5.2: Ligands and Nomenclature (2024)

Ligand Denticity

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

Figure \(\sf{\PageIndex{1}}\). (A) Ammonia is 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\(\sf{\PageIndex{2}}\).

Figure \(\sf{\PageIndex{2}}\). A selection of chelating ligands classified according to denticity. this work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

The classification of several of the ligands in Figure\(\sf{\PageIndex{2}}\) requires a bit of explanation, specifically as to why the denticity of the carboxylate-containing ligands is less than the total number of lone-pair bearing oxygen and nitrogen atoms present. This is because only one of the carboxylate oxygen atoms is counted. Only one is counted because in most cases only one oxygen binds to a metal at any given time. When that oxygen is bound the other oxygen faces away from the metal, as depicted for the iron complex shown in Figure\(\sf{\PageIndex{3A}}\). Because only one oxygen per carboxylate typically binds only one is counted when assigning a ligand's denticity.

Although only one carboxylate oxygen usually binds to a metal it is still possible to bind a metal using both oxygen atoms. As shown in Figure\(\sf{\PageIndex{3B}}\) complexes in which both carboxylates bind to a metal are known, and in fact are common in the active sites of some enzymes. It is just that the binding of both oxygen atoms gives a strained four-membered ring that is usually unstable.

Figure \(\sf{\PageIndex{3}}\). (A) Only one oxygen per carboxylate counts towards the denticity of EDTA since on binding the other oxygen generally points away from the metal center, as in the structure of Fe(EDTA)^-. This does not mean that both oxygens of a carboxylate can never both bind to metal centers in a complex. (B) Structures in which both oxygens of a carboxylate side chain bind to a metal are sometimes found in the active sites of some of the nonheme iron enzymes your body uses to break down amino acids. this work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

As was the case for carboxylate groups above, sometimes the classification of a group's denticity is based on experimental knowledge of their common binding modes. Carboxylates might commonly act as monodentate ligands but dithiocarbamates more commonly bind metals through both sulfur atoms (Figure\(\sf{\PageIndex{4}}\)) and are classified as bidentate.

Figure \(\sf{\PageIndex{4}}\). As in this complex, dithiocarbamates commonly bind metals through both sulfur atoms. Consequently, dithiocarbamates are classified as bidentate. this work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

Because of these factors it is technically more correct to say that carboxylates usually act as monodentate ligands and dithiocarbamates bidentate ones than it is to say that carboxylates are monodentate ligands and dithiocarbamates bidentate ones. So in other words the ligand classifications presented here are just represent common binding modes.

This experimentally-based classification of dithiocarbamates as bidentate and carboxylates as monodentate can be confusing to a beginner. Fortunately, such experimentally based classifications are embedded in the lists of common monodentate ligands are given in Table \(\sf{\PageIndex{1}}\) and common chelating ligands in Table \(\sf{\PageIndex{2}}\).

A perusal of the ligands in Table \(\sf{\PageIndex{1}}\) reveals that several can bind to a metal in multiple ways. For example, thiocyanate, SCN^- can bind metal's through its S or N atoms. Such ligands are called ambidentate ligands. When 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.

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

Table \(\sf{\PageIndex{2}}\). 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)	LigandType	structure or representative/parent structure (shown in the ionization state in which they bind to a metal)
bidentate ligands
acetylacetonato	2,4-pentanediono	acac	LX
2,2'-bipyridine	2,2'-bipyridine	bpy or bipy	L2
dialkyldithiocarbamato	dialkylcarbamodithiolato	R2NCS2- or dtc	LX
diphenylphosphinoethane or 1,2-(diphenylphosphino)ethane	Ethane-1,2-diylbis(diphenylphosphane)	dppe	L2
ethylenediamine	Ethane-1,2-diamine	en	L2
ethylenedithiolato	Ethane-1,2-dithiolato	C2H2S22-	X2
nacnac	N,N'-diphenyl-2,4-pentanediiminato	nacnac	LX
oxalato	oxalato	ox	X2
1,10-phenanthroline or o-phenanthroline	1,10-phenanthroline	phenor o-phen	L2
tridentate ligands
triazacyclononane	1,3,7-triazacyclononane	tacn	L3
diethylenetriamine	1,4,7-triazaheptane	dien	L3
pyrazoylborato (scorpionate)	hydrotris(pyrazo-1-yl)borato	Tp	L2X
terpyridine or 2,2';6',2"-terpyridine	12,22:26,32-terpyridine or 2,6-bis(2-pyridyl)pyridine, tripyridyl, 2,2′:6′,2″-terpyridine	tpy or terpy	L3
tetradentate ligands
\(\beta\), \(\beta\)', \(\beta\)''-triaminotriethylamine	\(\beta\), \(\beta\)', \(\beta\)''-tris(2-aminoethyl)amine	tren	L4
triethylenetetramine	1,4,7,10-tetraazadecane	trien	L4
corroles	variable and generally not used	cor or Cor	L2X2
12-crown-4	1,4,7,10-tetraoxacyclododecane	12-crown-4	L4
tetramethylcyclam	1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane	TMC or cyclam	L4
cyclam	1,4,8,11-tetraazacyclotetradecane	cyclam	L4
cyclen	1,4,7,10-tetraazacyclododecane	cyclen	L4
tris(2-pyridylmethyl)amine	1-pyridin-2-yl-N,N-bis(pyridin-2-ylmethyl)methanamine	tpa or TPA	L4
phthalocyanines	variable and generally not used	variable, usually a modified Pc	L2X2
porphyrins	variable and generally not used	variable, usually a modified por, Por, or P (e.g. TPP = tetraphenylporphyrin)	L2X2
salen	2,2'-ethylenebis(nitrilomethylidene)diphenoxido	salen	L2X2
pentadentate ligands
15-crown-5	1,4,7,10,13-Pentaoxacyclopentadecane	15-crown-5	L5
tetraethylenepentamine	1,4,7,10,13-pentaazatridecane	tepa or TEPA	L5
hexadentate ligands
18-crown-6	1,4,7,10,13,16-hexaoxacyclooctadecane	18-crown-6	L6
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	L6
ethylenediaminetetraaceto	2,2′,2″,2‴-(Ethane-1,2-diyldinitrilo)tetraaceto	EDTA, edta, Y4-	L2X4
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	L7
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	L8
pentetato acid or diethylenetriaminepentaacetato or DTPA	2-[bis[2-[bis(carboxylatomethyl)amino]ethyl]amino]acetato	DTPA	L3X5
DOTA or tetraxetan	1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid	Dota, DOTA	L4X4

