School Science Lessons
2023-08-16
Ligands, chelates, EDTA, Ion exchange resins
Contents
1.0 Ligands
2.0 Reactions of metals with ligands
3.0 List of ligands
4.0 EDTA
16.1.0 Neonicotinoid insecticides
1.0 Ligands
A ligand can be an ion or an atom that can form a co-ordination complex with a central atom, usually a metal.
A ligand is a molecule that selectively binds to another molecule.
Transition metals (transition elements) can form bonds called dative covalent bonds by accepting pairs of electrons (shown as :), from other ions or molecules.
Both the shared electrons come from the same atom.
The ion or molecule that forms the bond with the transition metal is called a ligand, e.g. :Cl-, :CN-, H2O:, :NH3.
More than one ligand can bind to a transition metal ion to form a complex ion.
The number of ligands so bonded is called the co-ordination number, for example:
co-ordination number 4: [CoCl4]2-,
co-ordination number 6: [CO(NH3)6]3+
Experiments
12.5.0 Chromium ions in solution (See: 6.)
1.1 Copper (II) sulfate solution with ammonia solution, ligand substitution
1.2 Copper (II) sulfate solution with concentrated hydrochloric acid, ligand substitution
1.1 Copper (II) sulfate solution with ammonia solution, ligand substitution
The copper ion, Cu2+ (aq), in aqueous solution, forms a blue complex ion, [Cu(OH2)6]2+, with 6 water molecules.
The blue complex is the cause of the blue colour of copper sulfate solution.
Ammonia solution contains hydroxide ions, OH-, and is alkaline.
Add ammonia solution drop by drop to the pale blue copper sulfate solution.
A pale blue precipitate of insoluble copper hydroxide forms.
Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)
Add more ammonia solution to dissolve the copper hydroxide to form the deep blue complex ion, [Cu(NH3)4(OH2)2]2+.
Four of the 6 water molecules in the blue complex ion, [Cu(OH2)6]2+ have been replaced by ammonia ions, leaving 2 water molecules still there.
So the number of ligands around the copper ion is still 6.
1.2 Copper (II) sulfate solution with concentrated hydrochloric acid, ligand substitution
The copper ion, Cu2+ (aq), in aqueous solution, forms a blue complex ion, [Cu(OH2)6]2+, with 6 water molecules.
The blue complex is the cause of the blue colour of copper sulfate solution.
Concentrated hydrochloric acid has a high concentration of chloride ions, which are better ligands than water,
because they are negatively charged ions and are attracted electronically to the copper ion, Cu2+.
Add concentrated hydrochloric acid drop by drop to the pale blue copper sulfate solution.
The solution turn yellow-green as the chloride ion ligands, Cl-, replace water in the complex ion.
Four chloride ions replace the 6 water molecules in the blue complex ion to form the yellow-green [CuCl4]2- complex ion.
So the number of ligands around the copper ion has dropped from 6 to 4.
2.0 Reactions of metals with ligands
See diagram 16.4.4: EDTA molecule
Metals and ligands form co-ordination bonds (co-ordination complexes), with both electrons coming from the ligand.
Ligands have a lone pair of electrons.
Metals do not have enough electrons to form covalent bonds by sharing one electron from the metal ion with one electron from the bonded atom.
The metals involved include Ag+, Al3+, Cu2+ and Fe3+.
Examples of ligands include: -NH3, -OH2, -Cl-, -OCOCH3-, -EDTA-4, -NTA-3.
Complexes include metal carbonyls, metal (CO)4, [Cu(H2O)6]2+, [PtCl4]2-.
Metals usually bond with 4 to 6 ligands.
Chelates are ligands that bind more than one compound.
Copper forms a series of ligands with ammonia.
Cu2+ + NH3 <--> CuNH32+
CuNH32+ + NH3 <--> Cu(NH3)22+
Cu(NH3)22+ + NH3 <--> Cu(NH3)32+
Cu(NH3)32+ + NH3 <--> Cu(NH3)42+
Ammonia is a monodentate (one tooth) ligand, because it forms one co-ordination bond with a metal.
Ethanediamine, (H2NCH2CH2NH2), is a bidentate (two tooth) ligand, because it forms two co-ordination bonds with a metal.
