6.6: Ligands

Denticity

Ligands are classified based on whether they bind to the metal center through a single site on the ligand or whether they bind at multiple sites. Ligands that bind through only a single site are called monodentate from the Latin word for tooth; in contrast, those which bind through multiple sites are called chelating after the Greek \(\chi \alpha \lambda \epsilon \) for “claw”. These relationships are summarized in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\). (A) Ammonia is a monodentate ligand while (B) ethylene diamine is a chelating ligand owing to its capacity to bind metals via its two amine functional groups. (C) Chelating ligands act like a lobster claw in attaching to the metal via multiple sites. The lobster claw image is adapted from https://www.clipart.email/download/1127636.html. Otherwise this work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

Following naturally from the classification of non-chelating ligands as monodentate, chelating ligands are further classified according to the number of sites which they can use to bind a metal center. This number of binding sites is called the denticity and ligands are referred to as monodentate (non- chelating), bidentate, tridentate, etc., based on the number of sites available. Ligands with two binding sites have a denticity of two and are said to be bidentate; those with three are tridentate, four tetradentate, and so on. To illustrate this classification system examples of chelating ligands classified according to denticity are given in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\). Because only one oxygen per carboxylate typically binds, only one is counted when assigning a ligand's denticity. This work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

Ambidentate Ligands

A perusal of the ligands in Table \(\PageIndex{1}\) reveals that several can bind to a metal in multiple ways. For example, thiocyanate, SCN^- can bind metals through its S or N atoms. Such ligands are called ambidentate ligands. In naming an ambidentate ligand, the atom through which it attaches to the metal is commonly specified after the ligand name using the italicized element symbol or, more formally, a \(\kappa\) followed by the italicized element symbol. An example is given in Figure \(\PageIndex{3}\).²

Figure \(\PageIndex{1}\): Two possible binding modes of nitrite acting as a ligand.³ This work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

Examples of Ligands

Table \(\PageIndex{1}\). Common monodentate ligands. Most chemists still prefer common names over the IUPAC ones.

Ligand	Common name	IUPAC name
H^- (H ligands are always considered anions for naming purposes)	hydrido	hydrido
F^-	fluoro	fluorido
Cl^-	chloro	chlorido
Br^-	bromo	bromido
I^-	iodo	iodido
CN^-, as M-CN	cyano	cyanido or cyanido-\(\kappa\)C or cyanido-C
CN^-, as M-NC	isocyano	isocyanido or cyanido-\(\kappa\)N or cyanido-N
CH3NC	methylisocyanide	methylisocyanide
N3^-	azido	azido
SCN^-, e.g. thiocyanate as M-SCN	thiocyanato	thiocyanato-\(\kappa\)S or thiocyanato-S
NCS^-, e.g. thiocyanate as M-NCS	isothiocyanato	thiocyanato-\(\kappa\)N or thiocyanato-N
CH3CO2^-	acetato	ethanoato
N^3-	nitrido	nitrido
NH^2^-	imido	azanediido
NH2^-	amido	azanido
NH3	ammine	ammine
RNH2, R2NH, R3N	alkylamine, dialkylamine, trialkalyamine (e.g. methylamine for CH3NH2)	alkylamine, dialkylamine, trialkalyamine (e.g. methylamine for CH3NH2)
, piperidine, abbreviated pip	piperidine	piperidine
, pyridine, abbreviated py	pyridine	pyridine
CH3CN, acetonitrile, abbreviated MeCN	acetonitrile	acetonitrile
P^3-	phosphido	phosphido
PH3	phosphine	phosphane
PR3	trialkylphosphine (e.g. trimethylphosphine for Me3P)	trialkylphosphane (e.g. trimethylphosphane for Me3P)
PAr3	triarylphosphine (e.g. triphenylphosphine for Ph3P)	triarylphosphine (e.g. triphenylphosphane for Ph3P)
, dimethylsulfoxide or DMSO or dmso	dimethylsulfoxide (sometimes called dimethylsulfoxo but this usage is rare and violates the nomenclature rules for neutral ligands)	(methanesulfinyl)methane or dimethyl(oxido)sulfur
, thiourea or tu	thiourea	thiourea
O^2-	oxo	oxido
OH^-	hydroxo	hydroxido
H2O	aqua	aqua
S^2-	sulfo	sulfo
HS^-	hydrosulfido	hydrosulfido
RS^-	alkanethiolate (e.g. ethanthiolate for EtS^-)	thioalkanoate
H2S	hydrogen sulfide	hydrogen sulfide
R2S	alkylsulfanylalkane (e.g. ethylsulfanylethane for Et2S)	dialkyl sulfide
O2	dioxygen	dioxygen
O2^-, superoxide	superoxido	dioxido(1-) or superoxido
O2^2^-, peroxide	peroxido	dioxido(2-) or peroxido
N2	dinitrogen	dinitrogen
NO (are always considered neutral for naming purposes)	nitrosyl	nitrosyl
CO	carbonyl	carbonyl
CS	thiocarbonyl	thiocarbonyl
SO, as M-SO	sulfino	sulfur monoxide-\(\kappa\)S or sulfur monoxide-S
NO2, as M-NO2	nitryl	nitrogen dioxide-\(\kappa\)N or nitrogen dioxide-N
CO3^2^-	carbonato	carbonato
NO2^-, as M-NO2	nitro or nitrito-N	nitrito-\(\kappa\)N or nitrito-N
NO2^-, as M-ONO	nitrito or nitrito-O	nitrito-\(\kappa\)O or nitrito-O
NO3^-	nitrato	nitrato
SO3^2^-	sulfito	sulfito
SO4^2^-	sulfato	sulfato
S2O3^2^-, as M-S-SO2-O^-	thiosulfato-S	thiosulfato-\(\kappa\)S or thiosulfato-S
S2O3^2^-, as M-O-SO2-S^-	thiosulfato-O	thiosulfato-\(\kappa\)O or thiosulfato-O

