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Date: 18-6-2019
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Complexes with metal–ligand π-bonding
The metal dxy, dyz and dxz atomic orbitals (the t2g set) are nonbonding in an [ML6]n+, σ-bonded complex (Figure 1.1) and these orbitals may overlap with ligand orbitals of the correct symmetry to give π-interactions (Figure 1.2).
Fig. 1.1 An approximate MO diagram for the formation of [ML6]n+ (where M is a first row metal) using the ligand group orbital approach. The bonding only involves M_L π-interactions.
Although π-bonding between metal and ligand d orbitals is sometimes considered for interactions between metals and phosphine ligands (e.g. PR3 or PF3), it is more realistic to consider the roles of ligand σ*-orbitals as the acceptor orbitals. Two types of ligand must be differentiated: π-donor and π-acceptor ligands.
Fig. 1.2 π-Bond formation in a linear L_M_L unit in which the metal and ligand donor atoms lie on the x axis: (a) between metal dxz and ligand pz orbitals as for L = I-, an example of a π-donor ligand; and (b) between metal dxz and ligand π*-orbitals as for L = CO, an example of a π-acceptor ligand.
A π-donor ligand donates electrons to the metal centre in an interaction that involves a filled ligand orbital and an empty metal orbital; a π-acceptor ligand accepts electrons from the metal centre in an interaction that involves a filled metal orbital and an empty ligand orbital. π-Donor ligands include Cl-, Br- and I- and the metal– ligand π-interaction involves transfer of electrons from filled ligand p orbitals to the metal centre (Figure 1.2a). Examples of π-acceptor ligands are CO, N2, NO and alkenes, and the metal–ligand π-bonds arise from the back donation of electrons from the metal centre to vacant antibonding orbitals on the ligand (for example, Figure 1.2b). π-Acceptor ligands can stabilize low oxidation state metal complexes. Figure 1.3 shows partial MO diagrams which describe metal–ligand π- interactions in octahedral complexes; the metal s and p orbitals which are involved in σ-bonding have been omitted. Figure 20.14a shows the interaction between a metal ion and six π-donor ligands; electrons are omitted from the diagram, and we return to them later. The ligand group π-orbitals are filled and lie above, but relatively close to, the ligand σ-orbitals, and interaction with the metal dxy, dyz and dxz atomic orbitals leads to bonding (t2g) and antibonding (t2g*) MOs. The energy separation between the t2g* and eg* levels corresponds to Δoct. Figure 1.3b shows the interaction between a metal ion and six π-acceptor ligands. The vacant ligand π* orbitals lie significantly higher in energy than the ligand σ-orbitals. Orbital interaction leads to bonding (t2g) and antibonding (t2g*) MOs as before, but now the t2g* MOs are at high energy and Δoct is identified as the energy separation between the t2g and eg* levels (Figure 1.3b).