Triethanetetramine (trien) and nitrilotriacetic acid (NTA), are tetradentate ligands, because they form one four co-ordination bonds with a metal.
Ethanediaminetetraacetate (EDTA4-), is a hexadentate ligand, because it forms six co-ordination bonds with a metal.
3.0 List of ligands
1,1-Bis(diphenylphosphino)methane (dppm)
1,10-Phenanthroline (phen), bidentate ligand
Phenanthroline
1,2-Bis(diphenylphosphino)ethane (dppe), bidentate ligand
2,2'-Bipyridine (bipy), bidentate ligand
2,2',5',2-Terpyridine (terpy), tridentate ligand
2,2,2-crypt hexadentate ligand (for alkali earth metal cations)
Acetonitrile (acetonitrilo), CH3CN, monodentate ligand
Acetonitrile (MeCN), monodentate ligand
Acetylacetonate (Acac), CH3-C(O)-CH-C(O)-CH3, bidentate ligand
Ammonia (NH3), monodentate ligand
Azide (azido), N-N2-, monodentate ligand
Benzene, C6H6 (and other arenes), ligands
Bromide (bromo), Br-, monodentate ligand
Carbon monoxide (carbonyl, CO), monodentate ligand
Chloride (chloro), Cl-, monodentate ligand
Corroles (aromatic organic corrin ring as in vitamin B12), C19H22N4, tetradentate ligands
Crown ether (18-crown-6), C12H24O6, (tests for alkali earth metal cations), ligands
Cyanide (cyano), CN-, monodentate ligand
Cyclopentadienyl groups (Cp).
[C5H5]-, ligands
Diethylenetriamine (dien), C4H13N3, tridentate ligand
Dimethylglyoximate (dmgH-), ligand
Ethylenediamine (en), bidentate ligand
Ethylenediaminetetraacetate (EDTA), hexadentate ligand
Ethylenediaminetriacetate (ED3A), pentadentate ligand
Fluoride (fluorido), F-, monodentate ligand
Glycinate (Gly), amino acid, NH2CH2COOH, bidentate ligand
Haeme A (cytochrome oxidase ligand complex), C49H56O6N4Fe, tetradentate ligand
Hydroxide (hydroxo), O-H-, monodentate ligand
Iodide (iodo), I-, monodentate ligand
Isothiocyanate (isothiocyanato), N=C=S-, monodentate ligand
Nitrate (nitroso), O-NO2-, monodentate ligand
Nitrite (nitrito), O-N-O-, monodentate ligand
Nitrite (nitro), N-O2-, monodentate ligand
Nitrosyl (NO+), cationic ligand
Nitrilotriacetic acid (NTA), C6H9NO6, tetradentate ligand
Oxalate (oxalato), [O-C(=O)-C(=O)-O]2-, bidentate ligand
Pyrazine (1, 4-diazine), N2C4H4, ligand
Pyridine (pyridino), C5H5N, monodentate ligand
ligands
Scorpionate ligands (Tp ligands), tris(pyrazolyl)hydroborates, tridentate ligand trispyrazolylborate ligand, Tp-, is an anionic tridentate and tripodal ligand,
Trispyrazolylborate: [HB(C3N2H3)3]-
Sulfide (thio), S2-, monodentate ligand or bidentate ligand
Sulfite, -SO23, monodentate ligand
Terpyridine, C15H11N3, tridentate ligand
Thiocyanate (thiocyanato), S-CN-, monodentate ligand
Tri(o-tolyl)phosphine, P(o-tolyl)3, monodentate ligand
Triazacyclononane (tacn) (C2H4)3(NR)3, tridentate ligand
Tricyclohexylphosphine (C6H11), monodentate ligand
Triethylenetetramine (trien), tetradentate ligand
Trimethylphosphine (PMe3), monodentate ligand
Triphenylphosphine (PPh3), monodentate ligand
Tris(2-diphenylphosphineethyl)amine (np3), tetradentate ligand
Tris(2-aminoethyl)amine (tren) (NH2CH2CH2)3N, tetradentate ligand
Tropylium, C7H7+, cationic ligand
Water (aqua), H-O-H, monodentate ligand
4.0 EDTA
EDTA, C10H14N2Na2O8.2H2O, Ethylenediaminetetraacetic acid disodium salt
EDTA- diammonium salt, EDTA- tetrasodium salt, EDTA- dipotassium salt, chelating agents
Ethylenediaminetetraacetic acid disodium salt, disodium ethylenediamine, disodium tetraacetic acid, EDTA, Toxic if ingested
Eriochrome black T (metal indicator for the titration of calcium and magnesium ions with EDTA)
1,2-diaminoethane, C2H8N2, C2H4(NH2)2, ethylenediamine, smells like ammonia, related to EDTA, Toxic by all routes, corrosive
1,2-diaminoethane, Solution < 1%, Not hazardous
EDTA, Ethylenediaminetetraacetic acid, edetic acid, [(HOOCCH2)2NCH2], C10H16N2O8,
chelating agent that binds calcium and heavy metal ions including lead so used to treat lead poisoning, food additive, anticoagulant, EDTA disodium, C10H14N2Na2O8.2H2O, edetic acid disodium salt.
