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A Qualitative Molecular Orbital Diagram for Ferrocene

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A Qualitative Molecular Orbital Diagram for Ferrocene Ferrocene Ferrocene was first accidently discovered sandwich compound in 1951 at Duquesne University in Pittsburgh, Pennsylvania by T.J. Kealy and P.L.Paulson. Cyclopentadienyl magnesium bromide (CpMgBr) was reacted with iron (II) chloride in attempt to create a fulvalene. When an orange complex formed instead, the chemists hypothesized that the iron was bound to one carbon in each ring. After one year, in 1952, G. Wilkinson and R. B. Woodward correctly deduced the sandwich structure: two anionic cyclopentadienyl (Cp) rings each donating 6π electrons to the Fe2+ cation between them. Fulvalene H MgBr + FeCl2 Fe H H Kealy & Paulson's Hypothesized Structure Fe Fe Actual Ferrocene structure Fig1. First synthesis of ferrocene Wilkinson shared the Nobel Prize for his work in this area in 1973. Ferrocene’s cyclopentadienyl rings are aromatic – each containing 6 delocalized π electrons like benzene. If a strong base is used to deprotonate cyclopentadiene (Cp), the H+ is removed from the only sp3 (tetrahedral) carbon in the structure. A lone pair of electrons is then formally assigned to a nonbonding p orbital of that carbon (which is now sp2 hybridized). How many resonance structures can be created for a cyclopentadienyl anion (Cp−)? That lone pair of electrons joins the 4 electrons from the two π bonds to create an aromaticity in the ring. :B H H H : Fig 2. Aromaticity of cyclopentadienyl ring Properties of Ferrocene: [1] Ferrocene is the first sandwitch compound discovered accidently in 1951. [2] Ferrocene is a diamagnetic yellow crystalline solid in which Fe is exist as zero oxidation state. [3] The melting point of ferrocene is 174 0C and boiling point is 249 0C. [4] Ferrocene is most stable compound among all the metallocene. [5] Ferrocene is insoluble in water but soluble in most of the organic solvent. [6] Ferrocene shows only one 1H NMR signal at δ4.4 ppm. (The entire protons are chemically and magnetically equivalent). [7] Ferrocene behaves as Lewis base. [8] It is present into two conformations. The eclipsed conformation exists in gas phase (at high temperature) and staggered conformation (at low temperature) exists in solid phase. if H is replaced by CH Fe 3 Fe Fe Eclipsed, D5h Staggered, D5d (in gas phase) (in solid phase) Staggered only TheElectronicStructureofFerrocene • 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 FeͲCp axis is very small (~ 4 kJmolͲ1) and ground state structures of ferrocene may show either of these conformations. • The primary orbital interactions that form the metalͲligand bonds in ferrocene occur between the Fe dͲorbitals and the SͲorbitals of the Cp ligand. • The D5d point group representations simplify the symmetry matching of ligand molecular orblbitalsand metal atomic orblbitals. • If D5d symmetry is assumed, so that there is a centre of symmetry in the ferrocene molecule through the Fe atom there will be centroͲsymmetric (g) and antiͲsymmetric (u) combinations. Ͳ • The five pͲorbitals on the planar Cp ring (D5h symmetry) can be combined to produce five molecular orbitals. Energy TheSͲmolecularorbitalsofthecyclopentadienyl 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 Ͳ Cp anion whereas the e2 orbitals are net antiͲbonding and are unfilled. 5 • For a bisͲcyclopentadienyl metal complex (K ͲCp)2M , such as ferrocene, the SͲ orbitals of the two Cp ligands are combined pairwise to form the symmetryͲadapted 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), (\1Ͳ\1); (\2+\2), (\2Ͳ\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; ¾ a low lying filled bonding pair of a1g and a2u symmetry ¾ a filled weakly bonding pair of e1g and e1u syyymmetry ¾ an unfilled antiͲbonding pair of e2g and e2u symmetry. 5 SALCorbitalsfora(K ͲCp)2Mcomplex;*S =A1g +A2u +E1g +E1u +E2g +E2u e2g e2u e1g e1u Z Y X a1g a2u • The metal orbitals transform as 2 2 2 A1g (dz ,s)+A2u (pz)+E1u (px,py)+E1g (dyz,dxz)+E2g(dx Ͳ dy ,dxy) • Reducible representation of SALC’s : *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. 2 • For example, the a1g SALC orbital can in theory overlap with the Fe 4s and 3dz orbitals as they are also of a1g symmetry. This interaction gives rise to the bonding * and antiͲbonding 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 antiͲbonding molecular orbital [(\ligand molecular orbital)Ͳ(\metal atomic orbital)] providing that the energies of the two component sets are sufficiently close for overlap. SymmetrymatchingofSALCorbitalswiththemetalatomicorbitals no metal no metal orbital of orbital of appropriate appropriate symmetry symmetry dxy dx2-dy2 available available e2g e2u dyz dxz p y p x e1g e1u Z Y X s dz2 p z a1g a1u Aqualitativemolecularorbitaldiagramforferrocene (D5d) p z p y p x a1u e1u dyz dxz e1g.
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