User:Nahoiona/2.6 Pi Donor and Acceptor Ligands

=== 2.6 Pi Donor and Acceptor Ligands ===

The nature of ligands coordinated to the centre metal along with other properties such as metal identity and its oxidation state are important features of a complex compound. More specifically, it is the identity and consequently the ability of the ligand to donate or accept electrons to the centre atom that will determine the molecular orbitals.

The spectralchemical series shows the trend of compounds as weak field to strong field ligands. Furthermore, ligands can be characterized by their π-bonding interactions. This interaction reveal the amount of split between e_g and t_2g energy levels of the molecular orbitals that ultimately dictates the strength of field of the ligands.

Examples of Weak Field Ligands:

X^-, OH^-, H₂O

Examples of Strong Field Ligands

H^-, NH₃, CO, PR₃

The difference in energy split is illustration in this MO diagram for both π donor and acceptor ligands.

In a π-donor ligand, the SALCs are occupied, hence it donates the electrons to the molecular σ σ* and π π* orbitals. The orbitals associated to e_g orbitals are not involved in πinteractions therefore it stays in the same energy level (figure 1). On the other hand, the occupied ligand SALCs t_2g orbitals thatwould form molecular orbitals with the metal t_2g orbitals (ie. d_xy, d_xz, d_xy) are lower in energy than its metal counterparts. The resulting MO has π* orbitals that are energetically lower than the σ* orbitals that are formed from the non-bonding orbitals (e_g). The difference between the t_2g π* and e_g σ orbitals is donated as Δ, split. In the π-donor case, the Δ is small due to the low π* level.

Conversely, the t_2g SALCs of a π accepting ligand are higher in energy than the metal t_2g orbitals because they are unoccupied. The resulting t_2g π* orbitals are higher than the σ* orbitals. This creates a larger Δ between the e_g and t_2g πorbitals, making these π-accepting orbitals high split ligands.

The metal d electron configurations for low and high spin.

Finally, the magnitude of Δ as influenced by the identity of the ligand will dictate how electrons are distributed in the metal d orbitals (figure 2). Weak field ligands produce a small Δ hence a high spin configuration. Strong field ligands produce a large Δ hence a low spin configuration on the d electrons. This property of the d electron configurations will have further implications discussed in future chapters.