Ligands are atoms or molecules that bind to a central metal atom in a coordination compound. The ligands can be classified into three types: simple, complex, and coordinative unsaturation. A simple ligand is an atom or molecule that binds directly to the metal ion. The most common simple ligands are oxygen, nitrogen, halogens, carbon monoxide and water. The complex ligands are those that bind to the metal ion through a pair of donor atoms or a donor atom and a lone electron pair. The most common examples for this type of ligand include ethylene dibromide (EDB) and nitriles. The coordinative unsaturation ligands are those that can bind to the metal ion through more than one donor atom. The most common example of this type of ligand is ammonia. A coordination compound is a type of compound where a central metal atom is surrounded by ligands. The ligands are the atoms or molecules that bind to the metal atom in this type of compound.
Ligand Binding to Metal Ions
As you know, a coordination complex forms when an ion or molecule that can donate one or more electron pairs reacts with a central positively charged metal ion. The ligands donate their electrons to the metal ion, forming a stable complex. There are three types of ligand-metal interactions: electrostatic, covalent, and coordinate covalent. The electrostatic interaction is the strongest type of interaction and occurs when the electron pairs on the ligand are attracted to the metal ion. The covalent interaction is the next strongest type of interaction and occurs when the ligand and metal ion share electrons. The coordinate covalent interaction is the weakest type of interaction and occurs when the ligands donate their electrons to the metal ion without forming any bonds.
JEE Chemistry Ligands and Its Types and Coordination
For understanding the meaning and characteristics of a ligand is, we first need to understand the meaning of coordination chemistry and coordination compounds.
Coordination chemistry is a branch of chemistry that deals with the study of coordination compounds. The presence of coordination groups significantly changes the chemistry of a molecule. This branch of chemistry studies these changes and how they reflect upon during the chemical reactions, and what can be utilized out of these different properties of these unique compounds.
These are the compounds that contain platinum, cobalt, and other transition metals and are made up of two parts – a central atom and ligands. The ligands are bound to the central atom via means of coordination bonds. In these compounds, an atom or a group of atoms (called ligands) is/are bound to the central atom by utilizing a shared pair of electrons supplied by the coordinated group and not by the central atom. These compounds have strange empirical formulas and unique properties.
They are often very brightly coloured compounds. Their major distinguishing feature is the presence of two, four, six, and sometimes even more chemical groups positioned geometrically around the metal ion (also known as the central atom). These groups (also known as the ligands) can be neutral molecules, cations or anions. Each co-co-ordinating group can be a separate entity, or all groups can be connected in one long, flexible molecule that wraps itself around the metal. Coordinating groups significantly change the chemical behaviour of metal.
The colours of the compounds provide clues about their electronic energy levels, e.g., every plant depends on the green magnesium coordinating complex known as chlorophyll for carrying out the process of photosynthesis in order to synthesize their own food using sunlight, carbon dioxide and water. The combination of magnesium and its coordinating groups in chlorophyll has electronic properties that the free metal or ion does not have and can absorb visible light and use the energy for chemical synthesis, which either the free metal or ions cannot do. Another example of this is cytochromes (the coordination compounds of iron) that are essential for every oxygen inhaling organism for the breakdown and combustion of food and the storage of the energy released upon the breakdown and metabolism of that food. Most of the larger organisms need haemoglobin, another iron coordination complex in which the coordinating groups enable the iron to bind oxygen molecules without being oxidized. In fact, large areas of biochemistry are really the application of these transition metal-based coordination compounds.
The neutral molecules or ions (or atoms or groups of atoms) which are directly attached to the central metal ion or atom through coordinate bonds in the complexion are called ligands or ligands. In other words, any species capable of donating a pair of electrons to metal is called a ligand. A ligand may be an ion, negatively or positively charged, or a neutral molecule.
There are a few requirements for an atom or a group of atoms or ions to behave as a ligand. These are:
Ligands should have at least one lone pair of electrons
Ligands should have the capability to donate their lone pair of electrons to the central metal atom or ion and form a coordinate covalent bond(s) with it
The Ligand behaves as a Lewis base while the metal atom or ion behaves as a Lewis acid, and a Lewis acid-base reaction takes place between them to form a coordination compound
Examples: Cl- , Br- , SO42- , NH2NH3+, NH3, H2O, NH2CH2CH2NH2 etc.
