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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.
In coordination chemistry, a ligand [a] is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs , often through Lewis bases . [ 1 ]
Ammonia is a pure σ-donor, in the middle of the spectrochemical series, and shows intermediate hard–soft behaviour (see also ECW model [7] [8]). Ammonia is classified as an L ligand in the Covalent bond classification method. In the usual electron counting method, it is a two-electron ligand. Ammonia is also compact such that steric effects ...
Weak field ligands, as judged by the spectrochemical series, favor tetrahedral geometry and strong field ligands favor the square planar isomer. Both weak field (Cl − ) and strong field (PPh 3 ) ligands comprise NiCl 2 (PPh 3 ) 2 , hence this compound is borderline between the two geometries.
The other form of coordination π bonding is ligand-to-metal bonding. This situation arises when the π-symmetry p or π orbitals on the ligands are filled. They combine with the d xy , d xz and d yz orbitals on the metal and donate electrons to the resulting π-symmetry bonding orbital between them and the metal.
Low-spin [Fe(NO 2) 6] 3− crystal field diagram. The Δ splitting of the d orbitals plays an important role in the electron spin state of a coordination complex. Three factors affect Δ: the period (row in periodic table) of the metal ion, the charge of the metal ion, and the field strength of the complex's ligands as described by the spectrochemical series.
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):
Trifluorophosphine (PF 3) is a strong π-acid with bonding properties akin to those of the carbonyl ligand. [8] In early work, phosphine ligands were thought to utilize 3 d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding. [ 9 ]