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

2022
Views:

206

Pages:

29
Author:

Leilani Contreras

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 h energy off rotation i about b the h Fe‐Cp C axis i is i very smallll (~ ( 4 kJmol k l‐11) and d ground d state structures of ferrocene may show either of these conformations. • There is also very little difference in electronic states between the D5h and D5d symmetries however the D5d point group irreducible representations are used here in the description of the electronic structure of ferrocene as they simplify the symmetry matching of ligand molecular orbitals (SALCs) and metal atomic orbitals. • The primary orbital interactions that form the metal‐ligand bonds in ferrocene occur between the Fe d orbitals and the ‐orbitals of the Cp ligand. • If D5d symmetry is assumed, assumed so that there is a centre of symmetry in the ferrocene molecule through the Fe atom there will be centro‐symmetric (g) and anti‐ symmetric (u) combinations.  MOs of Cyclopentadienyl, C5H5‐ • C5H5 , has three pairs of electrons delocalized in a  system extending around the pentagonal ring. • The five 2p orbitals perpendicular to the ring on the five carbon atoms combine to form three bonding (1, 2, 3) and three antibonding (4*, 5*, 6*) MOs. • The symmetries and forms of these MOs can be deduced by applying the operations of the point group D5h to a set of five vectors perpendicular to the ring, one at each carbon, b t generate to t a reducible d ibl representation t ti  .  MOs of Cyclopentadienyl, C5H5‐ D 5h A 1' A 2' E 1' E 2' A 1'' A 2'' E 1'' E 2'' 2C 52 5C 2 2S 5 2S 35 5 E 2C 5 5 0 0 -1 -5 0 0 1 5 5 10 10 5 5 10 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -5 5 0 0 -5 5 0 0 -5 -5 -10 -10 5 5 10 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 -5 0 0 -5 5 0 0 h d /h v 0 0 0 0 0 20 20 20 0 0 0 0 0 1 1 1 h = 20 • The five p‐orbitals on the planar Cp ring (D5h symmetry) can be combined to produce five molecular orbitals according to the reducible representation: • One combination has the full symmetry of the ring (a2’’) • There are two doubly degenerate combinations (e1’’ and e2’’) having one and two planar nodes at right angles to the plane of the ring. • The relative energies of these orbitals increase as the number of nodes increases. 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 anti‐bonding bonding and are unfilled. unfilled The ‐molecular orbitals of the cyclopentadienyl ring (D5h) E2'' (4 nodes) anti-bonding E1'' (2 nodes) weakly bonding A2'' (0 nodes) bonding • The five p‐orbitals on the planar Cp ring (D5h symmetry) can be combined to produce five molecular orbitals. • 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) which are described by the irreducible representations of the D5d point group. A1g E1g E2g A2u E1u E2u • The  SALCs of the (Cp)2 framework are also defined by the sum and difference of SALCs from the two contributing Cp ligands whose results must correspond to the irreducible components of  (Cp)2 , i.e. (1+1), ) (1‐ 1); (2+2), ) (2‐ 2) ...etc. etc where, for example, '1+1' gives rise to a molecular orbital of A1g symmetry. • This gives rise to three sets of ligand molecular orbitals of gerade (g) and g ((u)) symmetry y y with respect p to the centre off inversion;; ungerade  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. SALCs for a (5‐Cp)2M complex e2g e2u e1g e1u Z Y X a2u a1g A1g E1g E2g A2u E1u E2u • Using the reducible representation of SALCs the corresponding metal AOs are found A1g • E1g E2g A2u E1u E2u Thus, in D5d the bonding metal orbitals transform as A1g : (s , dz2) E1g : (dyz , dxz) E2g : (dx2 ‐ dy2, dxy) A2u : (pz) E1u : (px, py) • Byy consideringg the AO and SALC symmetries y and how overlap p can be affected the MO bonding picture of ferrocene can be constructed. • Each combination of AOs and SALCs leads to a bonding molecular orbital [(ligand anti‐bonding bonding molecular li d molecular l l orbital bi l)+(metall atomic i orbital bi l)] and a corresponding anti orbital [(ligand molecular orbital)‐(metal atomic orbital)] providing that the energies of the two component sets are sufficiently close for overlap. Symmetry matching of the SALC’s 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) FeII SALC's Fe pz py px a2u , e1u e1u a1u e2g , e2u a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe pz py px e1u a1u a2u , e1u e2g , e2u a1g a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g a1g , a2u a1g