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The magnitude of Δ o is determined by the field-strength of the ligand: strong field ligands, by definition, increase Δ o more than weak field ligands. Ligands can now be sorted according to the magnitude of Δ o (see the table below). This ordering of ligands is almost invariable for all metal ions and is called spectrochemical series.
A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element.For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the ligand-field splitting parameter in ligand field theory, or the crystal-field splitting parameter in crystal field theory.
As described above, π-donor ligands lead to a small Δ O and are called weak- or low-field ligands, whereas π-acceptor ligands lead to a large value of Δ O and are called strong- or high-field ligands. Ligands that are neither π-donor nor π-acceptor give a value of Δ O somewhere in-between.
Strong-field ligands, such as CN − and CO, increase the Δ splitting and are more likely to be low-spin. Weak-field ligands, such as I − and Br − cause a smaller Δ splitting and are more likely to be high-spin. Some octahedral complexes exhibit spin crossover, where the high and low spin states exist in dynamic equilibrium.
T. Tetraacetylethane; Tetradentate ligand; Transition metal acyl complexes; Transition metal arene complex; Transition metal azide complex; Transition metal carboxylate complex
Some ligands always produce a small value of Δ, while others always give a large splitting. The reasons behind this can be explained by ligand field theory. The spectrochemical series is an empirically-derived list of ligands ordered by the size of the splitting Δ that they produce (small Δ to large Δ; see also this table):
However, these ligands also form dative covalent bonds like the L-type. [2] This type of ligand is not usually used because in certain situations it can be written in terms of L and X. For example, if a Z ligand is accompanied by an L type, it can be written as X 2. Examples of these ligands are Lewis acids, such as BR 3. [3]
Many ligands other than CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than CO and ν NO is a diagnostic tool in metal–nitrosyl chemistry . Isocyanides , RNC, are another class of ligands that are capable of π-backbonding.