4.1 EDTA, C10H14N2Na2O8.2H2O, Ethylenediaminetetraacetic acid disodium salt
4.2 Ion exchange resins, deionized water
3.4.7 Prepare nylon polymer:
19.4.6 Sequestrants, food additives Experiments
9.9.18.5 Mineral deficiency experiment, hydroponics, EDTA
12.13.11 Tests for metal ions, using EDTA
12.13.12.0 Tests for water hardness, EDTA titration, Calmagite indicator
12.13.12.1 Tests for water hardness, EDTA titration, Eriochrome Black T indicator
4.1 EDTA
EDTA, ethylene diamine tetra acetic acid (HOOC.CH2)2N(CH2)2N(CH2.COOH)2
See diagram 16.4.4: EDTA molecule
1. Ethylenediaminetetraacetic acid (HOOCCH2)2N(CH2)2N(CH2COOH)2, edathamil, sequestering agent for metal ions, Mg2+, Ca2+.
Complexing agent for most metal ions, especially Ca2+ and Mg2+.
Used for water softening, remove scale from kettles, medical removal of excess metallic ions.
If a ligand is defined as a small molecule that binds to a larger molecule, then chelates can be said to bring about the complexation of a ligand.
The terms ligand, chelate, chelating agent and sequestering agent are used in slightly different ways in chemistry, medicine, and general industry.
If EDTA = H4Y, then the disodium dihydrate form = Na2H2Y.2H2O
H2Y2- + Ca2+ <--> CaY2- + 2H+
2. EDTA disodium salt, ethylenediaminetetraacetic acid disodium salt dihydrate, C10H14N2Na2O8.2H2O, EDTA disodium salt,
ethylenediaminetetraacetate dihydrate (HOOCCH2)2N(CH2)2N(CH2COONa)2.2H2O, (0.1 M, solution + sodium azide), for electrophoresis, molecular biology.
Electrophoresis, (Commercial)
3. A chelate is a metal ion bound to two or more atoms of a chelating agent (sequestering agent),
For example, the simple chelating agent 1,2-diaminoethane (ethylene diamine), NH2.CH2.CH2.NH2, forms bonds to a metal ion through its nitrogen atoms.
[Diaminoethane, ethylene diamine, C2H4(NH2)2, ligand, chelating agent, Toxic by all routes, Corrosive]
Porphyrin chelates include haeme, in haemoglobin, bonded to iron (II) ion, and chlorophyll bonded to Mg (II) ion.
Similarly vitamin B-12 has cobalt (II) ion bonded to a chelating agent.
4. The synthetic chelating agent EDTA can form complexes with calcium and magnesium ions.
So it can form the calcium complex [Ca(EDTA)]2-.
The sodium salt used as an antidote for metal poisoning, an anticoagulant, enzyme deactivation, bactericide, industrial processes.
EDTA disodium salt,   (HOOC.CH2)2N(CH2)2N(CH2.COO.Na)2.2H2O.
EDTA deactivates the enzymes containing metal ions that cause food spoilage, loss of colour and loss of flavour.
EDTA dissolves the calcium carbonate scale caused by hard water and prevents stored blood from clotting by sequestering calcium ions.
Calcium disodium EDTA is a treatment for lead poisoning by exchanging its chelated calcium for lead so lead chelate is excreted.