A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe pz py px a2u , e1u e1u a1u a1g* e2g 2 , e2u 2 a1g a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g a1g , a2u a1g A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe pz py px a2u , e1u e1u a1u a1g* e2g 2 , e2u 2 a1g a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g a1g , a2u a1g A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe a2u* pz py px a2u , e1u e1u a1u a1g* e2g 2 , e2u 2 a1g a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe a2u* pz py px e1u a1u a2u , e1u a1g* e2g , e2u a1g e1g* a1g a1g , e1g , e2g e1g , e1u dxz dyz e1g e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe e1u* a2u* pz py px e1u a1u a2u , e1u a1g* e2g , e2u a1g e1g* a1g a1g , e1g , e2g e1g , e1u e1u dxz dyz e1g e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe e1u* a2u* pz py px e1u a1u a2u , e1u a1g* e2g* e2g , e2u a1g e1g* a1g a1g , e1g , e2g e2g e1g , e1u e1u dxz dyz e1g e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe e1u* a2u* pz py px e1u a1u a2u , e1u a1g* e2g* e2g , e2u e2u 2 a1g e1g* a1g a1g , e1g , e2g e2g e1g , e1u e1u dxz dyz e1g e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe e1u* a2u* pz py px a2u , e1u e1u a1u a1g* e2g* e2g 2 , e2u 2 e2u a1g e1g* a1g a1g , e1g , e2g e2g e1g , e1u e1u dxz dyz e1g e1g a2u a1g a1g , a2u A qualitative molecular orbital diagram for ferrocene (D5d) FeII SALC's Fe e1u* a2u* pz py px e1u a1u a2u , e1u a1g* e2g* e2g , e2u e2u 2 a1g e1g* LUMO a1g HOMO a1g , e1g , e2g e2g e1g , e1u e1u dxz dyz e1g e1g a2u a1g a1g , a2u • Due to a difference in energies the lowest energy a1g molecular orbital is mainly ligand based with a slight admixture of the Fe 4s and 3dz2 orbitals. • Similarly the a2u level has little if any metal character due to higher lying Fe 4pz orbital with which it is formally able to combine. • The e1g molecular orbital arises from the bonding combination of the ligand e1g orbitals with the Fe 3dxz and 3dyz orbitals. This is the only symmetry combination of orbitals in the two Cp rings that has appreciable overlap with the metal 3d orbitals to act as an efficient donor and it is thus this interaction which is mainly responsible ibl for f the th stability t bilit off the th complex. l • The corresponding anti‐bonding orbitals, e1g*, are unfilled in the ground state of ferrocene but they are involved in excited state transitions. • The e1u bonding molecular orbitals are again mainly ligand based but with a small contribution from the higher energy Fe 3 px , py orbitals. • The a1g HOMO mostlyy consists of the Fe 3dz2 orbital as the a1g SALC and the metal dz2 orbital result in little or no overlap. • The e2g (dx2‐y2, dxy) metal orbitals are considered weakly‐bonding due to poor p with the e2g SALC orbitals. overlap • Since the occupied orbitals are of either a1g , e1g or e2g type symmetry no intrinsic barrier to internal rotation is predicted as each of these molecular orbitals are symmetric about the axis of rotation. • The very low values observed for this rotation ((~ 4 kJmol‐1) may be attributed to van der Waals forces between the two Cp rings. • The attachment of additional groups or ligands destroys the D5d/D5h symmetry of ferrocene thus significantly altering the MO diagram. The Electronic Structure of bisbenzenechromium Cr(6‐C C6H6)2 • Similar to ferrocene, primary orbital interactions that form the metal‐ligand bonds in occur between the Cr d orbitals and the ‐orbitals of the benzene ligand. • The two benzene rings of Cr(6‐C6H6)2 are ideally orientated in an eclipsed (D6h) conformation.  MOs of Benzene, C6H6 B2g (6 nodes) anti-bonding E2u (4 nodes) anti-bonding B2g E1g ( nodes)) (2 bonding A2u 2 (0 nodes) bonding E1g A2u + E2u • For (6‐C6H6)2M the ‐orbitals of the two benzene ligands are combined pairwise to form the SALCs which are described by the irreducible representations of the D6h point group. A1g B2g E1g E2g A2u B1u E1u E2u SALCs for a (5‐Cp)2M complex b2g b1u e2g e2u e1g e1u Z Y X a1g a2u • Using the reducible representation of SALCs the corresponding metal AOs are found • In D6h the bonding metal orbitals transform as A1g : (s , dz2) E1g : (dyz , dxz) E2g : (dx2 ‐ dy2, dxy) A2u : (pz) E1u : (px, py) • The metal s , p and d orbitals all transform in D6h in a similar manner as found for the D4d ferrocene metal AOs, thus a similar LCAO can be carried out. Symmetry matching of the SALC’s with the metal atomic orbitals

The Electronic Structure of Ferrocene

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