Coordination Compounds and Ligands

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Questions and Answers

Which statement accurately describes ligands in coordination compounds?

Each ligand has at least one nonbonded pair of electrons. (correct)

Ligands always consist of more than one atom.

Ligands can only be neutral molecules.

Ligands do not influence the stability of coordination compounds.

What is the significance of chelates in coordination chemistry?

Chelates are formed exclusively from cationic ligands.

Stability increases with five, six, or seven-membered rings. (correct)

Chelates are unstable ring structures in solutions.

Chelates involve only unidentate ligands.

Which of the following ligands would generally be considered most stable when forming a coordination compound?

A monodentate ligand from Group VIA.

An anionic ligand from Group VIIA. (correct)

A neutral ligand from Group II.

A bidentate ligand from Group IB.

What type of bonding is primarily involved when ligands coordinate with metal ions?

Coordinate covalent bonding primarily. Signup and view all the answers

Which characteristic describes polydentate ligands involved in chelation?

They attach to metal ions at multiple coordination sites. Signup and view all the answers

What is the term used for ligands that improve the solubility and stability of metal ions through chelation?

Chelating agents. Signup and view all the answers

What is the relationship between the basicity of ligands and their stability in coordination complexes?

Higher basicity generally correlates with greater stability. Signup and view all the answers

Which type of ligand cannot form a chelate with a metal ion?

Monodentate ligands. Signup and view all the answers

Which heavy metal poisoning is primarily treated with calcium disodium edetate (EDTA)?

Lead poisoning Signup and view all the answers

What commonly affects calcium levels when using calcium disodium edetate?

Hypocalcemia Signup and view all the answers

What is the preferred route of administration for calcium disodium edetate due to its poor gastrointestinal absorption?

Intravenous Signup and view all the answers

Which metal is not effectively treated by calcium disodium edetate?

Mercury Signup and view all the answers

What is a common physical state of dimercaprol (BAL) at room temperature?

Colorless liquid Signup and view all the answers

Which of the following agents is primarily used for treating copper and nickel poisoning?

Calcium disodium edetate Signup and view all the answers

What is the expected urinary excretion level indicative of lead toxicity?

500 pg/liter/24 hours Signup and view all the answers

What type of hybridization is assumed when six ligand atoms coordinate with a Cr(III) ion in a complex?

d2sp3 hybridization Signup and view all the answers

What is the pH range of an aqueous solution of disodium edetate?

4.0 to 6.0 Signup and view all the answers

What is the primary role of chelating agents in analytical chemistry?

To solubilize metal ions and stabilize their oxidation states Signup and view all the answers

How does the presence of CN- ligands affect the magnetic moment (MM) of an Fe(III) complex?

Reduces MM by pairing electrons in d orbitals Signup and view all the answers

What is the magnetic moment (MM) indicative of when it is found to be 6 in a complex?

Five unpaired electrons Signup and view all the answers

Which factors influence the configuration of a metal ion's d orbitals in a complex?

The electron count and ligand field strength Signup and view all the answers

What common analytical solutions utilize chelating agents for identifying reducing substances?

Benedict's solution and Fehling's solution Signup and view all the answers

Which statement about the effect of water and cyanato ligands on the MM of Fe(III) complexes is true?

Cyanato ligands create a lower MM than water ligands Signup and view all the answers

In complexation chemistry, which characteristic is NOT typically associated with chelating agents?

Formation of insoluble precipitates Signup and view all the answers

What is the primary mechanism by which heavy metals such as trivalent arsenic exert cellular toxicity?

Covalent bond formation with sulfhydryl groups Signup and view all the answers

In which situation is the use of dimercaprol contraindicated?

Iron poisoning Signup and view all the answers

Which route of administration is typically used for penicillamine?

Oral Signup and view all the answers

What is the significance of alkalinizing the urine during treatment with dimercaprol?

To avoid renal toxicity from free metals Signup and view all the answers

Which of the following is NOT a metal that penicillamine can chelate?

Iron Signup and view all the answers

What is the relationship between penicillamine's effectiveness and its chemical structure?

It lacks a hydrogen on the beta-carbon atom Signup and view all the answers

What is the stability period for an aqueous solution of deferoxamine at room temperature?

Two weeks Signup and view all the answers

Which type of solution is deferoxamine produced as after removing iron?

Methylsulfonate (mesylate) salt Signup and view all the answers

Study Notes

Coordination Compounds and Complexation

Metallic cations can bond with additional anions or neutral molecules beyond their normal valence requirements.
These additional bonding species are called ligands and bond directly to the metal cation.
The maximum number of ligands that can be accommodated by a metal ion is called the coordination number.
Coordination compounds consist of a metal ion and its associated ligands.
Some complexes are stable in crystalline form but decompose in solution, while others are stable only in solution.

Properties of Ligands

Ligands are usually anions or neutral molecules, not neutral atoms.
Ligands have at least one nonbonded pair of electrons to form a coordinate covalent bond with the metal ion.
More stable complexes are formed with anionic or molecular ligands containing elements from Groups VA, VIA, or VIIA.
Ligand stability generally follows the basicity of the ligand (Lewis base).
Ligands can be classified based on the number of positions on the molecule capable of coordinating with a metal (e.g., monodentate, bidentate, etc.).

