University:
Seattle Pacific University
Course:
CHM 3372 | Organic Chemistry II
Academic year:

2021
Views:

496

Pages:

17
Author:

Hallie Randolph

Organometallic Chemistry • Organometallic compounds combine an organic moiety with a metal in a molecule that has direct metal‐carbon bonds. • Ferrocene, first f prepared d in 1951, ushered h d in the h modern d era off organometallic ll chemistry. h • Originally called “dicyclopentadienyliron.” • Subsequent X‐ray analysis proved that ferrocene consisted of an iron(II) ion sandwiched between two parallel cyclopentadienyl (Cp) rings. • The cyclopentadienyl ligand is just one example of many where the ‐system of an organic compound binds directly to a metal atom via a d interaction. • The term hapticity, denoted , describes the number of ligand atoms coordinated to the central metal atom, e.g. (5‐Cp)2Fe  bonded ligands alkene complexes alkyne complexes allyl complexes diene complexes cyclopentadienyl complexes arene complexes metallacycles M • Common unsaturated  donating ligands encountered in organotransition‐metal chemistry together with the respective numbers of electrons relevant to the application of the 18 VE rule. Dewar‐Chatt‐Duncanson model • Zeise’s salt was as the first organometallic compound compo nd to be isolated in pure p re form (1825 by b William Zeise). The structure was not confirmed until 1838. • This discovery spawned a tremendous growth in organometallic chemistry and still serves as the simplest example of transition metal metal‐olefin olefin complexation. • Zeise’s salt has become one of the most cited examples of the Dewar‐Chatt‐Duncanson model for metal‐olefin complexation. • The Dewar‐Chatt‐Duncanson model explains the type of chemical bonding between an unsaturated d ligand l d and d a metall fforming a  complex. l • The ‐acid ligand donates electron density into a metal d‐orbital from a π‐symmetry bonding orbital between the carbon atoms. • The metal donates electrons back from a filled t2g d‐orbital into the empty π* antibonding orbital of the ligand (hence the description ‐acid ligand). • Both of these effects tend to reduce the C‐C bond order, leading to an elongated C‐C distance and a lowering lo ering of its vibrational ibrational frequency. freq enc • The interaction can cause carbon atoms to "rehybridize“, for e.g in metal alkene complexes from sp2 towards sp3, which is indicated by the bending of the hydrogen atoms on the ethylene back away from the metal. • Often the reactivity of the ligand is reversed from its free state – “umpolung”. Molecular geometry of Zeise’s salt (neutron diffraction) • The PtCI3 moiety forms a nearly planar group with the Pt atom. • The Pt‐CI bond trans to the ethylene group (2.340 Å) is significantly longer than the cis Pt‐CI bonds (2.302 and 2.303 Å) – trans effect !! • The C atoms are approximately equidistant from the Pt atom (2.128 and 2.135 Å). • The distance from the midpoint of the C‐C bond to the Pt atom is 2.022 Å. • The C‐C distance, 1.375 Å, is slightly longer than the value found in free ethylene (1.337 Å), indicating some d‐p* back‐bonding from the platinum atom to C2H4. • Back‐bonding is also indicated by a bending of the four hydrogen atoms away from the Pt atom. …some other th classic l i examples l off organometallic t lli bonding b di interactions: i t ti Ferrocene Synthesis • Cyclopentadiene (b.p. 42.5 °C) is produced by cracking dicyclopentadiene (b.p. 170 °C). • Cyclopentadiene slowly dimerizes back to dicyclopentadiene at room temperature. • The burgundy‐red burgundy red cyclopentadienyl ion, ion C5H5, can be produced by reaction of cyclopentadiene with KOH in solvent 1,2‐dimethoxyethane (ethylene glycol dimethyl ether) • A solution of FeCl2 in DMSO is slowly added to the solution containing C5H5– ions, resulting in yellow yellow‐orange orange ferrocene: • An inert atmosphere (N2) is necessary to avoid air oxidation of Fe2+ to Fe3+, which cannot effectively form ferrocene. • The FeCl2 solution is prepared by dissolving FeCl2.4H2O in