Types of Ligands
Ligands can be classified on the basis of many things. The most common classification of ligands is on the basis of their binding sites with the central metal atom or ion.
On the basis of the number of sites, ligands can be classified as monodentate, bidentate, polydentate etc. ligands.
Monodentate ligand: These are the ligands that coordinate to the only site of a metal ion. In other words, only one pair of electrons can be donated to the metal ion. For instance: Cl-,SO42-, Br-, NH3, NH2NH3+, H2O
Bidentate ligand: These are the ligands that occupy two sites of a metal ion. That is, it can be attached to two metal ion positions, e.g. NH2CH2CH2NH2 etc.
Polydentate ligands: These are the ligands that occupy many sites of the same metal ion. Example: EDTA etc. This category includes all the higher levels of dentate ligands above bidentate, e.g., tridentate, tetradentate, pentadentate, hexadentate etc.
Another classification of ligands is on the basis of the molecular complex structure they form after forming coordination compounds. According to this classification, ligands are divided into two types – chelating agents and ambident ligands:
Chelating Agents: These are the ligands that are bonded with the same central metal atom or ion and form a ring-type structure. Usually, bidentate or polydentate ligands fall under this category. Chelating ligands generally form a ring structure around the central metal atom or ion. The most common example of these types of ligands is EDTA (ethylene diamine tetraacetic acid).
Ambident Ligand: An ambident ligand is that ligand which binds with the central metal atom or ion through more than one site. Usually, monodentate ligands fall under this category of ligands. The most common examples of these types of ligands are cyanides (M–CN) and isocyanides (M–NC).
Another classification of ligands is on the basis of their chemical nature. According to this classification, ligands are divided into the following types – inorganic ligands, neutral organic ligands, anionic organic ligands and cationic organic ligands:
Inorganic Ligands: These are the ligands that are of either ionic nature or other inorganic forms of chemical compounds. The most common examples of these types of ligands are the halide ions (such as fluoride, chloride, bromide and iodide) and cyanometallates such as CN– and SCN–
Neutral Organic Ligands: These are ligands that are of organic nature by origin and do not possess any type of charge on them. These are hence molecules and notions. The most common example of this category of ligands includes pyrazine.
Anionic Organic Ligands: These are ligands that are organic in nature and possess a negative charge on them due to the presence of highly electronegative atoms in them, mainly oxygen or nitrogen. The most common example of this category of ligands includes oxalate.
Cationic Organic Ligands: These are ligands that are organic in nature and possess a positive charge on them due to the presence of pentavalent nitrogen atoms in them. The most common example of this category of ligands includes pyridine based ligands.
Another classification of ligands is based on the type of their covalent bonds. This classification is also sometimes referred to be based upon the LXZ Approach or the CBC Method (which stands for Covalent Bond Classification). This type of classification is mostly used in organometallic chemistry. According to this classification, the ligands are divided into three types – L ligand, X ligands and Z ligands:
L Ligands: The L ligands are derived from charge-neutral precursors: NH3, amines, N-heterocycles such as pyridine, PR3, CO, alkenes etc.
X Ligands: The X ligands are derived from anionic precursors: halides, hydroxide, alkoxide alkyls—species that are one-electron neutral ligands, but two-electron donors as anionic ligands. EDTA4- is classified as an L2X4 ligand, features four anions and two neutral donor sites. C5H5 is classified anL2X ligand.
Z Ligands: Z ligands are very rare in nature. They accept two electrons from the metal centre. They donate none. The "ligand" acts as a Lewis acid that accepts electrons instead of the X and L ligands ' Lewis bases that donate electrons.
Another classification of ligands is based on the type of donor orbital involved in the donation of electrons to the central metal atom or ion. According to this type of classification, the ligands are of the following type – σ ligands, σ+π ligands, σ+π*/σ* ligands and π + π* ligands:
σ ligands: These involve only sigma bonding between the li
gand and the central metal atom or ion.
σ + π ligands: These involve the donation of lone pairs through the ligand to the metal atom or ion.
σ + π*/ σ* ligands: These involve bonding to vacant π orbitals from σ.
π + π* ligands: These involve bonding to vacant π orbitals from π.