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Coordination Number and Geometry

Coordination Number and Geometry

For transition metal complexes, the coordination number determines the geometry around the central metal ion. Table 1 compares coordination numbers to molecular geometry. The most common structures of the complexes in coordination compounds are octahedral, tetrahedral, and square planar.

Valence Bond Theory

Valence Bond Theory

Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...

Colors and Magnetism

Colors and Magnetism

Color in Coordination Complexes
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human...

Crystal Field Theory - Tetrahedral and Square Planar Complexes

Crystal Field Theory - Tetrahedral and Square Planar Complexes

Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...

Crystal Field Theory - Octahedral Complexes

Crystal Field Theory - Octahedral Complexes

Crystal Field Theory
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...

Metal-Ligand Bonds

Metal-Ligand Bonds

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
In these complexes, transition metals form coordinate covalent bonds, a kind of Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in...

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Related Experiment Video

Updated: Jan 6, 2026

Thermochemical Studies of NiII and ZnII Ternary Complexes Using Ion Mobility-Mass Spectrometry

Thermochemical Studies of NiII and ZnII Ternary Complexes Using Ion Mobility-Mass Spectrometry

Published on: June 8, 2022

A hexagonal planar transition-metal complex.

Martí Garçon¹, Clare Bakewell¹, George A Sackman^2,3

¹Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, London, UK.

|October 11, 2019

Summary

This summary is machine-generated.

Researchers report the first hexagonal planar transition-metal complex. This novel structure, featuring palladium with hydride and magnesium ligands, expands coordination chemistry possibilities beyond traditional geometries.

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Area of Science:

Coordination Chemistry
Inorganic Chemistry
Materials Science

Background:

Transition-metal complexes are vital in catalysis, synthesis, and bioinorganic chemistry.
Established understanding links complex shape to properties via molecular-orbital theory.
Known six-coordinate geometries (octahedral, trigonal prismatic) are limited, with hexagonal planar rare and confined to specific phases or clusters.

Purpose of the Study:

To isolate and structurally characterize a simple coordination complex with a hexagonal planar geometry.
To challenge existing limitations in transition-metal complex geometries.
To explore new design principles for transition-metal complexes.

Main Methods:

Synthesis and isolation of a novel transition-metal complex.
Structural characterization using X-ray diffraction or similar techniques.
Analysis of bonding and electronic structure.

Main Results:

Successfully synthesized and characterized a transition-metal complex with a hexagonal planar arrangement.
The complex features a central palladium atom coordinated to three hydride and three magnesium-based ligands.
This represents the first reported instance of a simple coordination complex exhibiting this geometry.

Conclusions:

The discovery of a hexagonal planar complex expands the known coordination geometries for transition metals.
This finding offers new avenues for designing transition-metal complexes with unique properties.
Potential implications for catalysis, materials science, and other chemical disciplines.