 

Table \(\PageIndex{2}\). Common chelating ligands organized by denticity. Most chemists use the common names and abbreviations to describe these ligands.

Common Ligand name	IUPAC ligand name	abbreviation (if applicable)	structure or representative/parent structure (shown in the ionization state in which they bind to a metal)
Bidentate Ligands
acetylacetonato	2,4-pentanediono	acac
R-BINAP and S-BINAP	R- or S-2,2'-bis(diphenylphosphino)-1,1'-binapthyl	BINAP
2,2'-bipyridine	2,2'-bipyridine	bpy or bipy
cyclooctadiene	1,5-cyclooctadiene	COD	(binding to the metal occurs through the alkene \(\pi\) cloud)
dialkyldithiocarbamato	dialkylcarbamodithiolato	R2NCS2^- or dtc
dimethylgloximato	butanedienedioxime	Hdmg or DMG
diphenylphosphinoethane or 1,2-(diphenylphosphino)ethane	Ethane-1,2-diylbis(diphenylphosphane)	dppe
ethylenediamine	Ethane-1,2-diamine	en
ethylenedithiolato	Ethane-1,2-dithiolato	C2H2S2^2^-
nacnac	N,N'-diphenyl-2,4-pentanediiminato	nacnac
oxalato	oxalato	ox
1,10-phenanthroline or o-phenanthroline	1,10-phenanthroline	phen or o-phen
phenylpyridinato	2-phenylpyridinato-C^2,N or 2-phenylpyridinato-\(\kappa\)C^2,N	ppy
Tridentate Ligands
triazacyclononane	1,4,7-triazacyclononane	tacn
diethylenetriamine	1,4,7-triazaheptane	dien
pyrazoylborato (scorpionate)	hydrotris(pyrazo-1-yl)borato	Tp
terpyridine or 2,2';6',2"-terpyridine	1^2,2^2:2^6,3^2-terpyridine or 2,6-bis(2-pyridyl)pyridine, tripyridyl, 2,2′:6′,2″-terpyridine	tpy or terpy
Tetradentate Ligands
\(\beta\), \(\beta\)', \(\beta\)''-triaminotriethylamine	\(\beta\), \(\beta\)', \(\beta\)''-tris(2-aminoethyl)amine	tren
triethylenetetramine	1,4,7,10-tetraazadecane	trien
corroles	variable and generally not used	cor or Cor
12-crown-4	1,4,7,10-tetraoxacyclododecane	12-crown-4
tetramethylcyclam	1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane	TMC or cyclam
cyclam	1,4,8,11-tetraazacyclotetradecane	cyclam
cyclen	1,4,7,10-tetraazacyclododecane	cyclen
tris(2-pyridylmethyl)amine	1-pyridin-2-yl-N,N-bis(pyridin-2-ylmethyl)methanamine	tpa or TPA
phthalocyanines	variable and generally not used	variable, usually a modified Pc
porphyrins	variable and generally not used	variable, usually a modified por, Por, or P (e.g. TPP = tetraphenylporphyrin)
salen	2,2'-ethylenebis(nitrilomethylidene)diphenoxido	salen
Pentadentate Ligands
15-crown-5	1,4,7,10,13-Pentaoxacyclopentadecane	15-crown-5
