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Lecture 4 Transition MetAL Organometallics LIGANDS

Lecture 4 Transition MetAL Organometallics LIGANDS. TRANSITION METALS. EARLY TRANSITION METALS Groups 3 , 4. Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18 e Polar and very reactive M-C bonds (to alkyl and aryl).

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Lecture 4 Transition MetAL Organometallics LIGANDS

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  1. Lecture 4Transition MetALOrganometallics LIGANDS

  2. TRANSITION METALS

  3. EARLY TRANSITION METALSGroups 3, 4 • Strongly electrophilic and oxophilic • Few redox reactions (exception: Ti) • Nearly always < 18e • Polar and very reactive M-C bonds (to alkyl and aryl)

  4. EARLY TRANSITION METALSGroups 3, 4 • Few d-electrons: • preference for "hard" s-donors (N/O/F) • weak complexation of p-acceptors (olefins, phosphines) • Typical catalysis: Polymerization

  5. "MIDDLE" TRANSITION METALS Groups 5-7 • Many accessible oxidation states • Mostly 18e • Ligands strongly bound • Strong, not very reactive M-C bonds

  6. "MIDDLE" TRANSITION METALS Groups 5-7 • Preference for s-donor/p-acceptor combinations (CO!) • Typical catalysis: Alkene and alkyne metathesis

  7. LATE TRANSITION METALSGroups 8-10 (and 11) • Many accessible oxidation states • Mostly 18e or 16e16e common for square-planar complexes • Easy ligand association/dissociation • Weak, not very reactive M-C bonds • Even weaker, reactive M-O/M-N bonds

  8. LATE TRANSITION METALSGroups 8-10 (and 11) • Preference for s-donor/weak p-acceptor ligands (phosphines) • Typical catalysis: Hydroformylation

  9. ROWS Transition-metal Organometallics

  10. GOING DOWN 1st row: • often unpaired electrons • different spin states (HS/LS) accessible • "highest possible" oxidation states not very stable • MnO4- is a strong oxidant 2nd/3rd row: • nearly always "closed shell" • virtually same atomic radii (except Y/La) • highest oxidation states fairly stable • ReO4- is hardly oxidizing • 2nd row often more reactive than 3rd

  11. THE CARBONYL LIGAND • In 1884 Ludwig Mond found his nickel valves were being eaten away by CO. An experiment was designed where he deliberately heated Ni powder in a CO stream thus forming the volatile compound, Ni(CO)4, the first metal carbonyl. It was also found that upon further heating Ni(CO)4 decomposes to give pure nickel. This Ni refining process still used today is known as the Mondprocess. • Having no net dipole moment, intermolecular forces are relatively weak, allowing Ni(CO)4 to be liquid at room temperature.

  12. THE CARBONYL LIGAND • CO groups have a high tendency to stabilize M−M bonds; not only are CO ligands relatively small but they also leave the metal atom with a net charge similar to that in its elemental form (electroneutrality principle). “Stable complexes are those with structures such that each atom has only a small electric charge. Stable M-L bond formation generally reduces the positive charge on the metal as well as the negative charge and/or e- density on the ligand. The result is that the actual charge on the metal is not accurately reflected in its formal oxidation state” - Pauling; The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.

  13. CARBONYL • CO also has the ability to stabilize polyanionic species by acting as a strong  acceptor and delocalizing the negative charge over the CO oxygens. • Na4[Cr(CO)4] has the extraordinarily low ν(CO) of 1462 cm−1, the extremely high anionic charge on the complex, and ion pairing of Na+ to the carbonyl oxygen contribute to the reduced CO bond order by favoring the MC−ONa resonance

  14. THE CARBONYL LIGAND • As the CO ligand is small and strongly bound, many will usually bind as are required to achieve coordinative saturation, e.g. V(CO)7 • Metal carbonyls, in common with metal hydrides, show a strong preference for the 18e configuration.

  15. METAL CARBONYLS – STRUCTURE and BONDING • CO is an unsaturated ligand, by virtue of the CO multiple bond. • CO is classed as a soft ligand because it is capable of accepting metal d electrons by back bonding, i.e. it is a -donor -acceptor ligand. This contrasts to hard ligands, which are σ donors, and often  donors, too. • CO can act as a spectator or an actor ligand. Overview of Organometallic Chemistry

  16. METAL CARBONYLS – STRUCTURE and BONDING

  17. The CARBONYL LIGAND • In the CO molecule both the C and the O atoms are sphybridized. • The singly occupied sp and pz orbitals on each atom form a σ and a bond, respectively. Frontier orbitals of free CO showing the polarization of the z orbital.

  18. THE CARBONYL LIGAND • This leaves the C py orbital empty, and the O py orbital doubly occupied, and so the second bond is formed only after we have formed a dative bond by transfer of the lone pair of O py electrons into the empty C py orbital.

  19. This transfer leads to a C−−O+ polarization of the molecule, which is almost exactly canceled out by a partial C+−O− polarization of all three bonding orbitals because of the higher electronegativity of oxygen. • The free CO molecule therefore has a net dipole moment very close to zero.

