1. What Are Ligands
2. Dmg Is Which Type Of Ligand C
3. Dmg Is Which Type Of Ligand Name
4. Types Of Ligands Biology
5. Dmg Is Which Type Of Ligand Good

Coordination compounds, such as the FeCl₄^- ion and CrCl₃ 6 NH₃, are called such because they contain ions or molecules linked, or coordinated, to a transition metal. They are also known as complex ions or coordination complexes because they are Lewis acid-base complexes. The ions or molecules that bind to transition-metal ions to form these complexes are called ligands (from Latin, 'to tie or bind'). The number of ligands bound to the transition metal ion is called the coordination number.

A ligand exchange (also ligand substitution) is a type of chemical reaction in which a ligand in a compound is replaced by another. One type of pathway for substitution is the ligand dependent pathway. In organometallic chemistry this can take place via associative substitution or by dissociative substitution. As a member of the wwPDB, the RCSB PDB curates and annotates PDB data according to agreed upon standards. The RCSB PDB also provides a variety of tools and resources. Users can perform simple and advanced searches based on annotations relating to sequence, structure and function. These molecules are visualized, downloaded, and analyzed by users who range from students to specialized scientists. The molecules or ions surrounding the central metal ion are called ligands. The nature of ligands. Simple ligands include water, ammonia and chloride ions. What all these have got in common is active lone pairs of electrons in the outer energy level. These are used to form co-ordinate bonds with the metal ion. Denticity refers to the number of donor groups in a single ligand that bind to a central atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate (sometimes called unidentate).

1. It is a bi dentate ligand although it is an unsymmetrical ligand but its square planar complex can not show geometrical isomerism becoz of intra molecular H - bonding See also the structure of Ni.
2. The ions or molecules which surround a metal ion or metal atom are called ligands. They are attached to central metal atom or central metal ion by co-ordinate bond. They donate at least one electron pair to central metal atom or ion.

Although coordination complexes are particularly important in the chemistry of the transition metals, some main group elements also form complexes. Aluminum, tin, and lead, for example, form complexes such as the AlF₆^3-, SnCl₄^2- and PbI₄^2- ions.

Alfred Werner developed a model of coordination complexs which explains the following observations.

• At least three different cobalt(III) complexes can be isolated when CoCl₂ is dissolved in aqueous ammonia and then oxidized by air to the +3 oxidation state. A fourth complex can be made by slightly different techniques. These complexes have different colors and different empirical formulas.

• The reactivity of the ammonia in these complexes has been drastically reduced. By itself, ammonia reacts rapidly with hydrochloric acid to form ammonium chloride.

NH₃(aq) + HCl(aq) NH₄⁺(aq) + Cl^-(aq)

These complexes don't react with hydrochloric acid, even at 100^oC.

• Solutions of the Cl^- ion react with Ag⁺ ion to form a white precipitate of AgCl.

Ag⁺(aq) + Cl^-(aq) AgCl(s)

When excess Ag⁺ ion is added to solutions of the CoCl₃ 6 NH₃ and CoCl₃ 5 NH₃ H₂O complexes, three moles of AgCl are formed for each mole of complex in solution, as might be expected. However, only two of the Cl^- ions in the CoCl₃ 5 NH₃ complex and only one of the Cl^- ions in CoCl₃ 4 NH₃ can be precipitated with Ag⁺ ions.

• Measurements of the conductivity of aqueous solutions of these complexes suggest that the CoCl₃ 6 NH₃ and CoCl₃ 5 NH₃ H₂O complexes dissociate in water to give a total of four ions. CoCl₃ 5 NH₃ dissociates to give three ions, and CoCl₃ 4 NH₃ dissociates to give only two ions.

Werner explained these observations by suggesting that transition-metal ions such as the Co³⁺ ion have a primary valence and a secondary valence. The primary valence is the number of negative ions needed to satisfy the charge on the metal ion. In each of the cobalt(III) complexes previously described, three Cl^- ions are needed to satisfy the primary valence of the Co³⁺ ion.

The secondary valence is the number of ions of molecules that are coordinated to the metal ion. Werner assumed that the secondary valence of the transition metal in these cobalt(III) complexes is six. The formulas of these compounds can therefore be written as follows.