tetraethylenepentamine	1,4,7,10,13-pentaazatridecane	tepa or TEPA
Hexadentate Ligands
18-crown-6	1,4,7,10,13,16-hexaoxacyclooctadecane	18-crown-6
2,1,1-cryptand	4,7,13,18-Tetraoxa-1,10-diazabicyclo[8.5.5]icosane	2,1,1-crypt or [2.1.1]-cryptand kryptofix 211 and variations thereof
ethylenediaminetetraaceto	2,2′,2″,2‴-(Ethane-1,2-diyldinitrilo)tetraaceto	EDTA, edta, Y^4-
Heptadentate Ligands
2,2,1-cryptand	4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]icosane	2,2,1-crypt or [2.2.1]-cryptand kryptofix 221 and variations thereof
Octadentate Ligands
2,2,2-cryptand	4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan	2,2,2-crypt or [2.2.2]-cryptand kryptofix 222 and variations thereof
pentetato acid or diethylenetriaminepentaacetato or DTPA	2-[bis[2-[bis(carboxylatomethyl)amino]ethyl]amino]acetato	DTPA
DOTA or tetraxetan	1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid	Dota, DOTA

Problems

Exercise \(\PageIndex{1}\)

Determine the denticity of each ligand in the list below and classify them as monodentate, tridentate, etc.

Answer

(a) bidentate

(b) tridentate

(c) bidentate

(d) tridentate (only the lower N on each ring has a lone pair that can be used to bind the metal)

(e) hexadentate (remember that each carboxylate only counts as one point of attachment)

(f) bidentate

(g) monodentate (through the lone pair on the isocyanide C)

(h) bidentate

References

1. International Union of Pure and Applied Chemistry Nomenclature of Inorganic Chemistry Cambridge, UK, 2005.

2. The structure and name is taken from Choudhury, S. B.; Allan, C. B.; Maroney, M.; Wodward, A. D.; Lucas, C. R. Inorg. Synth. 1998, 32, 98-107.

3. Haas, K. Naming Transition Metal Complexes. https://chem.libretexts.org/Courses/Saint_Mary's_College%2C_Notre_Dame%2C_IN/CHEM_342%3A_Bio-inorganic_Chemistry/Readings/Week_2%3A_Introduction_to_Metal-Ligand_Interactions_and_Biomolecules/2.1_Transition_metal_complexes/2.1.6%3A_Naming_Transition_Metal_Complexes

Contributors and Attributions

Stephen Contakes, Westmont College, to whom comments, corrections, and criticisms should be addressed.

with some examples taken from Naming Transition Metal Complexes by Kathryn Haas.

Consistent with the policy for original artwork made as part of this project, all unlabeled drawings of chemical structures are by Stephen Contakes and licensed under a Creative Commons Attribution 4.0 International License.