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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.
Weak field ligands: H 2 O, F −, Cl −, OH −. Strong field ligands: CO, CN −, NH 3, PPh 3. Ligands arranged on the left end of this spectrochemical series are generally regarded as weaker ligands and cannot cause forcible pairing of electrons within the 3d level, and thus form outer orbital octahedral complexes that are high spin. Ligands ...
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.
Most homoleptic complexes of NCS − feature isothiocyanate ligands ... complexes would be tetrahedral since isothiocyanate is a weak-field ligand. Two examples are ...
For example, NO 2 − is a strong-field ligand and produces a large Δ. ... which cause a small splitting Δ of the d-orbitals are referred to as weak-field ligands ...
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.
Common functional groups such as ketones usually are only weak ligands and thus often are poor DGs. This problem is solved by the use of a transient directing group. Transient DGs reversibly convert weak DGs (e.g., ketones) into strong DG's (e.g., imines) via a Schiff base condensation. Subsequent to serving their role as DGs, the imine can ...
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]