Coordination geometry

Coordination geometry refers to the spatial arrangement of ligands that are attached to a central atom or ion in a coordination complex. This geometry is a key factor in determining the properties and reactivity of coordination compounds. The central atom, often a metal ion, is surrounded by a number of ligands, which can be molecules like water or ammonia, or ions such as chloride or hydroxide. The specific arrangement of these ligands around the central atom is what defines the coordination geometry.

Types of Coordination Geometries

Several common coordination geometries are observed in coordination compounds, including linear, trigonal planar, tetrahedral, square planar, trigonal bipyramidal, octahedral, and square pyramidal among others. The geometry primarily depends on the number of ligands and the electronic configuration of the central atom or ion.

Linear

In a linear geometry, two ligands are positioned directly opposite each other with respect to the central atom. This geometry is common for d^10 metal ions such as Ag^(I) and Cu^(I).

Trigonal Planar

Trigonal planar geometry involves three ligands arranged around the central atom in a plane, at angles of 120° to each other. This geometry is often seen in compounds of metals with d^8 configuration.

Tetrahedral

A tetrahedral geometry has four ligands positioned at the corners of a tetrahedron with respect to the central atom. This is a common geometry for many coordination compounds, including those of Zn^(II), Cu^(II), and Ni^(II).

Square Planar

Square planar geometry is characterized by four ligands arranged at the corners of a square plane around the central atom. This geometry is typical for d^8 metal ions, including Pt^(II) and Ni^(II) complexes.

Trigonal Bipyramidal

In trigonal bipyramidal geometry, five ligands are arranged with three in a plane and two positioned above and below this plane. This geometry is common for PF_5 and other penta-coordinated compounds.

Octahedral

Octahedral geometry involves six ligands arranged at the corners of an octahedron around the central atom. This is one of the most common geometries in coordination chemistry, especially for transition metal complexes with a d^6 configuration.

Square Pyramidal

Square pyramidal geometry has five ligands, with four at the corners of a square and one above the square plane. This geometry is less common but can be found in certain transition metal complexes.

Factors Influencing Coordination Geometry

Several factors influence the coordination geometry of a complex, including the size and electronic configuration of the central atom or ion, the size and charge of the ligands, and the electronic and steric requirements of both the ligands and the central atom or ion.

Importance of Coordination Geometry

The coordination geometry of a complex can significantly affect its physical and chemical properties, including color, magnetism, and reactivity. For example, the color of a coordination compound can change with different geometries due to changes in the ligand field, which affects the d-orbital splitting of the central metal ion. Similarly, the geometry can influence the reactivity of the complex in chemical reactions, including catalysis.