# Metal-Coordinated Polymer-Inorganic Hybrids: Synthesis, Properties, and Application
## 1 Introduction  
Organic-inorganic hybrid materials (OIHs) have gained increasing attention due to their ability to combine the diverse properties of organic materials with the unique characteristics of inorganic components. Over the past two decades, these materials have been the focus of extensive research exploring a wide range of applications, including coatings, catalysis, optics, optoelectronics, and biomedical fields. Polymer-based hybrids are particularly desired due to the excellent processability of polymers [1, 2, 3]. The properties of OIHs are strongly influenced by the types and nature of the organic and inorganic precursors used in their synthesis. Inorganic precursors, such as metal salts or metal oxides, interact with organic polymers through functional groups like carboxylates, sulfur groups, and amines [4]. These inorganic components not only provide structural reinforcement but also introduce specialized functionalities, such as catalytic activity, luminescence, or conductivity, while enhancing the mechanical and thermal stability of the material. For instance, Zr(IV) and Ti(IV) ions create highly stable frameworks through strong coordination with organic linkers, making them well suited for harsh environments or high-temperature applications [5]. Additionally, the interaction between inorganicnanostructures and polymer matrices can enhance ionic conductivity by forming transport channels and amorphous regions that facilitate ion dissociation [6; 7]. The coordination geometry and oxidation state of the metal ions also play a critical role in determining the structure and properties of the hybrid material. For example, Cu(II) ions are known to form square planar complexes that contribute to redox-active frameworks, while Fe(II) and Fe(III) ions provide tunable redox activity, leading to materials with unique magnetic and electronic properties [8]. Furthermore, the ionic radius of the metal ion affects the porosity of the hybrid material. Lanthanide-based hybrids, characterized by larger ionic radii, exhibit open architectures and greater spatial flexibility, making them more porous than those formed with transition metals like Zn(II) or Cu(II) [9; 10]. Inorganic-organic hybrid materials can be categorized based on the dominant phase, either as organic-inorganic or inorganic-organic materials. However, this classification is not universally accepted due to the large number of intermediate cases. Unlike general nanocomposites, hybrid materials are distinguishable by the presence of chemical reactions between the organic and inorganic phases rather than being simple physical mixtures. Consequently, hybrid materials are typically classified according to the type of interaction occurring between the organic and inorganic components [3; 11], namely Class I and Class II hybrids. Class I hybrids are characterized by weak interactions, such as Van der Waals forces, electrostatic interactions, or hydrogen bonds between organic and inorganic components. These hybrids often have a core-shell structure, where inorganic nanostructures are embedded in an organic matrix, making them suitable for applications like biosensors [12], photocatalysis [13], and optoelectronics [14]. However, to prevent phase separation, compatible functional groups on both the organic and inorganic components are essential [15]. In contrast, Class II hybrids feature strong chemical bonds, including covalent, ionic-covalent, or coordination bonds, between the organic and inorganic phases. These hybrids can include metal-carbon bonds and alkoxy groups (R-OM) that undergo hydrolysis-condensation reactions, resulting in enhanced stability [3]. Early examples of Class II hybrids include peptide-based macromolecules such as metallo-proteins [16] and synthetic water-soluble polymers, which are used for applications such as batteries [17], filtration systems [18], and the development of hydrophilic and hydrophobic materials [3].