![]() The metal ions can also be arranged in order of increasing Δ, and this order is largely independent of the identity of the ligand. Ligands that do this very effectively include CN −, CO, and many others. This addition to the bonding scheme increases Δ. When ligands have vacant π* and d orbitals of suitable energy, there is the possibility of pi backbonding, and the ligands may be π acceptors. Most halide ligands as well as OH − are primary examples of π donor ligands. These types of ligands tend to donate these electrons to the metal along with the σ bonding electrons, exhibiting stronger metal-ligand interactions and an effective decrease of Δ. Ligands that have occupied p orbitals are potentially π donors. ![]() Another example of a σ bonding ligand would be ethylenediamine however, ethylenediamine has a stronger effect than ammonia, generating a larger ligand field split, Δ. Bonding by these ligands to metals is relatively simple, using only the σ bonds to create relatively weak interactions. Some, like NH 3, are σ bond donors only, with no orbitals of appropriate symmetry for π bonding interactions. The order of the spectrochemical series can be derived from the understanding that ligands are frequently classified by their donor or acceptor abilities. However, it is known that "the spectrochemical series is essentially backwards from what it should be for a reasonable prediction based on the assumptions of crystal field theory." This deviation from crystal field theory highlights the weakness of crystal field theory's assumption of purely ionic bonds between metal and ligand. On the other hand, ligands lying at the right end are stronger ligands and form inner orbital octahedral complexes after forcible pairing of electrons within 3d level and hence are called low spin ligands. 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. Strong field ligands: CO, CN −, NH 3, PPh 3 Weak field ligands: H 2O, F −, Cl −, OH − I − < Br − < S 2− < SCN − (S–bonded) < Cl − < NO 3 – < N 3 − < F − < OH − < C 2O 4 2− < H 2O < NCS − (N–bonded) < CH 3CN < py ( pyridine) < NH 3 < en ( ethylenediamine) < bipy ( 2,2'-bipyridine) < phen (1,10- phenanthroline) < NO 2 − (N–bonded) < PPh 3 ( Triphenylphosphine) < CN − < CO Ī partial spectrochemical series listing of ligands from small Δ to large Δ is given below. ![]() The spectrochemical series was first proposed in 1938 based on the results of absorption spectra of cobalt complexes. The splitting parameter is reflected in the ion's electronic and magnetic properties such as its spin state, and optical properties such as its color and absorption spectrum. 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. List of ligands in coordination compounds topic of Inorganic chemistryĪ spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element.
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