Complex (chemistry)

Nomenclature

Names of Ligands

The name of the anion ligands always finish in " o"

Examples:

F $^-$ : fluoro
Cl $^-$ : chloro
Br $^-$ : bromo
I $^-$ : iodo

The noun of the neutral ligands are not modified, except for:

H₂O: aqua
NH₃: amine
CO: carbonyl

The number of ligands is specified by the prefix: mono, di, sorting, will tétra, penta, hexa, etc…

Name of the formed Complexes

One writes the number and the nature of the ligands, the nature of the ion (or atom) central and a Roman numeral which specify the number of oxidation of metal.

If the load of the complex is negative, the termination " is added; ate". In the contrary case, one notes the name of the métal.

Examples:

^-: ion dicyanoargentate (I)
²⁺: ion hexaaquafer (II)

Ligands

The Ligand S are various types; among most current one finds molecules like water H₂O or the NH₃ ammonia or of the anions like the chlorides Cl^-, the cyanides CN^-, the OH^- hydroxides… At first approximation, the capacity of a ligand to be bound to a metal is correlated with its nucleophilicity in the organic reactions

The molecules bearing several chemical functions being able to play the part of ligands are ligands polydentes (their denticity , i.e. the number of atoms likely to bind to metal, is higher than 1). The connection of the ligands polydentes is favoured entropiquement compared to that of the ligands monodentes. Indeed, in the reactions of displacement of N ligands monodentes around a central ion by 1 ligand polydente there is increase in the total number of molecules of n-1 , which is favorable entropiquement. So ligands at least bidentate can form cycles with 5 or 6 with the central cation (these cycles are, as in Organic chemistry, thermodynamically favoured), the stability of the complex is improved. It is about the effect chelate. The etymology of this word reveals its direction: indeed it derives from the Greek khêlê : “grip”. Stabilization additional due to the chelate effect draws its origin owing to the fact that the connection with the central cation from the first function being able to be used as ligand place near the cation the other free doublets of the molecule, which is favorable entropiquement.

Geometrical structure of the complexes

Number of coordination

The structure of a complex depends on its number of coordination, equal to the number of connections σ between the ligands and the central atom). The number of coordination of a ligand lies between 2 and 9, but the complexes include/understand a great number of ligands (higher than 6). The most frequent numbers of coordination are 4 and 6. The number of connections metal-ligand depends on the size, the load and the electronic Configuration of the metal ion. The majority of the ions can accept several numbers of coordination, adopting different geometries then.

The chemistry of the complexes is dominated by the interactions between the molecular orbital S and p of the ligands and the atomic orbital D (or F ) of the central ion. Orbital the S , p and D of metal can accept a total of 18 electron S (for the elements of the block F , this maximum increases with 32 electrons). The maximum number of coordination thus depends on the electronic configuration of metal (more particularly of the number of orbital vacant which can generate a connection σ ligand-metal). However, contrary to the Rule of the byte in organic chemistry, the rule of the 18 electrons is not absolute and of many stable complexes do not respect it.

The number of coordination of a complex also depends on the size on the ligands and the metal cation. Small ligands around a large cation will involve a steric compactness, which leads to great numbers of coordination.

Example: ^4-

Small cations surrounded by large ligands will have low numbers of coordination.

Example: Pt₂

For metals of transition from series 3D , which includes metals of biological interest (and which are most abundant on Earth) such as the Fer, the Manganèse, the Zinc, the Cuivre… the number of coordination usually is included/understood between 4 and 6. Of share their big size, the Lanthanide S, the Actinide S and the metals of transition from the series 4d and 5d will be able to have large numbers of coordination (> 6).

Various possible geometries

The space arrangement of the ligands depends on the number of coordination (NC) of the complex.

For the majority of the structures, one places the metal ion in the center of a sphere where are placed different the ligands (one then regards the distance ion-ligand as identical). The electrostatic coverings orbitalaires ligand-metal and repulsions between the ligands tend to form regular geometrical structures. The metal complexes respect theory VSEPR except when fine electronic factors (which can be related to distortion due to the Effet Jahn-Teller), which is the case for example for the complexes of Cu (II) and Nor (III) which are often in octahedral geometry with a distortion tétragonale (2 connections in an axis longer or shorter than the 4 others) or plan square (thorough distortion tétragonale ad infinitum), which are not canonical geometries predicted by the theory VSEPR. The steric obstruction due to the coordination of ligands encumbered can also modify the geometry of the complexes.

One gathered below the list of the most widespread structures according to the number of coordination (NC) (or Coordinence):

NC = 2: linear,
NC = 3: trigonal planes,
NC = 4: tetrahedral or plane square
NC = 5: bipyramidale at triangular or pyramidal base at carée base
NC = 6: octahedral or prism trigonal
NC = 7: bipyramidale at pentagonal base

In many cases, the real geometry deviates from the theoretical structure. For example, the complex can comprise different ligands (the lengths of the connections ion-ligand are not identical any more, and the structure is not any more that of a regular Polyèdre). The size of the ligands can modify the structure of the complex of share pressures steric too important. Also in the case of complex with polydentes, the structure of the molecules carrying the electronic doublets ensuring coordination metal can be incompatible with the geometrical requirements of coordination (it results from it from the distorted complexes).

The idealized descriptions off 5, 7, 8, and 9 - coordination indistinct are often geometrically from alternate structures with slightly different L-M-L angles. The classic example off this is the difference between pyramidal public garden and trigonal bipyramidal structures.

Due to special electronic effects such ace (second-order) Jahn-Teller stabilization, unquestionable geometries are stabilized relative to the other possibilities, e.g for trigonal sum compounds the prismatic geometry is stabilized relative to octahedral structures for six-coordination. -->

Isomerism

Geometrical isomerism

The geometrical Isomérie takes place in the octahedral and square complexes plane but not in the tetrahedral complexes. When, in such complexes, ligands are in adjacent positions, one uses the descriptor cis , and when they are in opposed positions, the descriptor trans

When three ligands identical or the three coordinating functions of a tridentate ligand occupy a face of octahedral, one speaks about facial isomer ( FAC ), if they occupy an edge of the octèdre, one speaks about southernmost isomer ( sea ).

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