5. Industrial synthesis of EDTA
NH2.CH2.CH2.NH2 + 4 H.CHO + 4 Na.CN + 4 H2O
(Na.OOC.CH2)2N(CH2)2N(CH2.COO.Na)2 + 4 NH3
1, 2-diaminoethane (ethylenediamine) + methanal (formaldehyde) + sodium cyanide + water -->
sodium salt + ammonia (Na.OOC.CH2)2N(CH2)
2N(CH2.COO.Na)2 + 4 HCl -->
(HOOC.CH2)2N(CH2)2N(CH2.COOH)2 + 4 NaCl
sodium salt + hydrochloric acid --> EDTA + sodium chloride
4.2 Ion exchange resins, deionized water
Let RZ = the resin, an organic polymer matrix.
Charged groups are bound to the resin.
Cation exchange resin, H+ form, to remove cations, e.g. Ca2+, from solution.
2RZ-SO3- H+ + Ca2+ <--> RZSO3-)2Ca2+ + 2H+
Anion exchange resin, OH- from, to remove anions, e.g. Cl-, from solution
RZ-N(CH3)3+ OH- + Cl- -->RZ-N(CH3)3+ Cl- + OH-
To "soften" water, usually only a cation exchange resin is used.
If both a cation exchange resin and an anion exchange resin are used with tap water to remove ionic salts by ion exchange, the resulting solution is deionized water,
a cheaper alternative to distilled water.
Neonicotinoid insecticides
Neonicotinoid insecticides, neonics, are systemic insecticides, so taken to all parts of the plant, used for seed treatment, pot plant soil treatment, turf spray,
foliar spray in glass houses for fruit trees.
They have been blamed for the present decline in the bee population and so have been partially banned in some countries.
15/03/2013
The European Food Safety Authority gave the most compelling evidence that toxic chemicals "neonicotinoid pesticides" could be responsible for the bee deaths.
Italy has banned some uses of these bee killing pesticides and has already seen it's bee populations come back,
However, Bayer and Sygenta lobbied to prevent a Euro-wide ban, for fears it would harm their global business.
It seems they're close to having the support of the UK, Spain, and Germany, who want to protect their biggest chemical corporations.
Now the issue is coming to a boil.
Just weeks ago, Avaaz delivered a petition signed by over 2.5 million of us to the European Commission, who proposed a ban day later.
EU parliamentarians stepped up their pressure and several other European governments have announced plans to push ahead with new legislation to ban the pesticides.
16.1.0 Neonicotinoid insecticides:
Neonicotinoid insecticides
Acetamiprid, C10H11ClN4
Clothianidin, C6H8ClN5O2S
Fipronil, C12H4Cl2F6N4OS
Imidacloprid, C9H10ClN5O2
Thiacloprid, C10H9ClN4S
Thiamethoxam, C8H10ClN5O3S
Acetamiprid
Acetamiprid, C10H11ClN4, neonicotinoid insecticide, has population level effects on honeybees by interfering with honeybee reproduction and navigation.
Clothianidin
Clothianidin, C6H8ClN5O2S, neonicotinoid insecticide, is neurotoxic and is highly toxic to bees and other non-target insects.
In 2008 a massive bee die-off occurred in Germany, which was subsequently associated with clothianidin.
Fipronil
Fipronil, C12H4Cl2F6N4OS, neonicotinoid insecticide, is used for indoor and turf pest control.
It causes reduced behavioural function and learning performances in honeybees.
Imidacloprid
Imidacloprid, C9H10ClN5O2, neonicotinoid insecticide, is used in agriculture as foliar and seed treatments, for indoor and outdoor insect control.
It is the most popular neonicotinoid.
It has been found to be highly toxic to bees and other beneficial insects.
It is also toxic to upland game birds, is generally persistent in soils and can leach to groundwater.
It cause disruptions in mobility, navigation, and feeding behaviour.
Thiacloprid
Thiacloprid, C10H9ClN4S, neonicotinoid insecticide, is used to control sucking and biting insects in cotton, rice, vegetables, pome fruit, sugar beet, potatoes and ornamentals.
Low doses are highly toxic to honeybees and fish.
Thiamethoxam
Thiamethoxam, C8H10ClN5O3S, neonicotinoid insecticide, is a systemic insecticide absorbed and transported to all parts of the plant.
If bees eat the pollen, it interferes with nerve cell information exchange, paralysing the insect.