1.2 Ligands and Coordination Numbers
3 min read•august 14, 2024
Ligands and coordination numbers are key players in coordination chemistry. They determine how metal complexes form and behave. Ligands can be simple or complex, with varying charges and binding abilities.
Coordination numbers tell us how many ligand atoms are attached to a metal center. This affects the shape and properties of the complex. Understanding these concepts is crucial for predicting and explaining metal complex behavior.
Ligand Denticity and Charge
Types of Ligands
Top images from around the web for Types of Ligands
5.2: Ligands and Nomenclature - Chemistry LibreTexts View original
Is this image relevant?
Coordination Chemistry of Transition Metals | Chemistry: Atoms First View original
Is this image relevant?
Coordination Compounds | Boundless Chemistry View original
Is this image relevant?
5.2: Ligands and Nomenclature - Chemistry LibreTexts View original
Is this image relevant?
Coordination Chemistry of Transition Metals | Chemistry: Atoms First View original
Is this image relevant?
1 of 3
Top images from around the web for Types of Ligands
5.2: Ligands and Nomenclature - Chemistry LibreTexts View original
Is this image relevant?
Coordination Chemistry of Transition Metals | Chemistry: Atoms First View original
Is this image relevant?
Coordination Compounds | Boundless Chemistry View original
Is this image relevant?
5.2: Ligands and Nomenclature - Chemistry LibreTexts View original
Is this image relevant?
Coordination Chemistry of Transition Metals | Chemistry: Atoms First View original
Is this image relevant?
1 of 3
- Ligands are ions or molecules that bond to a central metal atom or ion to form a coordination complex
- ligands bind to the central metal atom through a single donor atom (, , , , )
- ligands bind through multiple donor atoms ( (en), ethylenediaminetetraacetate (EDTA4-))
Ligand Charge and Denticity
- Ligands can be neutral, anionic (negatively charged), or cationic (positively charged)
- The charge of a ligand affects the overall charge of the coordination complex
- The denticity of a ligand refers to the number of donor atoms that coordinate to the central metal atom
- ligands have two donor atoms
- ligands have three donor atoms
- Tetradentate and higher denticity ligands also exist (EDTA4- is )
Coordination Number of Central Metal
Definition and Range
- The coordination number is the number of ligand donor atoms bonded to the central metal atom in a coordination complex
- Coordination numbers typically range from 2 to 9
- 4 and 6 are the most common coordination numbers
- Examples: [Cu(NH3)4]2+ (), [Co(NH3)6]3+ ()
Determining Coordination Number
- The coordination number is determined by counting the total number of ligand donor atoms, regardless of the ligand's denticity
- A complex with two bidentate ligands and two monodentate ligands has a coordination number of 6
- Example: [Co(en)2Cl2]+ has two bidentate en ligands and two monodentate Cl- ligands, giving a coordination number of 6
Denticity and Coordination Number Relationship
Polydentate Ligands and Coordination Sites
- The denticity of the ligands and the coordination number of the central metal atom are related but not always equal
- Polydentate ligands can occupy multiple coordination sites on the central metal atom, reducing the total number of ligands needed to achieve a given coordination number
- A complex with three bidentate ligands has a coordination number of 6 (e.g., [Co(en)3]3+)
- A complex with six monodentate ligands also has a coordination number of 6 (e.g., [Co(NH3)6]3+)
Maximum Number of Polydentate Ligands
- The maximum number of polydentate ligands that can bind to a central metal atom depends on the coordination number and the ligand's denticity
- For a coordination number of 6, up to three bidentate ligands, two tridentate ligands, or one hexadentate ligand can be accommodated
- Examples: [Co(en)3]3+ (three bidentate ligands), [Co(dien)2]3+ (two tridentate ligands), [Co()]- (one hexadentate ligand)
Coordination Geometry Prediction
Factors Determining Coordination Geometry
- The coordination geometry refers to the spatial arrangement of the ligand donor atoms around the central metal atom
- The coordination geometry is determined by the coordination number and the presence of sterically demanding ligands
- Sterically demanding ligands, such as polydentate or bulky ligands, can distort the coordination geometry from the ideal arrangement
- Example: [Cu(NH3)4]2+ has a geometry due to the Jahn-Teller effect, while [Ni(NH3)4]2+ has a geometry
Common Coordination Geometries
- Linear (coordination number 2)
- Example: [Ag(NH3)2]+
- Trigonal planar (coordination number 3)
- Example: [HgCl3]-
- Tetrahedral or square planar (coordination number 4)
- Examples: [Zn(NH3)4]2+ (tetrahedral), [Pt(NH3)4]2+ (square planar)
- Trigonal bipyramidal or square pyramidal (coordination number 5)
- Examples: [Fe(CO)5] (trigonal bipyramidal), [VO(acac)2] (square pyramidal)
- (coordination number 6)
- Example: [Co(NH3)6]3+
Key Terms to Review (28)
Anionic Ligands: Anionic ligands are negatively charged ions that bond to a central metal atom in coordination compounds, playing a crucial role in determining the properties and behavior of these complexes. Their charge and geometry influence the overall structure of the coordination complex, including its stability and reactivity. Understanding anionic ligands is essential for correctly naming coordination compounds and recognizing their coordination numbers.