Rules for Naming Coordination Compounds

There are well-established rules for both naming and writing the formulae of coordination compounds. The purpose of these rules is to facilitate clear and precise communication among chemists. As with all such rules some are more burdensome than others to employ and some serve more crucial roles in the communication process while others are more peripheral.

Coordination Complexes are named as the ligand derivatives of a metal

A variety of systems have been used for naming coordination compounds since the development of the discipline in the time of Alfred Werner. In this section the most common approaches as they are currently used by practicing chemists will be described. Those who need a more thorough and accurate acquaintance with the full IUPAC nomenclature rules are encouraged to consult the IUPAC brief guide to inorganic nomenclature followed by complete guidelines, commonly known as the IUPAC red book.The systems for naming coordination compounds used at present are additive, meaning that they consider coordination compounds as comprised of a central metal to which are added ligands. To specify the structure and bonding in this metal-ligand complex then involves

1. When there are several different ways of attaching the metal and ligands, specifying the structural or stereoisomer

2. systematically listing the ligands in a way that, as necessary, conveys information about how they are linked to the metal and their stereochemistry

3. providing the identity of the metal and its valence, or if the valenceis unclear, at least the overall charge on the complex

4. specifying any counterions present

Since the stereochemistry of coordination compounds forms is the subject subsequentsections, in this section it will be addressed by simply giving the prefixes that designate stereochemistry as if they were self-evident. Do not worry about these for now. They will make sense after you have learned more about stereochemistry in the next sections. At that time, you can go back over the examples in thissection to solidify your understanding of how to name coordination compounds.

Before going into these rules it is worth pointing out a few things.

1. It is easiest to learn these rules by starting with one or two of the rules, learning how to apply them, and then adding additional rules one at a time.

2. The rules also assume some familiarity with common coordination geometries and patterns of isomerism in metal complexes. Again, we will learn more about these in a later section. For now it is safe to assume

• complexes in which the metal has a coordination number of six are octahedral
• complexes in which the metal has a coordination number of five are trigonal bipyramidal
• complexes in which Pt^II , Pd^II , or Rh^I, or Ir^I have a coordination number of four are square planar
• other complexes in which the metal has a coordination number of four are tetrahedral

Like all assumptions these don't always work in real life but they should be good enough for now.

Rule 2: When multiple isomers are possible, designate the particular isomer in italics at the front of the name of each complex

When a complex might exist as one of two stereosisomers prefixes are commonly used to designate which isomer is present. The most common cases are listed in Table \(\sf{\PageIndex{3}}\).

Table\(\sf{\PageIndex{3}}\). Prefixes used to specify isomerism about a metal center when naming and writing coordination compounds' formulae.

Type of isomerism	Graphical reminder	Prefixes
Geometric, cis- , trans-		cis- or trans-
Geometric, fac- /mer-		fac- or mer-
Enantiomers, \(\Lambda\)-, \(\Delta\)-		\(\Lambda\)- or \(\Delta\)-

Examples of how isomerism about a metal center is designated are given in Figure\(\sf{\PageIndex{5}}\).

Figure\(\sf{\PageIndex{5}}\). Application of nomenclature rules for stereosimerism about a metal.¹

There are a number of other cases where it might be advisable to specify the stereochemistry of a complex. These cases involve specifying

• the coordination geometry about a metal center (octahedral, trigonal prismatic, tetrahedral, square planar, etc. )
• the geometry cannot be unambigouously described by a single cis/trans or fac/mer relaionship of ligands

These cases may also be handled by using a designator to specify the coordination geometry and, as necessary, giving the position of ligated atoms in terms of designated numbered positions for that geometry. See the IUPAC red book for details as such cases fall outside the scope of thiscourse.