The cobalt ion is coordinated to a total of six ligands in each complex, which satisfies the secondary valence of this ion. Each complex also has a total of three chloride ions that satisfy the primary valence. Some of the Cl^- ions are free to dissociate when the complex dissolves in water. Others are bound to the Co³⁺ ion and neither dissociate nor react with Ag⁺.

The [Co(NH₃)₆]Cl₃ complex dissociates in water to give a total of four ions, and all three Cl^- ions are free to react with Ag⁺ ion.

One of the chloride ions is bound to the cobalt in the [Co(NH₃)₅Cl]Cl₂ complex. Only three ions are formed when this compound dissolves in water, and only two Cl^- ions are free to precipitate with Ag⁺ ions.

Once again, the three Cl^- ions are free to dissociate when [Co(NH₃)₅(H₂O)]Cl₃ dissolves in water, and they precipitate when Ag⁺ ions are added to the solution.

Two of the chloride ions are bound to the cobalt in [Co(NH₃)₄Cl₂]Cl. Only two ions are formed when this compound dissolves in water, and only one Cl^- ion is free to precipitate with Ag⁺ ions.

Werner assumed that transition-metal complexes had definite shapes. According to his theory, the ligands in six-coordinate cobalt(III) complexes are oriented toward the corners of an octahedron, as shown in the figure below.

Any ion or molecule with a pair of nonbonding electrons can be a ligand. Many ligands are described as monodentate (literally, 'one-toothed') because they 'bite' the metal in only one place. Typical monodentate ligands are given in the figure below.

Other ligands can attach to the metal more than once. Ethylenediamine (en) is a typical bidentate ligand.

Each end of this molecule contains a pair of nonbonding electrons that can form a covalent bond to a metal ion. Ethylenediamine is also an example of a chelating ligand. The term chelate comes from a Greek stem meaning 'claw.' It is used to describe ligands that can grab the metal in two or more places, the way a claw would.

What Are Ligands

Linking ethylene- diamine fragments gives tridentate ligands and tetradentate ligands, such as diethylenetriamine (dien) and triethylenetetramine (trien). Adding four -CH₂CO₂^- groups to an ethylenediamine framework gives a hexadentate ligand, which can single-handedly satisfy the secondary valence of a transition-metal ion.

Typical Coordination Numbers

Transition-metal complexes have been characterized with coordination numbers that range from 1 to 12, but the most common coordination numbers are 2, 4, and 6. Examples of complexes with these coordination numbers are given in the table below.

Dmg Is Which Type Of Ligand C

Examples of Common Coordination Numbers

Note that the charge on the complex is always the sum of the charges on the ions or molecules that form the complex.

Cu²⁺ + 4 NH₃ Cu(NH₃)₄²⁺

Pb²⁺ + 2 OAc^- Pb(OAc)₂

Fe²⁺ + 6 CN^- Fe(CN)₆^4-

Note also that the coordination number of a complex often increases as the charge on the metal ion becomes larger.

Cu⁺ + 2 NH₃ Cu(NH₃)₂⁺

Cu²⁺ + 4 NH₃ Cu(NH₃)₄²⁺

G. N. Lewis was the first to recognize that the reaction between a transition-metal ion and ligands to form a coordination complex was analogous to the reaction between the H⁺ and OH^- ions to form water. The reaction between H⁺ and OH^- ions involves the donation of a pair of electrons from the OH^- ion to the H⁺ ion to form a covalent bond.

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The H⁺ ion can be described as an electron-pair acceptor. The OH^- ion, on the other hand, is an electron-pair donor. Lewis argued that any ion or molecule that behaves like the H⁺ ion should be an acid. Conversely, any ion or molecule that behaves like the OH^- ion should be a base. A Lewis acid is therefore any ion or molecule that can accept a pair of electrons. A Lewis base is an ion or molecule that can donate a pair of electrons.

When Co³⁺ ions react with ammonia, the Co³⁺ ion accepts pairs of nonbonding electrons from six NH₃ ligands to form covalent cobalt-nitrogen bonds as shown in the figure below.

The metal ion is therefore a Lewis acid, and the ligands coordinated to this metal ion are Lewis bases.

The Co³⁺ ion is an electron-pair acceptor, or Lewis acid, because it has empty valence-shell orbitals that can be used to hold pairs of electrons. To emphasize these empty valence orbitals we can write the configuration of the Co³⁺ ion as follows.