Bidentate: Bidentate refers to a type of ligand that has two donor atoms capable of coordinating to a central metal atom in a complex. This dual attachment allows for the formation of more stable chelate complexes compared to monodentate ligands, which only have one binding site. Bidentate ligands enhance the overall stability and geometry of coordination compounds, significantly influencing their chemical properties and reactivity.
Cl-: Cl- is the chloride ion, a negatively charged ion derived from chlorine. It plays a crucial role as a ligand in coordination chemistry, forming complexes with metal ions and influencing their properties. The presence of Cl- in coordination compounds can affect the geometry, stability, and reactivity of these complexes, making it an important player in the study of ligands and their coordination numbers.
Cn-: The term 'cn-' refers to a coordination number in coordination chemistry that indicates the total number of ligand donor atoms bonded to a central metal atom or ion in a complex. This number is critical as it influences the geometry and stability of the complex, determining how the ligands arrange around the metal center and how they interact with each other.
Co: The term 'co' in the context of coordination chemistry refers to the bidentate ligand that forms two coordinate bonds with a metal center. This term is critical because it describes how certain ligands can simultaneously interact with a central metal ion, thereby influencing the structure and stability of coordination complexes. Understanding 'co' helps in grasping the broader implications of ligand behavior and coordination numbers within coordination compounds.
Coordination Number 4: Coordination number 4 refers to the number of ligand atoms that are bonded to a central metal atom in a coordination complex. This configuration can influence the geometry of the complex, leading to various arrangements such as tetrahedral or square planar structures, which are essential in understanding the bonding and behavior of coordination compounds.
Coordination Number 6: Coordination number 6 refers to the arrangement of six ligands around a central metal atom in a coordination complex. This specific coordination number often leads to an octahedral geometry, where the ligands are symmetrically positioned around the central metal. Understanding this concept is crucial in theories of bonding in coordination compounds, as it illustrates how ligands interact with the metal and influence the compound's properties.
Covalent ligands: Covalent ligands are molecules or ions that bond to a central metal atom in a coordination complex through the sharing of electrons, forming a covalent bond. These ligands can significantly influence the properties of the metal complex, including its stability, reactivity, and geometry. The nature of the bonding between the covalent ligand and the metal center is crucial for understanding the overall behavior and characteristics of coordination compounds.
Crystal Field Splitting: Crystal field splitting refers to the energy difference that occurs when transition metal ions are surrounded by ligands in a coordination complex, causing the degenerate d-orbitals to split into different energy levels. This phenomenon is crucial for understanding how ligands influence the electronic structure of transition metals, which in turn affects their chemical properties and reactivity. The extent of this splitting is influenced by the type of ligands and their arrangement around the metal ion, which is often described using coordination numbers and determines the color and magnetic properties of the complexes.
EDTA: EDTA (ethylenediaminetetraacetic acid) is a versatile chelating agent that forms stable complexes with metal ions, significantly impacting various fields such as biochemistry and analytical chemistry. Its ability to bind to metal ions makes it essential in understanding ligand behavior, coordination numbers, stability constants, and the chelate effect. Additionally, EDTA plays a critical role in biological systems by regulating metal ion availability and function.
Ethylenediamine: Ethylenediamine is a bidentate ligand with the chemical formula C2H8N2, consisting of two amine groups (-NH2) connected by an ethylene bridge. This structure allows ethylenediamine to form chelate complexes with metal ions, where it can bind through both nitrogen atoms. The ability of ethylenediamine to coordinate with metals plays a crucial role in the nomenclature of coordination compounds and their classification based on coordination numbers.
Frederick E. E. van der Waals: Frederick E. E. van der Waals was a Dutch physicist known for his pioneering work in the field of molecular physics and intermolecular forces. His contributions significantly enhanced the understanding of gas behavior and led to the formulation of the van der Waals equation, which describes how real gases deviate from ideal behavior, particularly in relation to coordination numbers and ligand interactions.
H2O: H2O, commonly known as water, is a simple chemical compound composed of two hydrogen atoms covalently bonded to one oxygen atom. This molecule is essential for life and plays a critical role in various chemical and biological processes, particularly in the context of ligands and coordination chemistry, where it can act as both a solvent and a ligand in coordination complexes.
Hexadentate: Hexadentate refers to a type of ligand that has six donor atoms capable of coordinating to a central metal ion in a complex. These ligands can form stable chelate complexes due to their ability to wrap around the metal, often leading to enhanced stability and unique properties in coordination chemistry. Understanding hexadentate ligands is crucial for grasping concepts related to coordination numbers and ligand behavior in metal complexes.
