Classification of Ligands

CLASSIFICATION OF LIGANDS On the Basis of Charge on Ligands (1) Anionic ligands are characterized by their negative charge. Examples of atomic anions include halide ions, oxides, and nitrides. (ii) Neutral ligands possess a lone pair of electrons that they can offer to the metal. Common examples of neutral ligands include water, carboxyl, nitrosyl, and others. (iii) Cationic ligands refer to ligands that carry a positive charge. Examples of such ligands include NO+, N*H_{2} - tilde N H 3 s and others with similar properties. On the Basis of Denticity Denticity refers to the quantity of coordinate bonds established by a single ligand. (0) Monodentate ligands feature a solitary donor atom, such as Cl, H_{2}*O NH, NH, NH, etc., and are commonly termed unidentate ligands. (ii) Bidentate ligands have the capability to bind through two donor atoms simultaneously. CH2-NH2 CH₂-NH2 0 0- en(ethylene-1,2-diamine) ox (oxalate) NH CH₂ 0- 0 0- gly (glycinate) (iii) Polydentate ligands are molecules that coordinate with a central ion by utilizing more than two donor atoms. These ligands vary in their dentate nature, being tridentate, pentadentate, or hexadentate, possessing three, five, or six donor sites, respectively. A prominent example of a hexadentate ligand is ethylenediamine tetra acetic acid (EDTA). In the case of EDTA, the central metal ion forms bonds with two nitrogen atoms and four oxygen atoms from the four COOH groups of the molecule. Other notable polydentate ligands include terpyridine and diethylenetriamine. (iv) Ambidentate ligands are characterized by their ability to form coordinate bonds through two distinct atoms, albeit only one at a time. Examples of such ligands include N*O_{2} and SCN-. MN-0- M-O-N=0 M = metal atom/ion) (nitrito-N) (nitrito-0) (-) M SCN (thiocyanato) M: NCS) (isothiocyanato (v) Flex dentate ligands are ligands that exhibit variable denticity, meaning their ability to form a certain number of coordinate bonds depends on the nature of the metal ion they are interacting with. An example of a flex dentate ligand is EDTA, which can exhibit a denticity of either 4 or 6 depending on the specific conditions and the metal ion involved in the coordination complex. On the Basis of Type of Donation of Lone Pair (i) o-donor ligands are those that donate a lone pair of electrons to the central atom/ion, forming a o-bond directly between the ligand and the central atom/ion. Examples of such ligands include H_{2}*O and N*H_{3} (ii) σ-donor n-acceptor ligands, also known as n-acid ligands, are ligands that donate a lone pair of electrons to the central atom/ion via a a-bond while simultaneously accepting a significant amount of electron density from the metal atom/ion into its vacant or pi ^ * orbital. Common examples of these ligands include CO and NO. (iii) n-donor n-acceptor ligands are characterized by their ability to donate and accept -electrons through -bonds with the central atom/ion. Examples of such ligands include HC = CH C_{2} H 4 2 and C6H6. List of Ligands Denticity Name Ligand Charge Name of the ligand in the complex Donor atom Neutral ligands Water H2O 0 Aqua/aquo 0 Ammonia NH3 0 Ammine N Carbon monoxide CO 0 Carbonyl C Nitrogen oxide NO 0 Nitrosyl N Thiocarbonyl CS 0 Thiocarbonyl S Thionitrosyl NS 0 Thionitrosyl N Monodentate Pyridine (py) 0 Pyridine (py) N Methyl amine 0 Methylamine N Dinitrogen 0 Dinitrogen N Dioxygen 0 Dioxygen 0 Phosphine 0 Bidentate Ethylenediamine (en) NH 2 (C*H_{2}) 2 NH 2 0 Phosphine Ethylenediamine (en) P 2 N-atoms Tridentate Diethylene triamine 0 Diethylene triamine (diene) 3 N-atoms H 2 C- overline NH - C*H_{2} H 2 C- tilde N tilde N - CHz Tetradentate Tri ethylene tetramine 0 Tri ethylene tetramine (trien) H 2 C- dot N H (C*H_{2}) 2 dot N H 2 H 2 C- tilde N H (C*H_{2}) 2 NH 2 4 N-atoms Positive ligands Monodentate Nitrosonium ion N O^ oplus +1 Nitrosonium N Nitronium ion +1 Nitronium N Hydrazinium ion +1 Hydrazinium N Negative ligands Monodentate Hydride ion -1 Hydrido H Oxide ion 02- -2 Oxo 0 Peri oxide ion -2 Peroxo 0 Nitrido ion -3 Nitrido N Bidentate Sulphate ion -2 Sulphato 20-atoms Thiosulphate ion -2 Thiosulphato 20-atoms H ^ oplus N*H_{2}*N H 3 ^ oplus O_{2} ^ 2- S_{2} * O_{3} ^ 2 N O 2 ^ oplus S O 4 ^ 2- N_{2} O_{2} P*H_{3} C*H_{3}*N*H_{2} N ^ 3+ C_{5}*H_{5}*N