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COMPLEX COMPOUNDS AND LIGANDS: TYPES OF INTERACTIONS
Saydullayeva Guliraykhon
https://doi.org/10.5281/zenodo.16931058
Annotation
This article provides a comprehensive overview of complex compounds, focusing on the roles
of central atoms, ligand types, and the interaction mechanisms leading to complex formation.
The discussion covers electrostatic and donor–acceptor interactions, the chelate effect with
polydentate ligands, and the structural diversity of mono- and polynuclear complexes. Practical
applications in catalysis, medicine, and biological systems are highlighted, making the article
relevant for both academic and applied chemistry studies.
Keywords
Complex compounds; ligands; donor–acceptor interaction; chelates; coordination chemistry;
transition metals; polynuclear complexes; bioinorganic chemistry; catalysis; stability constants
Complex compounds are molecular entities capable of independent existence, consisting
of a central atom (often referred to as the complexing agent or coordination center) surrounded
by one or more ligands. The central atom, typically a transition metal ion, possesses empty
orbitals that enable coordination with negatively charged species or neutral molecules
containing donor atoms. These compounds are fundamental in fields such as catalysis,
bioinorganic chemistry, and materials science [1].
Central Atom
The central atom is usually a metal ion from the d-block elements, although elements such
as Si, P, or As can also serve this role in specific systems. The effective charge of the central atom
commonly ranges from +0.3 to +2 units of electronic charge, depending on its oxidation state
and the surrounding environment [2].
Ligands
Ligands are ions or molecules that donate electron pairs to the central atom to form
coordinate covalent bonds. They can be classified based on their charge and structure:
Anionic ligands:
such as Cl⁻, CN⁻, OH⁻.
Neutral (molecular) ligands:
such as H₂O, NH₃, CO.
Chelating (cycle-forming) ligands:
such as ethylenediamine (en) or EDTA, which can
bind through multiple donor atoms.
Macrocyclic ligands:
including porphyrins and crown ethers, which form highly stable
complexes due to the preorganized cyclic structure [3].
The coordination number and geometry of complexes depend on the size, charge, and
electronic configuration of the central atom, as well as the steric and electronic properties of
the ligands.
Types of Interactions Leading to Complex Formation
Complex formation arises from two primary interaction mechanisms:
1.
Electrostatic interactions:
These involve purely ionic attractions between the central
ion and counter-ions, leading to the formation of ion-associate complexes. These are less
stable in polar solvents like water but are more significant in non-polar or organic
solvents [4].
2.
Donor–acceptor (coordinate bond) interactions:
In these cases, the ligand donates a
lone pair of electrons to an empty orbital of the central atom. If charge neutralization
occurs, the resulting bond is often described as covalent; otherwise, it remains a donor–
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acceptor interaction. These interactions are fundamental in forming stable complexes in
aqueous environments [5].
Polydentate Ligands and Chelate Complexes
Ligands containing multiple donor sites, known as polydentate ligands, can form chelate
complexes by simultaneously binding to the central atom through two or more coordination
sites. Common functional groups in such ligands include –OH, –SH, –NH₂, –COOH, and –SO₃H.
Chelate complexes are known for their enhanced stability, often referred to as the "chelate
effect" [6]. An example is the nickel dithiooximate complex, which demonstrates both covalent
and donor–acceptor interactions within the same chelating framework.
Poly- and Mononuclear Complexes
Complexes can be categorized based on the number of coordination centers:
Mononuclear complexes
have a single central atom.
Polynuclear complexes
involve two or more central atoms connected via bridging
ligands or direct metal-metal bonds, leading to unique structural and electronic
properties that are critical in catalysis and materials applications [7].
Applications and Importance
Complex compounds are critical in various industries and biological systems. They are
utilized in homogeneous and heterogeneous catalysis, metal extraction and separation, and
therapeutic agents (e.g., cisplatin as an anticancer drug). Biologically, metalloproteins such as
hemoglobin and cytochromes rely on complex coordination chemistry for their function [8].
References:
Используемая литература:
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3.
Crabtree, R. H. (2009).
The Organometallic Chemistry of the Transition Metals
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4.
Housecroft, C. E., & Sharpe, A. G. (2012).
Inorganic Chemistry
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5.
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Inorganic Chemistry: Principles of Structure
and Reactivity
. HarperCollins.
6.
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Metal Complexes in Aqueous Solutions
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Mingos, D. M. P. (2016).
Essential Trends in Inorganic Chemistry
. Oxford University Press.
8.
Lippard, S. J., & Berg, J. M. (1994).
Principles of Bioinorganic Chemistry
. University Science
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