Kinetic Stability: Kinetic stability refers to the tendency of a chemical species, such as a coordination compound or an organometallic compound, to resist change or decomposition over time due to the energy barrier associated with reactions. It indicates how long a compound can exist without undergoing a reaction, which is influenced by factors such as bond strength and the nature of ligands. Understanding kinetic stability helps explain the reactivity and behavior of coordination and organometallic compounds in various chemical contexts.
L. Pauling: Linus Pauling was an influential American chemist, biochemist, and peace activist known for his work in the fields of quantum chemistry and molecular biology. He introduced the concept of hybridization and significantly advanced the understanding of chemical bonding and coordination chemistry, which are crucial for understanding ligands and coordination numbers in complex formation.
Ligand Field Stabilization Energy (LFSE): Ligand Field Stabilization Energy (LFSE) is the energy difference that arises from the interaction between a central metal ion and its surrounding ligands, specifically reflecting the stabilization provided by the arrangement of d-orbitals in a coordination complex. This concept is crucial for understanding how different ligands influence the electronic structure and stability of metal complexes, impacting their chemical behavior. The LFSE can be quantified based on the geometry of the complex and the nature of the ligands involved, revealing the relationship between coordination numbers, ligand types, and stability of transition metal complexes.
Monodentate: Monodentate refers to a type of ligand that can bind to a central metal atom or ion at only one site. This single attachment point means that monodentate ligands can form coordinate bonds with metal centers, playing a critical role in the formation of coordination compounds. Understanding monodentate ligands is essential for exploring how they influence the overall geometry and reactivity of coordination complexes.
Neutral Ligands: Neutral ligands are molecules or ions that have no overall charge and are capable of forming coordinate covalent bonds with a central metal atom in a coordination complex. These ligands can vary widely in their structure and can coordinate to the metal center through lone pairs of electrons, contributing to the stability and properties of the resulting complex.
NH3: NH3, commonly known as ammonia, is a colorless gas with a distinct pungent odor and is composed of one nitrogen atom covalently bonded to three hydrogen atoms. It serves as a crucial ligand in coordination chemistry, often acting as a donor to transition metals and influencing the coordination number of metal complexes. Ammonia's basicity and ability to form coordinate bonds allow it to stabilize various metal ions in solution.
Octahedral: Octahedral refers to a specific geometric arrangement in which a central atom is surrounded by six ligands positioned at the corners of an octahedron. This spatial arrangement is significant in understanding the structure and bonding of coordination compounds, influencing their physical and chemical properties.
Polydentate: Polydentate refers to a type of ligand that has multiple donor atoms capable of coordinating to a central metal atom or ion. These ligands can attach at several points, creating more stable complexes due to the chelation effect, where the formation of a ring structure enhances stability. The number of donor atoms involved in bonding determines the denticity of the ligand, which can significantly influence the coordination number and geometry of the resulting metal complex.
Square Planar: Square planar refers to a molecular geometry where four ligands are arranged around a central metal atom in a flat square shape, with bond angles of 90 degrees. This configuration often arises in coordination compounds, particularly when the central metal has a coordination number of four and is typically associated with d8 transition metal complexes.
Tetrahedral: Tetrahedral refers to a molecular geometry in which a central atom is surrounded by four other atoms, forming a shape like a tetrahedron. This geometry is significant in understanding the arrangement of ligands around a central metal ion in coordination compounds, influencing their chemical properties and behavior.
Thermodynamic stability: Thermodynamic stability refers to the stability of a system in terms of its energy state, where a lower energy state indicates greater stability. In the context of chemical compounds, particularly coordination compounds and organometallics, thermodynamic stability is essential as it influences reaction pathways, ligand binding, and overall reactivity. Compounds that are thermodynamically stable tend to resist change and do not easily undergo transformations under standard conditions.
Tridentate: Tridentate refers to a type of ligand that can form three coordinate bonds with a central metal atom in a coordination complex. This capability allows tridentate ligands to create stable complexes, as they can effectively wrap around the metal center, enhancing both stability and geometry. Understanding tridentate ligands is essential for grasping the broader concepts of coordination chemistry, as they play a significant role in determining the properties and reactivity of metal complexes.
π-backbonding: π-backbonding is a type of chemical bonding where electron density from a filled d-orbital of a metal center is donated back to an empty π* orbital of a ligand. This interaction enhances the stability of the metal-ligand complex and influences properties such as bond strength and geometry, linking it closely with the behavior of ligands and coordination numbers as well as the 18-electron rule.
σ-donation: σ-donation refers to the process in which a ligand donates a pair of electrons to a metal center, forming a coordinate bond. This interaction is fundamental to the formation of coordination complexes and plays a crucial role in determining the electronic properties and stability of these complexes. Understanding σ-donation helps explain how ligands interact with metal ions and the resulting influence on coordination numbers and geometry.