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Organometallic M T Complexes. M T Organometallics. Organometallic compounds of the transition metals have unusual structures, and practical applications in organic synthesis and industrial catalysis. M T Organometallics.
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MT Organometallics Organometallic compounds of the transition metals have unusual structures, and practical applications in organic synthesis and industrial catalysis.
MT Organometallics One of the earliest compounds, known as Zeise’s salt, was prepared in 1827. It contains an ethylene molecule π bonded to platinum (II).
Zeise’s Salt The bonding orbital of ethene donates electrons to the metal. The filled d orbitals (dxz or dyz) donate electrons to the antibonding orbital of ethene.
Square Planar Complexes The complexes of platinum(II), palladium(II), rhodium(I) and iridium(I) usually have 4-coordinate square planar geometry. These complexes also typically contain 16 electrons, rather than 18. The stability of 16 electron complexes, especially with σ-donor π-acceptor ligands, can be understood by examining a MO diagram.
Square Planar Complexes The electron pairs from the 4 ligands used in σ bonding occupy the bonding orbitals.
Square Planar Complexes The dxy, dxz, dyz and dz2 orbitals are either weakly bonding, non-bonding, or weakly antibonding.
Square Planar Complexes The dx2-y2 orbital is anti-bonding, and if filled, will weaken the σ bonds with the ligands.
Square Planar Complexes As a result, 16 electrons will produce a stable complex.
Catalysis of Square Planar Compounds Square planar complexes are often involved as catalysis for reactions. The four-coordinate complexes can undergo addition of organic molecules or hydrogen, and then be regenerated as the organic product is released from coordination to the catalyst.
Catalysis – aldehyde formation Pd(II) undergoes addition of an alkene which is subsequently converted to an alcohol. Addition of a hydrogen atom to the metal with subsequent migration to the alcohol produces an aldehyde.
Bonding of Hydrocarbons Hydrocarbons can bond to transition metals via σ bonds or π bonds. Wilkinson’s catalyst, [RhCl(PPh3)] is used to hydrogenate a wide variety of alkenes using pressures of H2 at 1 atm or less. During the hydrogenation, the alkene initially π bonds to the metal, and then accepts a hydrogen to σ bond with the metal.
Hydrogen Addition Square planar complexes are known to react with hydrogen, undergoing addition, and breaking the H-H bond.
Hydrogen Addition The hydrogen bonding orbital donates electron density into an empty p or dorbital on the metal. M
Hydrogen Addition The loss of electron density in the bonding orbital weakens the H-H bond. M
Hydrogen Addition The metal can donate electron density from a filled d orbital (dxz or dyz) to the antibonding orbital on hydrogen, thus weakening or breaking the H-H bond.
The Template Effect A metal ion can be used to assemble a group of organic ligands which then undergo a condensation reaction to form a macrocyclic ligand. Nickel (II) is used in the scheme below.
MT Carbonyls Metal carbonyl compounds were first synthesized in 1868. Although many compounds were produced, they couldn’t be fully characterized until the development of X-ray diffraction, and IR and NMR spectroscopy.
MT Carbonyls Metal carbonyl compounds typically contain metals in the zero oxidation state. In general, these compounds obey the “18 electron rule.” Although there are exceptions, this rule can be used to predict the structure of metal carbonyl cluster compounds, which contain metal-metal bonds.
The 18 Electron Rule Many transition metal carbonyl compounds obey the 18-electron rule. The reason for this can be readily seen from the molecular orbital diagram of Cr(CO)6. The σ donor and π acceptor nature of CO as a ligand results in an MO diagram with greatest stability at 18 electrons.
The eg* orbitals are destabilizing to the complex. Since the 12 bonding orbitals are filled with electrons from the CO molecules, 6 electrons from the metal will produce a stable complex.
MT Carbonyls The CO stretching frequency is often used to determine the structure of these compounds. The carbon monoxide molecule can be terminal, or bridge between 2 or 3 metal atoms. The CO stretching frequency decreases with increased bonding to metals. As the π* orbital on CO receives electrons from the metal, the CO bond weakens and the ν decreases.
MT Carbonyls As the π* orbital on CO receives electrons from the metal, the CO bond weakens and the ν decreases.
MT Carbonyls Mn2(CO)10 Fe2(CO)9
MT Carbonyls Co4(CO)12
MT Carbonyls ν for free CO = 2143 cm-1
MT Carbonyls ν for free CO = 2143 cm-1
MT Carbonyls The CO stretching frequency will also be affected by the charge of the metal. Compoundν (cm-1) [Fe(CO)6]2+ 2204 [Mn(CO)6)]+ 2143 Cr(CO)6 2090 [V(CO)6]- 1860 [Ti(CO)6]2- 1750
MT Carbonyls The IR spectra of transition metal carbonyl compounds are consistent with the predictions based on the symmetry of the molecule and group theory. The more symmetrical the structure, the fewer CO stretches are observed in the IR spectra.
MT Carbonyls If there is a center of symmetry, with CO ligands trans to each other, a symmetrical stretch will not involve a change in dipole moment, so it will be IR inactive. An asymmetric stretch will be seen in the IR spectrum. As a result, trans carbonyls give one peak in the IR spectrum.
MT Carbonyls If CO ligands are cis to each other, both the symmetric stretch and the asymmetric stretch will involve a change in dipole moment, and hence two peaks will be seen in the IR spectrum.
MT Carbonyls Metal carbonyls with a center of symmetry typically show only 1 C-O stretch in their IR spectra, since the symmetric stretch doesn’t change the dipole moment of the compound. Combined with the Raman spectrum, the structure of these compounds can be determined.
Nomenclature for Ligands The hapticity of the ligand is the number of atoms of the ligand which directly interact with the metal atom or ion. It is indicated using the greek letter η (eta) with the superscript indicating the number of atoms bonded.
Cyclopentadienyl Compounds The ligand C5H5 can bond to metals via a σ bond (contributing 1 electron), or as a π bonding ligand. As a π bonding ligand, it can donate 3, or more commonly 5 electrons to the metal.
Cyclopentadienyl Compounds W(η3-C5H5)(η5-C5H5)(CO)2 has two π bonded cyclopentadienyl rings. One donates 3 electrons, and the other donates 5.
Counting Electrons There are two common methods for determining the number of electrons in an organometallic compound. One method views the cylcopentadienyl ring as C5H5-, a 6 electron donor. CO and halides such as Cl- are viewed as 2 electron donors. The oxidation state of the metal must be determined to complete the total electron count of the complex.
Counting Electrons The other method treats all ligands as neutral in charge. η5-C5H5 is viewed as a 5 electron donor, Cl is viewed as a chlorine atom and a 1 electron donor, and CO is a 2 electron donor. The metal is viewed as having an oxidation state of zero in this method.
Counting Electrons In either method, a metal-metal single bond is counted as one electron per metal. Metal-metal double bonds count as two electrons per metal, etc.
Ferrocene Fe(η5-C5H5)2 , ferrocene, is known as a “sandwich” compound. In the solid at low temperature, the rings are staggered. The rotational barrier is very small, with free rotation of the rings.
Ferrocene The cyclopentadienyl rings behave as an aromatic electron donor. They are viewed as C5H5- ions donating 6 electrons to the metal. The iron atom is considered to be Fe(II).
Bonding of Ferrocene Group theory is used to simplify the analysis of the bonding. First, consider just a single C5H5 ring. Determine Τπ by considering only the pz orbitals which are perpendicular to the 5-membered ring.