  20. CARBONYL LIGAND • A metal orbital forms a bond with HOMO orbital of CO. • The HOMO is a s orbital based on C (due to the higher electronegativity of O its orbitals have lower energy). • The metal orbitals form a bond with the CO * LUMO (again polarized toward C) • The metal HOMO, the filled M dorbital, back donates to the CO LUMO increasing electron density at both C and O because CO * has both C and O character. • The result is that C becomes more positive on coordination, and O becomes more negative. This translates into a polarization of the CO on binding.

  21. THE CARBONYL LIGAND • This metal-induced polarization chemically activates the CO ligand. • It makes the carbon more sensitive to nucleophilic and the oxygen more sensitive to electrophilic attack. • The polarization will be modulated by the effect of the other ligands on the metal and by the net charge on the complex. • In LnM(CO), the CO carbon becomes particularly + in character if the L groups are good  acids or if the complex is cationic, e.g. Mo(CO)6 or [Mn(CO)6]+, because the CO-to-metal -donor electron transfer will be enhanced at the expense of the metal to CO back donation.

  22. CARBONYL • If the L groups are good donors or the complex is anionic, e.g. Cp W(CO) or 2[W(CO)5]2−, back donation will be encouraged, the CO carbon will lose its pronounced + charge, but the CO oxygen will become significantly −. • The range can be represented in valence bond terms the extreme in which CO acts as a pure s donor, through to the extreme in which both the ∗xand ∗y are both fully engaged in back bonding.

  23. CARBONYLs and IR BANDS • The high intensity of the CO stretching bands (a result of polarization on binding) means that IR spectroscopy is extremely useful. • From the band position, we can tell how good the metal is as a  base. • From the number and pattern of the bands, we can tell the number and stereochemistry of the CO’s present.

  24. CARBONYL LIGAND • We can tell the bond order of the CO ligand by recording the M-CO IR spectrum. The normal range of the C-O stretching frequency, (CO) is 1820–2150 cm−1. Free C-O stretch at 2143 cm-1. Lower energy for stretching mode means C-O bond is weaker. • As the metal to CO *back bonding becomes more important, we populate an orbital that is antibonding with respect to the C=O bond, and so we lengthen and weaken the CO bond, i.e. the M−C bond is made at the expense of the C=O bond.

  25. CARBONYL LIGAND • Strong donor co-ligands or a negative charge on the metal result in CO stretches at lower frequency. Why? v(CO) cm-1 [V(CO)6]- 1859 Cr(CO)6 2000 [Mn(CO)6 ]+ 2100 [Fe(CO)6]2+ 2204 • The greater the ability of a metal to donate electrons to the * orbitals of CO, the lower the energy of the C-O stretching vibration.

  26. SAMPLE EXERCISE • On the basis of the carbonyl complexes in the table shown, predict the approximate position (in cm-1) of the C-O streching band in [Ti(CO)6]2-

  27. CARBONYL LIGANDS • Carbonyls bound to very poor -donor metals have very high frequency ν(CO) bands as a result of weak back donation. • When these appear to high energy of the 2143 cm−1 band of free CO, the complexes are sometimes called non-classical carbonyls. • Even d0 species can bind CO, for example, the nonclassical, formally d0Zr(IV) carbonyl complexes, [Cp2Zr(S2)(CO)] has a ν(CO) stretching frequency of 2057 cm−1.

  28. CARBONYL LIGANDS The highest oxidation state carbonyl known is trans-[OsO2(CO)4]2+ with ν(CO) = 2253 cm−1. Carbonyls with exceptionally low ν(CO) frequencies are found for negative oxidation states (e.g., [Ti(CO) ]2−; ν(CO) = 1747 cm−1) or where a single CO is accompanied by non -acceptor ligands (e.g., [ReCl(CO)(PMe3)4]; ν(CO) = 1820 cm−1); these show short M−C and long C−O bonds.

  29. CARBONYL LIGAND • One of the most extreme weak -donor examples is [Ir(CO)6]3+ with ν(CO) bands at 2254, 2276, and 2295 cm−1. • The X-ray structure of the related complex [IrCl(CO)5]2+ shows the long M−C [2.02(2)A° ] and short C−O [1.08(2)A° ] distances expected. Overview of Organometallic Chemistry

  30. SYNTHESIS • Direct reaction of a transition metal with CO. Ni + 4CO Ni(CO)4 This method requires that the metal already be in a reduced state because only -basic metals can bind CO.

  31. SYNTHESIS 2. Reductive carbonylation (reducing agent plus CO gas): CrCl3 + 6CO + Al Cr(CO)6 + AlCl3 (catalyzed by AlCl3) Re2O7 + 17CO Re2(CO)10 + 7 CO2 (CO as RA) NiSO4 + CO + S2O42- Ni(CO)4

  32. SYNTHESIS 3. Thermal or photochemical reaction of other binary carbonyls. Fe(CO)5hv Fe2(CO)9 Lesser known: From organic carbonyls.

