Bonding in bis(cyclopentadienyl)metal complexes

Key points: The MO picture of bonding in bis (Cp) metal complexes shows that the frontier orbitals are neither strongly bonding nor strongly antibonding; thus, complexes that do not obey the 18-electron rule are possible. We start by looking at ferrocene, where, although details of the bonding are not settled, the molecular orbital energy level diagram shown in Fig. 22.13 accounts for a number of experimental observations. This diagram refers to the eclipsed (D_5h) form of the complex which, in the gas phase, is about 4 kJ mol 1 lower in energy than the staggered conform ation (Section 22.19c). We shall focus our attention on the frontier orbitals. As shown in Fig. 22.13, the e₁ symmetry-adapted linear combinations of ligand orbitals have the same

Figure 22.13 Molecular orbital energy diagram for a M(Cp)₂ with D5h symmetry. The energies of the symmetry-adapted π orbitals of the C₅H₅ ligands are shown on the left, relevant d orbitals of the metal are on the right, and the resulting molecular orbital energies are in the centre. Eighteen electrons can be accommodated by filling the molecular orbitals up to and including the a₁’ orbital in the box. The box denotes the orbitals typically regarded as frontier orbitals in these molecules.

symmetry as the dzx and dyz orbitals of the metal atom. The lower energy frontier orbital (a₁) is composed of dz2 and the corresponding SALC of ligand orbitals. However, there is little interaction between the ligands and the metal orbitals because the ligand π orbitals happen to lie, by accident, in the conical nodal surface of d_z2 orbital of the metal atom. In ferrocene and the other 18-electron bis(cyclopentadienyl) complexes, the a1 frontier orbital and all lower orbitals are full but the e1 frontier orbital and all higher orbitals are empty. The frontier orbitals are neither strongly bonding nor strongly antibonding. This characteristic permits the possibility of the existence of bis(cyclopentadienyl) complexes that diverge from the 18-electron rule, such as the 17-electron complex [Fe(5-Cp)₂] and the 20-electron complex [Ni( 5-Cp)₂]. Deviations from the 18-electron rule, however, do lead to significant changes in MC bond lengths that correlate fairly well with the molecular orbital scheme (Table 22.8). Similarly, the redox properties of the complexes can be under stood in terms of the electronic structure. We have already mentioned that ferrocene is fairly readily oxidized to the ferrociniumion, [Fe( 5-Cp)₂ ] . From an orbital viewpoint, this oxidation corresponds to the removal of an electron from the nonbonding a₁ orbital. The 19-electron complex [Co(5-Cp)₂] is much more readily oxidized than ferrocene because the electron is lost from the antibonding e₁ orbital to give the 18-electron [Co(5-Cp)₂] ion. A useful comparison can be made with ocatahedral complexes. The e₁ frontier orbital of a metallocene is the analogue of the eg orbital in an octahedral complex, and the a₁ orbital plus the e₂ pair of orbitals are analogous to the t_2g orbitals of an octahedral complex. This formal similarity extends to the existence of high- and low-spin bis(cyclopentadienyl) complexes.

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مواضيع ذات صلة

Isolobal analogies

Structure of clusters

Bent metallocene complexes

Fluxional behaviour of metallocenes

Synthesis and reactivity of cyclopentadienyl compounds

Spectroscopic properties of carbonyl compounds

Reactions of the CO ligand

Metal carbonyl basicity

Oxidation and reduction of carbonyls

Properties of homoleptic carbonyls

Synthesis of homoleptic carbonyls

Homoleptic carbonyls