond length upon coordination to a metal indicates that the metal donates electrons into the π* UM of the ethylene.. onsequently, the = bond weakens, which makes the = 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: ther igands. Dinitrogen often forms an η 1-bond with a metal, analogous to : M-N 2.. Dihydrogen, on the other hand, forms an η 2-bond with a metal.

12 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.. ovalent counting assumes the metal is in the zero oxidation state and the ligands are neutral.

13 The covalent counting method lasses of igands. We classify ligands by the number of electrons they donate to the metal: ne Electron Donors Two Electron Donors Three Electron Donors Alkyls, R Phosphines, PR3, PAr3 inear nitrosyl, N Aryls, Ar Phosphites, P(R)3 η 3-allyls, R2-R=R2 Bent nitrosyl, N arbonyl, η 3-p Hydride, H η 2-olefins, R2=R2 η 1-ligands Halides η 2-alkynes, RαR Amines, R3N-M Nitriles, RN-M Isonitriles, RN-M arbenes, M=R2. Notice that the hapticity of the ligand is generally equal to its electron count.. ertain 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.

14 Example: Which of the following is coordinatively unsaturated: pfe()2(propyl) or Rh(PPh3)3l?

15 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: ationic 2e- donor: N (nitrosyl) Neutral 2e- donors: PR3 (phosphines), (carbonyl), R2=R2 (alkenes), R R (alkynes, can also donate 4 e-), N R (nitriles) Anionic 2e- donors: l (chloride), Br (bromide), I (iodide), H3 (methyl), R3 (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): R (alkoxide), SR (thiolate), NR2 (inorganic amide), PR2 (phosphide) Anionic 4e- donors: 3H5 (allyl), 2 (oxide), S2 (sulfide), NR2 (imide), R22 (alkylidene) and from the previous list: R (alkoxide), SR (thiolate), NR2 (inorganic amide), PR2 Anionic 6e- donors: p (cyclopentadienyl), 2 (oxide) Please note that we are using the Ionic Method of electron-counting. 95% of inorganic/organometallic chemists use the ionic method.

16 Example: R 3 P H 3 Re PR 3 1) There is no overall charge on the complex 2) There is one anionic ligand (H3, 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) d6 2 PR3 4e- 2 4e- H3 2e- H2=H2 2e- Total: 18e-

17 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 d 3 d 4 d 5 d 6 d 7 d 8 d 9 d 10 d 10 s Sc Ti V r Mn Fe o Ni u Scandium Titanium Vanadium hromium Manganese Iron obalt Nickel opper Y Zr Nb Mo Tc Ru Rh Pd Ag Yttrium Zirconium Niobium Molybdenum Technetium Ruthenum Rhodium Palladium Silver a Hf Ta W Re s Ir Pt Au anthanum Hafnium Tantalum Tungsten Rhenium smium Iridium Platinum Gold Early Transition Metals Middle Transition Metals ate Transition Metals 16e and sub-16e configurations are common 18e configurations are common 16e and sub-16e configurations are common oordination geometries higher than 6 oordination geometries of 6 are common oordination geometries of 5 or lower

18 Fundamental Reactions of Transition Metal omplex 1. igands Substitution 2. xidative Addition 3. Insertion 4. Transmetallation 5. Reductive Elimination 6. Dehydrometallation (Elimination of Hydrogen) 7. Nucleophilic Attack on igands oordinated to Transition Metals Figure 2.10 A hypothetical catalytic cycle with the precatalyst Mn_1 and four catalytic intermediates.

19 igands Substitution M n + xp M n-x P x + x The mechanism of this substitution will almost always depend on whether the parent Mn complex is coordinatively saturated or not! Substitutions reactions occur by a combination of ligand addition and ligand dissociation reactions. Mo - Mo +PMe 3 Mo PMe 3 18e- saturated complex 16e- unsaturated complex 18e- saturated complex igand Addition (association): this is when an incoming ligand coordinates to a metal center that has one or more empty orbitals available. Ph 3 P Ph 3 P Rh l + Ph 3 P Rh l PPh 3 This Rh(+1) complex is d 8 and only 14e-. Adding a ligand takes one to the more stable 16e- square-planar complex. igand 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. igand Dissociation Ph 3 P Ph 3 P Rh l PPh 3 Ph 3 P Ph 3 P Rh l + PPh 3 The steric hindrence of the three bulky PPh 3 ligands favors dissociation of one to form the 14e- Rhl(PPh 3 ) 2 complex. The moderate electron-donating ability of the PPh 3 ligand (not a strongly coordinating ligand) makes this fairly facile.

20 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. onsider, for example, the following equilibrium: K D Ni(PR 3 ) 4 Ni(PR 3 ) 3 + PR 25º 3 igand: P(Et) 3 P(-p-tolyl) 3 P(-i-Pr) 3 P(-o-tolyl) 3 PPh 3 one angle: 109º 128º 130º 141º 145º K D : < x x x 10 2 > 1000 Solvent Effects l PPh 3 Pt Ph 3 P l -l l PPh 3 Pt Ph 3 P + solvent - solvent l PPh 3 Pt Ph 3 P solvent +Br l PPh 3 Pt Ph 3 P 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

21 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 ommon oordinating Solvents: acetone, THF, DMS, 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. l PEt 3 Pt Et 3 P -l Et 3 P Pt PEt 3 N N Et 3 P Pt PEt 3 Relative rate of substitution based on trans ligand : l = 1, Ph = 100, H 3 = 10 3, H = 10 4 There is a cis effect, but it is much weaker and basically ignored: The allyl anion has a similar facile ability to switch between η 3 and η 1 coordination modes that can promote ligand additions and/or substitutions. Mn Mn + η 3 η 1 η 1 η 3 usually not observed experimentally Mn can be stable and isolated - Mn