Co³⁺: [Ar] 3d⁶ 4s⁰ 4p⁰

There is room in the valence shell of this ion for 12 more electrons. (Four electrons can be added to the 3d subshell, two to the 4s orbital, and six to the 4p subshell.) The NH₃ molecule is an electron-pair donor, or Lewis base, because it has a pair of nonbonding electrons on the nitrogen atom.

According to this model, transition-metal ions form coordination complexes because they have empty valence-shell orbitals that can accept pairs of electrons from a Lewis base. Ligands must therefore be Lewis bases: They must contain at least one pair of nonbonding electrons that can be donated to a metal ion.

Dmg Is Which Type Of Ligand Name

Denticity refers to the number of donor groups in a single ligand that bind to a central atom in a coordination complex.^[1][2] In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate (sometimes called unidentate). Ligands with more than one bonded atom are called polydentate or multidentate. The word denticity is derived from dentis, the Latin word for tooth. The ligand is thought of as biting the metal at one or more linkage points. The denticity of a ligand is described with the Greek letter κ ('kappa').^[3] For example, κ⁶-EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Denticity is different from hapticity because hapticity refers exclusively to ligands where the coordinating atoms are contiguous. In these cases the η ('eta') notation is used.^[4]Bridging ligands use the μ ('mu') notation.^[5][6]

Classes[edit]

Polydentate ligands are chelating agents^[7] and classified by their denticity. Some atoms cannot form the maximum possible number of bonds a ligand could make. In that case one or more binding sites of the ligand are unused. Such sites can be used to form a bond with another chemical species.

• Bidentate (also called didentate) ligands bind with two atoms, an example being ethylenediamine.

• Tridentate ligands bind with three atoms, an example being terpyridine. Tridentate ligands usually bind via two kinds of connectivity, called 'mer' and 'fac.' 'fac' stands for facial, the donor atoms are arranged on a triangle around one face of the octahedron. 'mer' stands for meridian, where the donor atoms are stretched out around one half of the octahedron. Cyclic tridentate ligands such as TACN and 9-ane-S3 bind in a facial manner.
• Tetradentate ligands bind with four donor atoms, an example being triethylenetetramine (abbreviated trien). For different central metal geometries there can be different numbers of isomers depending on the ligand's topology and the geometry of the metal center. For octahedral metals, the linear tetradentate trien can bind via three geometries. Tripodal tetradentate ligands, e.g. tris(2-aminoethyl)amine, are more constrained, and on octahedra leave two cis sites (adjacent to each other). Many naturally occurring macrocyclic ligands are tetradentative, an example being the porphyrin in heme. On an octahedral metal these leave two vacant sites opposite each other.
• Pentadentate ligands bind with five atoms, an example being ethylenediaminetriacetic acid.
• Hexadentate ligands bind with six atoms, an example being EDTA (although it can bind in a tetradentate manner).
• Ligands of denticity greater than 6 are well known. The ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) and diethylene triamine pentaacetate (DTPA) are octadentate. They are particularly useful for binding lanthanide ions, which typically have coordination numbers greater than 6.

Stability constants[edit]

In general, the stability of a metal complex correlates with the denticity of the ligands, which can be attributed to the chelate effect. Polydentate ligands such as hexa- or octadentate ligands tend to bind metal ions more strongly than ligands of lower denticity, primarily due to entropic factors. Stability constants are a quantitative measure to assess the thermodynamic stability of coordination complexes.

See also[edit]

Types Of Ligands Biology

External links[edit]

• EDTA chelation lecture notes. 2.4MB PDF - Slide 3 on denticity

References[edit]

1. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'denticity'. doi:10.1351/goldbook.D01594
2. ^von Zelewsky, A. 'Stereochemistry of Coordination Compounds' John Wiley: Chichester, 1995. ISBN047195599X.
3. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'κ (kappa) in inorganic nomenclature'. doi:10.1351/goldbook.K03366
4. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'η (eta or hapto) in inorganic nomenclature'. doi:10.1351/goldbook.H01881
5. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'bridging ligand'. doi:10.1351/goldbook.B00741
6. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'µ- (mu) in inorganic nomenclature'. doi:10.1351/goldbook.M03659
7. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'chelation'. doi:10.1351/goldbook.C01012

Dmg Is Which Type Of Ligand Good