Chelation

Polydentate ligands form ring structures when they complex a metal ion, creating chelates.
Five, six, or seven-membered rings (including the metal) are the most stable chelates.
Chelates are important in pharmaceuticals and drug therapy.
Polydentate ligands used for chelate formation are known as chelating agents.
Sequestering agents are polydentate ligands used to improve metal ion solubility and stability through chelation (sequestration).

Bonding in Complexes

The (n-1)d, ns, and np orbitals of a metal ion can hybridize into six bonding orbitals directed along the axes occupied by ligands.
These hybrid orbitals are called d2sp3 hybrids and are equivalent when all ligands are identical.
For example, in the Cr(III) complex with six CN-, the Cr(III) ion has three electrons in its d orbitals, leaving two d orbitals, one s orbital, and three p orbitals available for bonding with the six cyanide groups.
When metal ions have four or more electrons in their d orbitals, complex formation may alter the ground state configuration.
This change can be measured experimentally by determining the magnetic moment (MM) of the complex, which reflects the unpaired electrons.
In the Fe(III) complex with six water molecules, the MM is 6, indicating five unpaired electrons. This suggests outer orbital hybridization (HP), where the 4d orbital is included in the hybrid along with the 4s and 4p orbitals.
If water is replaced by six CN-, the MM is about 2, indicating one unpaired electron. This occurs because the strong negative field of the CN- ligand repels the electrons in the 3d orbitals (dx2-y2 and dz2), forcing them to pair in other 3d orbitals.

Complexes and Chelating Agents in Analytical Chemistry

Complexation is crucial in analytical chemistry, where concentrations of metals can be determined by titration with complexing agents.
Chelates are used to solubilize metals and stabilize their oxidation states in analytical solutions, as seen in Benedict's solution and Fehling's solution.
Chelating agents are also used as preservatives to prevent decomposition caused by trace metals.

Chelating Agents in Drug Therapy

Chelating agents are essential in treating heavy metal poisoning from elements like lead, mercury, iron, etc.
They are also used for metabolic disorders where metals like iron and copper accumulate in abnormal amounts in tissues.
Common chelating agents include calcium disodium edetate (EDTA), dimercaprol (BAL), penicillamine, and deferoxamine.

Calcium Disodium Edetate (EDTA)

Exists as a white crystalline granule, odorless, slightly hygroscopic, and freely soluble in water.
Used to treat heavy metal poisoning, primarily lead poisoning, by forming insoluble complexes excreted by the kidney.
Has a strong affinity for calcium, so the disodium calcium form is used to avoid hypocalcemia (low serum calcium).
Effective against copper, nickel, cadmium, zinc, chromium, and manganese poisoning but ineffective against mercury, arsenic, or gold toxicity.
Poorly absorbed from the gastrointestinal tract, so administered intravenously (I.V.).
Intramuscular (I.M.) administration is used for diagnosing metal poisonings.

Disodium Edetate

White crystalline powder, soluble in water, with a pH between 4.0 and 6.0.
Chelates the same metals as the disodium calcium form.
Limited by the risk of hypocalcemia.
Primarily used for hypercalcemic states (high serum calcium).
May be useful in treating occlusive vascular disease and cardiac arrhythmias associated with high blood calcium levels.
Ineffective in aiding dissolution of urinary calculi (calcium-containing stones in the urinary tract).
Administered intravenously.

Dimercaprol (BAL)

Colorless or almost colorless liquid with a disagreeable odor.
Soluble in water, alcohol, and benzyl benzoate.
Neutralizes heavy metals like trivalent arsenic by competing with enzymes for their sulfhydryl (—SH) groups, which are critical for oxidation-reduction reactions in tissues.
Effective in arsenic, mercury, and gold poisoning.
Contraindicated in poisoning by iron, cadmium, or selenium because the resulting complexes are more renal toxic than the free metals.
Dimercaprol-metal chelates dissociate in acidic media, so urine should be alkalinized to prevent the release of free metal and renal toxicity.
Administered intramuscularly.

Penicillamine

White or off-white crystalline powder with a slight odor.
Freely soluble in water and slightly soluble in alcohol.
Chelates copper, iron, mercury, lead, gold, and other metals.
Used to improve copper excretion in Wilson's disease (degenerative changes in the brain associated with increased copper levels and liver degeneration).
Effective due to its resistance to deactivation by amino acid oxidase.
Used to treat gold dermatitis in patients receiving chronic gold therapy.
Administered orally.

Deferoxamine Mesylate

White, crystalline, lyophilized powder.
Soluble in water, stable at room temperature for two weeks.
Produced naturally by Streptomyces pilosus as a ferric [Fe(III)] complex.
Purified as the methylsulfonate (mesylate) salt after chemical removal of iron.

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Description

Explore the fascinating world of coordination compounds and ligands in this quiz. Understand how metallic cations bond with ligands, the role of coordination numbers, and the properties that influence ligand stability. Test your knowledge on the essential concepts of complexation chemistry.