COMPLEX COMPOUNDS AND LIGANDS: TYPES OF INTERACTIONS

Аннотация

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– 67 YOSH OLIMLAR ILMIY-AMALIY KONFERENSIYASI in-academy.uz/index.php/yo 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].