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Ionic Crystal Structures

Ionic Crystal Structures

Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
Most monatomic ions behave as charged spheres, and their attraction for ions of opposite charge is the same in every direction. Consequently, stable structures for ionic compounds result (1) when ions of one charge are surrounded by as many ions as possible of the opposite...

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...

Coordination Number and Geometry

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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.

Crystal Field Theory - Octahedral Complexes

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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...

Crystal Field Theory - Tetrahedral and Square Planar Complexes

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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,...

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...

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Sulfate Separation by Selective Crystallization with a Bis-iminoguanidinium Ligand

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Published on: September 8, 2016

Sulfate Local Coordination Environment in Schwertmannite.

Xiaoming Wang^1,2, Chunhao Gu¹, Xionghan Feng²

¹Department of Ecosystem Science and Management, University of Wyoming , Laramie, Wyoming 82071, United States.

Environmental Science & Technology

|July 29, 2015

Summary

Schwertmannite

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Area of Science:

Environmental geochemistry
Mineralogy
Spectroscopy

Background:

Schwertmannite is a key ferric oxyhydroxy-sulfate mineral in acidic, sulfate-rich environments.
Understanding its sulfate coordination is crucial for environmental applications.
Previous studies lacked clarity on the sulfate's local atomic environment.

Purpose of the Study:

To elucidate the sulfate coordination environment in schwertmannite.
To investigate the influence of pH and drying on sulfate complexation.
To refine models of schwertmannite structure and formation.

Main Methods:

Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy.
Extended X-ray absorption fine structure (EXAFS) spectroscopy.
Infrared spectroscopy on wet and air-dried samples.

Main Results:

Sulfate exists as both inner- and outer-sphere complexes.
EXAFS confirmed bidentate-binuclear sulfate inner-sphere complexes (S-Fe distance: 3.22-3.26 Å).
Inner-sphere complexation decreased with increasing pH; dried samples showed higher inner-sphere complexation than wet samples.

Conclusions:

Sulfate inner-sphere complexation occurs on defects within the Fe octahedra chains of the schwertmannite structure.
Ligand exchange with sulfate involves singly-Fe coordinated hydroxyl groups.
Drying reveals readily exchangeable water molecules within the mineral's tunnels.