organometallic compounds

OrganometallicMT Complexes

MT OrganometallicsOrganometallic compounds of the transition metals have unusual structures, and practical applications in organic synthesis and industrial catalysis.

MT OrganometallicsOne of the earliest compounds, known as Zeises salt, was prepared in 1827. It contains an ethylene molecule bonded to platinum (II).

Zeises SaltThe 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 ComplexesThe 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 -donor -acceptor ligands, can be understood by examining a MO diagram.

Square Planar ComplexesThe electron pairs from the 4 ligands used in bonding occupy the bonding orbitals.

Square Planar ComplexesThe dxy, dxz, dyz and dz2 orbitals are either weakly bonding, non-bonding, or weakly antibonding.

Square Planar ComplexesThe dx2-y2 orbital is anti-bonding, and if filled, will weaken the bonds with the ligands.

Square Planar ComplexesAs a result, 16 electrons will produce a stable complex.

Catalysis of Square Planar CompoundsSquare 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 formationPd(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.

Catalysis

Bonding of HydrocarbonsHydrocarbons can bond to transition metals via bonds or bonds. Wilkinsons 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 bonds to the metal, and then accepts a hydrogen to bond with the metal.

Wilkinsons Catalyst

Hydrogen AdditionSquare planar complexes are known to react with hydrogen, undergoing addition, and breaking the H-H bond.

Hydrogen AdditionThe hydrogen bonding orbital donates electron density into an empty p or d orbital on the metal.M

Hydrogen AdditionMThe loss of electron density in the bonding orbital weakens the H-H bond.

Hydrogen AdditionThe 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 EffectA 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 CarbonylsMetal carbonyl compounds were first synthesized in 1868. Although many compounds were produced, they couldnt be fully characterized until the development of X-ray diffraction, and IR and NMR spectroscopy.

MT CarbonylsMetal 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 RuleMany 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 donor and 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 CarbonylsThe 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 * orbital on CO receives electrons from the metal, the CO bond weakens and the decreases.

MT CarbonylsAs the * orbital on CO receives electrons from the metal, the CO bond weakens and the decreases.

MT CarbonylsMn2(CO)10

Fe2(CO)9

MT CarbonylsCo4(CO)12

MT Carbonyls for free CO = 2143 cm-1

MT CarbonylsThe CO stretching frequency will also be affected by the charge of the metal.Compound (cm-1)[Fe(CO)6]2+2204[Mn(CO)6)]+2143Cr(CO)62090[V(CO)6]-1860[Ti(CO)6]2-1750

MT CarbonylsThe 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 CarbonylsIf 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 CarbonylsIf 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 CarbonylsMetal carbonyls with a center of symmetry typically show only 1 C-O stretch in their IR spectra, since the symmetric stretch doesnt change the dipole moment of the compound. Combined with the Raman spectrum, the structure of these compounds can be determined.

Nomenclature for LigandsThe 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 (eta) with the superscript indicating the number of atoms bonded.

Cyclopentadienyl CompoundsThe ligand C5H5 can bond to metals via a bond (contributing 1 electron), or as a bonding ligand. As a bonding ligand, it can donate 3, or more commonly 5 electrons to the metal.

Cyclopentadienyl CompoundsW(3-C5H5)(5-C5H5)(CO)2 has two bonded cyclopentadienyl rings. One donates 3 electrons, and the other donates 5.

Counting ElectronsThere 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 ElectronsThe other method treats all ligands as neutral in charge. 5-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 ElectronsIn 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.

FerroceneFe(5-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.

FerroceneThe 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 FerroceneGroup theory is used to simplify the analysis of the bonding. First, consider just a single C5H5 ring. Determine by considering only the pz orbitals which are perpendicular to the 5-membered ring.

Bonding of Ferrocene

D5hE2C52C525C2h2S52S535 v


D5hE2C52C525C2h2S52S535 v5


D5hE2C52C525C2h2S52S535 v50


D5hE2C52C525C2h2S52S535 v500


D5hE2C52C525C2h2S52S535 v500-1


D5hE2C52C525C2h2S52S535 v500-1-5


D5hE2C52C525C2h2S52S535 v500-1-50


D5hE2C52C525C2h2S52S535 v500-1-500


D5hE2C52C525C2h2S52S535 v500-1-5001

Bonding of Ferrocene reduces to: A1, E1 and E2

Group theory can be used to generate drawings of the molecular orbitals.

D5hE2C52C525C2h2S52S535 v500-1-5001

Bonding of Ferrocene reduces to: A1, E1 and E2

E2

E1

A1


The totally bonding orbital (A1) has no nodes, and is lowest in energy.


The middle set of orbitals (E1) are degenerate, with a single node. These orbitals are primarily bonding orbitals.

Bonding of FerroceneThe upper set of orbitals (E2) are degenerate, with two nodes. These orbitals are primarily anti-bonding orbitals.

Bonding of FerroceneOnce the molecular orbitals of the cyclopentadienyl ring has been determined, two rings are combined, and matched with symmetry appropriate orbitals on iron.

Bonding of Ferrocene The A1 orbitals on the two cyclopentadienyl rings have the same symmetry as the dz2 orbital on iron. Since the metal orbital is located in the center of the C5H5 rings, this is essentially a non-bonding orbital.

Bonding of Ferrocene The E1 orbitals on the rings have the same symmetry as the dxz and dyz orbitals of the iron.

Bonding of Ferrocene The E2 orbitals on the rings have the same symmetry as the dxy and dx2-y2 orbitals of the iron.

Bonding of FerroceneThese are the bonding orbitals of ferrocene. If the upper cyclopentadienyl ring is flipped over, a set of antibonding orbitals results.

MO DiagramThe frontier orbitals are neither strongly bonding nor strongly antibonding. As a result, metallo-cene compounds often diverge from the 18 electron rule.

MO DiagramIf the complex has more than 18 electrons, the e1u orbitals, which are slightly antibonding (the dxzand dyz), become occupied. This lengthens the M-C distance.

Electron Count and Stability(5-Cp)2Me- countM-C(pm)Hdissoc.* Fe 18206.41470 kJ/mol Co 19211.91400 Ni 20219.61320

* Hdissoc refers to the complex dissociating to M2+ and 2C5H5-

organometallic compounds

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