Rule 3: Specify the identity, number, and as appropriate, isomerism of the ligands present in alphabetical order by ligand name.

Before specifying the metal, the ligands are written as prefixes of the metal.

In specifying the ligands several rules are followed.

1. The ligands are written in alphabetical order by the ligand name only; symbols are not considered and prefixes do not count in determining alphabetical order.

Example: In the name of the complex ion [Co(NH)₃Cl]²⁺, pentamminechlorocobalt(II), the ammine ligand is named before the chloro ligand because the order is alphabetical by the ligand name by virtue of which ammine comes before chloro.

2. Prefixes are used to indicate the number of each ligand present. Specifically, di-, tri-, tetra- , penta-, hexa-, etc. prefixes are used to indicate multiple ligands of the same type EXCEPT when the ligand is polydentate or its name already has a di-, tri-, tetra- etc. In that case bis-,tris-, tetrakis-, etc. are used instead. These prefix rules are summarized in Table \(\sf{\PageIndex{3}}\).

   Table \(\sf{\PageIndex{3}}\). Prefixes used to specify the number of a given ligand present.

Number of identical ligands	prefix used when the ligand name is simple	prefix used when the ligand is polydentate or its name already has a di-, tri-, tetra- etc.
2	di-	bis-
3	tri-	tris-
4	tetra-	tetrakis-
5	penta-	pentakis-
6	hexa-	hexakis-
7	hepta-	heptakis-
8	octa-	octakis-
9	nona-	nonakis-
10	deca-	decakis-

An example of the application of the prefix rule is given in Figure\(\sf{\PageIndex{6}}\).

Figure \(\sf{\PageIndex{6}}\). Example of the use of prefixes to specify the number of ligands of each type in a complex.¹

3. Ligand names are based on their charge

• L-typeligand names are the same as the names of their neutral compounds with two caveats

1. names that involve spaces should either be put in parentheses or the spaces should be eliminated (preferred)

Example: cis-dichlorobis(dimethyl sulfoxide)platinum(II) or cis-dichlorobis(dimethylsulfoxide)platinum(II)

See Also

What Is a Charge Nurse? Duties, Pay, and How to Become One

2. A few ligands are given common names.

• H₂O = aqua
• NH₃ = ammine (notice that there ate two n's)
• CO = carbonyl
• CS = thiocarbonyl
• NO = nitrosyl

• For X-type ligands, the ending is typically changed to end inan -”o”

Examples: Cl= chloro, NH₂= amido, N₃= azido

Caveat: some X-type ligands have common names that may also be used

Examples:

I= iodo or iodino

CN= cyano or cyanido

O= oxo or oxido

The IUPAC and common names of many ligands are given in Tables \(\sf{\PageIndex{1}}\). and \(\sf{\PageIndex{2}}\).

4. When an ambidentate ligand is present the atom through which it is bound to the metal is indicated by giving either its element symbol or a \(\kappa\) and its element symbol in italics after the ligand name

Example:

M-SCN is thiocyanato-S or thiocyanato-\(\kappa\)S

M-NCS = thiocyanato-N or thiocyanato-\(\kappa\)N

The use of \(\kappa\) and an element symbol to indicate how a ligand and metal are linked is called a k-term. More complex k-terms might also involve specifying the atoms by number, though their use is outside the scope of this text.

5. As appropriate, additional information about the way a ligand is bound to the metal center and/or its stereochemistry is specified using a prefix. The prefixes to provide linkage and stereochemistry for ligands are given in Table\(\sf{\PageIndex{4}}\).

Table\(\sf{\PageIndex{4}}\). Prefixes used to specify ligands' isomerism when naming and writing coordination compounds' formulae. Some of these types of isomerism will be discussed in later pages.

Type of isomerism	Graphical reminder	Prefixes
when a multidentate ligand binds through less than the full number of atoms		\(\kappa\)n where n is the number of attached atoms; used when the attached atoms are not directly connected by a chemical bond. The metal-ligand bonding usually involves \(\sigma\)-type coordination.
bridging ligands		\(\mu\)n where n is the number of atoms bridged. The number n is usually omitted when n =2.
chelating ligand ring twist		\(\lambda\)- or \(\delta\)-

A example showing how the nomenclature rule is applied to a ligand that can have two coordination modes is given in Figure\(\sf{\PageIndex{7}}\).

Figure \(\sf{\PageIndex{7}}\). Use of the \(\kappa\) notation to specify the number of attached groups in a multidentate ligand.

6. If desired, parentheses may be used to delineate a ligand name to make it easier to identify in the name. This can be particularly helpful when the name contains a lot of information to keep track of. An example is given in Figure\(\sf{\PageIndex{8}}\).

Figure\(\sf{\PageIndex{8}}\). When naming the complex shown cis-diaquabis(ethylenediamine)chromium(III) nitrate is easier to read than cis-diaquabisethylenediaminechromium(III) nitrate.

References

1. 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

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.

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 aCreative Commons Attribution 4.0 International License.