  33. BRIDGING MODES CO has a high tendency to bridge two metals (μ2-CO) Electron count here is unchanged either side of equilibrium In most cases the M−M bond accompanies the CO bridging group. The CO stretching frequency in the IR spectrum falls to 1720–1850 cm−1 on bridging. Type of CO v(CO) cm -1 Free CO 2143 terminal M-CO 1850-2120 bridging CO 1700-1850 Overview of Organometallic Chemistry

  34. BRIDGING • Consistent with the idea of a nucleophilic attack by a second metal, a bridging CO is more basic at O than the terminal ligand. • Thus a bridging CO ligand will bind a Lewis acid more strongly than a terminal CO ligand. • Equilibrium can therefore be shifted in the previous reaction scheme.

  35. BRIDGING • Triply and even quadruply bridging CO groups are also known in metal cluster compounds. For example, (Cp∗Co)3(μ3-CO)2 • These have CO stretching frequencies in the range of 1600–1730 cm−1.

  36. REACTIONS OF METAL CARBONYLS All reactions of the CO ligand depend on the polarization of the CO upon binding, and so change in importance as the co-ligands and net charge change. 1. Nucleophilic attack at the Carbon:

  37. Reactions Hydride attack at the C atom of CO here produces the unusual formylligand, which is important in CO reduction to MeOH. It is stable in this case because the final 18e complex provides no empty site for rearrangement to a hydridocarbonyl complex (a-elimination).

  38. Reactions 2. Electrophilic attack at Oxygen 3. Migratory Insertion:

  39. BRIDGING • There also exists the semi-bridging carbonyl in which the CO is neither fully terminal nor fully bridging but intermediate between the two. • This is one of the many cases in organometallic chemistry where a stable species is intermediate in character between two bonding types. • Below each semi-bridging CO is bending in response to the second metal atom

  40. LIGANDS SIMILAR TO CO CS, Cse, CTe do not exist as a stable free molecule and therefore do not provide a ready ligand source. CN- and N2 complexes CN- - stronger  donor than CO - very similar to CO when it interacts with metal orbitals - weaker  acceptor (consequence of the negative charge) Dinitrogen is a weaker  donor and  acceptor vs. CO. However, still of interest in reactions that might stimulate nitrogen fixation.

  41. PROBLEM SETS From Spessard and Meissler. 4-1 to 4-5, 4-8, 4-10, 4-11, 4-13 5-1, 5-3, 5-6, 5-7, 5-9, 5-13 to 5-14 6-1, 6-2, 6-3, 6-4, 6-6, 6-8

  42. IR SPECTRA No. of Bands: Monocarbonyl complexes have single possible C-O stretching mode – single IR band Dicarbonyl complexes: Linear and bent

  43. Complexes with 3 or more carbonyls Prediction of exact no. of carbonyl bands complex but can be determined using group theory. For convenience refer to a table. Although can predict using a table: - some bands may overlap - may have very low intensity - if isomers present, difficult to sort out

  44. POSITIONS OF IR BANDS: • Increase in negative charge of the complex, causes a reduction in the energy of the CO band. • The bonding mode of the CO terminal CO > double bridged (2) > triply bridged (3)

  45. POSITION OF IR BANDS • Other ligands present also affects position of IR bands. For example for the complex Ni(CO)3L: L v(CO), cm-1 PF3 2111 PCl3 2097 PPh3 2069 The greater the electron density on the metal, the greater the back bonding to CO, the lower the energy of the carbonyl stretching vibration.

  46. MAIN GROUP vs. BINARY CARBONYL COMPLEXES

  47. Cl vs. Co(CO)4

  48.  BONDED Ligands Alkene Complexes Alkyne Complexes Allyl Complexes Diene Complexes Cyclopentadienyl Complexes Arene Complexes Metallacycles Overview of Organometallic Chemistry

  49. TRANSITION METAL – ALKENE COMPLEXES • The report in 1825 by William Zeise of crystals with composition, KCl.PtCl2.Ethylene, prepared from KPtCl4 and EtOH was a topic of controversy for many years due to the nature of Zeise’s structure - only possible by the dehydration of EtOH. • Proof of Zeise’s formulation came 13 years later when Birnbaum isolated the complex from a solution of platinic acid, H2PtCl6.6H20, treated with ethylene. • Zeise’ssalt was the first organometallic compound to be isolated in pure form. • This discovery spawned a tremendous growth in organometallic chemistry and still serves as the simplest example of transition metal-olefin complexation. Overview of Organometallic Chemistry

  50. TRANSITION METAL – ALKENE COMPLEXES • The -acid ligand donates electron density into a metal d-orbital from a -symmetry bonding orbital between the carbon atoms. • The metal donates electrons back from a filled d-orbital into the empty * antibonding orbital of the ligand (similar to dihydrogen s-complexes) • Both of these effects tend to reduce the C-C bond order, leading to an elongated C-C distance and a lowering its vibrational frequency.

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