Types of Ligands and the Spectrochemical Series

The halide ions have ligand field strengths in the order 

Ligands that have occupied p orbitals are potentially tt donors. They tend to donate these electrons to the metal along with the cr-bonding electrons. As shown in Section 10-4-3, this TT-donor interaction decreases A. As a result, most halide complexes have high-spin configurations. Other primarily a-donor ligands that can also act as tt donors include H2O, OH, and RC02. They fit into the series in the order 

When ligands have vacant it or d orbitals, there is the possibility of tt back-bonding, and the ligands are tt acceptors. This addition to the bonding scheme increases A. Ligands that do this very effectively include CN , CO, and many others. A selected list of these ligands in order is 

When the two lists of ligands are combined, thiocyanate turns out to have a smaller effect than ammonia. This list is called the spectrochemical series and runs roughly in order from strong Tr-acceptor effect to strong ir-donor effect  

In Table 10-13, ligands in each group are listed in their order in the spectrochemical series. For example, for octahedral complexes of Cr, CN is listed first it is the 

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