DMSO. • An open bottle of FeCl2.4H2O (pale green) oxidizes over time to form brown Fe(III). (Do not leave container open and unattended). • The solution must be prepared and held under an inert atmosphere (N2) to avoid oxidation. • FeCl2.4H2O dissolves slowly ‐ start preparing the solution early and use mild heating to speed up the process. process The Electronic Structure of Ferrocene • The two cyclopentadienyl (Cp) rings of ferrocene may be orientated in the two extremes of either an eclipsed (D5h) or staggered (D5d) conformation. • The energy of rotation about the Fe‐Cp axis is very small (~ 4 kJmol‐1) and ground state structures of ferrocene may show either of these conformations. • The primary orbital interactions that form the metal metal‐ligand ligand bonds in ferrocene occur between the Fe d‐orbitals and the ‐orbitals of the Cp ligand. • The D5d point group representations simplify the symmetry matching of ligand molecular orbitals b l and d metall atomic orbitals. b l • If D5d symmetry is assumed, so that there is a centre of symmetry in the ferrocene molecule g the Fe atom there will be centro‐symmetric y (g) and anti‐symmetric y ((u)) combinations. through • The five p‐orbitals on the planar Cp‐ ring (D5h symmetry) can be combined to produce five molecular orbitals. Energy The ‐molecular orbitals of the cyclopentadienyl ring (D5h) • One combination has the full symmetry of the ring (a2) • There are two doubly degenerate combinations (e1 and e2) having one and two nodal planes orthogonalto the plane of the ring. • The relative energies of these orbitals increase as the number of nodal planes (0, 1, 2) increases. • The a2 and e1 orbitals are both fully occupied in the electronic configuration of the Cp‐ anion whereas the e2 orbitals are net anti‐bonding and are unfilled. • For a bis‐cyclopentadienyl metal complex (5‐Cp)2M , such as ferrocene, the ‐ orbitals of the two Cp ligands are combined pairwise to form the symmetry‐adapted linear combination of molecular orbitals (SALC’s). • To form the SALC orbitals, the sum and difference of corresponding molecular orbitals on the Cp ligand must be taken, e.g. (1+1), (1‐1); (2+2), (2‐2) etc. • For example, '1+1' gives rise to a molecular orbital of a1g symmetry. • Overall, this gives rise to three sets of ligand molecular orbitals of gerade (g) and ungerade (u) symmetry with respect to the centre of inversion;  a low lying filled bonding pair of a1g and a2u symmetry pair of e1g and e1u symmetry y y  a filled weaklyy bondingg p  an unfilled anti‐bonding pair of e2g and e2u symmetry. SALC orbitals for a (5‐Cp)2M complex;  = A1g + A2u + E1g + E1u + E2g + E2u e2g e2u e1gg e1u Z Y X a1g a2u • The metal orbitals transform as A1g (dz2, s) + A2u (pz) + E1u (px, py) + E1g (dyz, dxz) + E2g (dx2 ‐ dy2, dxy) • Reducible representation p of SALC’s :  = A1g + A2u + E1g + E1u + E2g + E2u • By considering the SALC orbitals and how overlap with metal atomic orbitals can be affected the molecular orbital bonding picture of ferrocene can be constructed. • For example, the a1g SALC orbital can in theory overlap with the Fe 4s and 3dz2 orbitals as they are also of a1g symmetry. This interaction gives rise to the bonding and anti‐bonding molecular orbitals of the complex a1g and a1g* respectively. • Each combination of ligand molecular orbitals and metal molecular orbitals leads to a bonding molecular orbital [(ligand molecular orbital)+(metal atomic orbital)] and a corresponding anti‐bonding molecular orbital [(ligand molecular orbital)‐(metal atomic orbital)] providing that the energies of the two component sets are sufficiently close for overlap. Symmetry matching of SALC orbitals with the metal atomic orbitals no metal orbital of appropriate symmetry available dx2-dy2 dxy e2g no metal orbital of appropriate symmetry available e2u py dxz dyz px e1u e1gg Z Y X s a1g pz dz2 a1u A qualitative molecular orbital diagram for ferrocene (D5d) pz py px e1u a1u dxz dyz e1g

Organometallic Chemistry

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