To understand ambidentate ligands, one must first understand what a ligand is. A ligand is a molecule or ion a functional group that can bind to a central metal atom which can be in a zero, negative, or positive oxidation state — this bonding usually involves the ligand donating one or more electron pairs. This means that ligands act as Lewis bases because they donate a pair of electrons , and the central atom acts as a Lewis acid because they accept a pair of electrons. All ligands must have at least one donor atom with an electron pair which can be used to form a covalent bond with the central atom.
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The key breakthrough occurred when Alfred Werner reconciled formulas and isomers. He showed, among other things, that the formulas of many cobalt III and chromium III compounds can be understood if the metal has six ligands in an octahedral geometry. The first to use the term "ligand" were Alfred Werner and Carl Somiesky, in relation to silicon chemistry. The theory allows one to understand the difference between coordinated and ionic chloride in the cobalt ammine chlorides and to explain many of the previously inexplicable isomers.
He resolved the first coordination complex called hexol into optical isomers, overthrowing the theory that chirality was necessarily associated with carbon compounds. This is because the ligand and central metal are bonded to one another, and the ligand is providing both electrons to the bond lone pair of electrons instead of the metal and ligand each providing one electron.
Bonding is often described using the formalisms of molecular orbital theory. Metal ions preferentially bind certain ligands. Binding of the metal with the ligands results in a set of molecular orbitals, where the metal can be identified with a new HOMO and LUMO the orbitals defining the properties and reactivity of the resulting complex and a certain ordering of the 5 d-orbitals which may be filled, or partially filled with electrons.
In an octahedral environment, the 5 otherwise degenerate d-orbitals split in sets of 2 and 3 orbitals for a more in depth explanation, see crystal field theory. This ordering of ligands is almost invariable for all metal ions and is called spectrochemical series.
For complexes with a tetrahedral surrounding, the d-orbitals again split into two sets, but this time in reverse order. When the coordination number is neither octahedral nor tetrahedral, the splitting becomes correspondingly more complex. The absorption of light what we perceive as the color by these electrons that is, excitation of electrons from one orbital to another orbital under influence of light can be correlated to the ground state of the metal complex, which reflects the bonding properties of the ligands.
The relative change in relative energy of the d-orbitals as a function of the field-strength of the ligands is described in Tanabe—Sugano diagrams. In cases where the ligand has low energy LUMO, such orbitals also participate in the bonding.
The metal—ligand bond can be further stabilised by a formal donation of electron density back to the ligand in a process known as back-bonding. In this case a filled, central-atom-based orbital donates density into the LUMO of the coordinated ligand.
Carbon monoxide is the preeminent example a ligand that engages metals via back-donation. Complementarily, ligands with low-energy filled orbitals of pi-symmetry can serve as pi-donor. Metal— EDTA complex, wherein the aminocarboxylate is a hexadentate chelating ligand.
Cobalt III complex containing six ammonia ligands, which are monodentate. The chloride is not a ligand. Classification of ligands as L and X[ edit ] Main article: Covalent bond classification method Especially in the area of organometallic chemistry , ligands are classified as L and X or combinations of the two.
Green and "is based on the notion that there are three basic types [of ligands] Example is alkoxy ligands which is regularly known as X ligand too. L ligands are derived from charge-neutral precursors and are represented by amines , phosphines , CO , N2, and alkenes.
X ligands typically are derived from anionic precursors such as chloride but includes ligands where salts of anion do not really exist such as hydride and alkyl. Cp is classified as an L2X ligand. Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom. Ligands that bind via more than one atom are often termed chelating.
A ligand that binds through two sites is classified as bidentate , and three sites as tridentate. The " bite angle " refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers. A classic example of a polydentate ligand is the hexadentate chelating agent EDTA , which is able to bond through six sites, completely surrounding some metals.
In practice, the n value of a ligand is not indicated explicitly but rather assumed. The binding affinity of a chelating system depends on the chelating angle or bite angle. Complexes of polydentate ligands are called chelate complexes. They tend to be more stable than complexes derived from monodentate ligands.
This enhanced stability, the chelate effect , is usually attributed to effects of entropy , which favors the displacement of many ligands by one polydentate ligand. When the chelating ligand forms a large ring that at least partially surrounds the central atom and bonds to it, leaving the central atom at the centre of a large ring. The more rigid and the higher its denticity, the more inert will be the macrocyclic complex. Heme is a good example: the iron atom is at the centre of a porphyrin macrocycle, being bound to four nitrogen atoms of the tetrapyrrole macrocycle.
The very stable dimethylglyoximate complex of nickel is a synthetic macrocycle derived from the anion of dimethylglyoxime.
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