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Editing Inorganic Chemistry
- Dec 24, 2016 -

Theoretical inorganic chemistry
An alternative perspective on the area of inorganic chemistry begins with the [[Bohr model]] of the atom and, using the tools and models of [[theoretical chemistry]] and [[computational chemistry]], expands into bonding in simple and then more complex molecules.  Precise quantum mechanical descriptions for multielectron species, the province of inorganic chemistry, is difficult.  This challenge has spawned many semi-quantitative or semi-empirical approaches including [[molecular orbital theory]] and [[ligand field theory]], In parallel with these theoretical descriptions, approximate methodologies are employed, including [[density functional theory]].

Exceptions to theories, qualitative and quantitative, are extremely important in the development of the field.  For example, [[Copper(II) acetate|Cu<sup>II</sup><sub>2</sub>(OAc)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]] is almost diamagnetic below room temperature whereas Crystal Field Theory predicts that the molecule would have two unpaired electrons.  The disagreement between qualitative theory (paramagnetic) and observation (diamagnetic) led to the development of models for "magnetic coupling."  These improved models led to the development of new magnetic materials and new technologies.
Qualitative theories
[[Image:Ferricyanide-3D.png|thumb|right|200px|[[Crystal field theory]] explains why [[Ferricyanide|[Fe<sup>III</sup>(CN)<sub>6</sub>]<sup>3−</sup>]] has only one unpaired electron]]
Inorganic chemistry has greatly benefited from qualitative theories.  Such theories are easier to learn as they require little background in quantum theory.  Within main group compounds, [[VSEPR]] theory powerfully predicts, or at least rationalizes, the [[Chemical structure|structures]] of main group compounds, such as an explanation for why NH<sub>3</sub> is pyramidal whereas ClF<sub>3</sub> is T-shaped.  For the transition metals, [[crystal field theory]] allows one to understand the magnetism of many simple complexes, such as why [[Ferricyanide|[Fe<sup>III</sup>(CN)<sub>6</sub>]<sup>3−</sup>]] has only one unpaired electron, whereas [Fe<sup>III</sup>(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> has five.  A particularly  powerful qualitative approach to assessing the structure and reactivity begins with classifying molecules according to [[electron counting]], focusing on the numbers of [[valence electron]]s, usually at the central atom in a molecule.

Molecular symmetry group theory
[[Image:Nitrogen-dioxide-3D-vdW.png|thumb|right|200px|[[Nitrogen dioxide]], NO<sub>2</sub>, exhibits [[Symmetry group|''C<sub>2v</sub>'' symmetry]]]]
A central construct in inorganic chemistry is the theory of [[molecular symmetry]].<ref>{{cite book | author = Cotton, F. A. | title = Chemical Applications of Group Theory | publisher = John Wiley & Sons | location = New York | year = 1990 | edition = 3rd | isbn = 978-0471510949}}</ref>  Mathematical [[group theory]] provides the language to describe the shapes of molecules according to their [[Point groups in three dimensions|point group symmetry]].  Group theory also enables factoring and simplification of theoretical calculations.

Spectroscopic features are analyzed and described with respect to the symmetry properties of the, ''inter alia'', vibrational or electronic states. Knowledge of the symmetry properties of the ground and excited states allows one to predict the numbers and intensities of absorptions in vibrational and electronic spectra.  A classic application of group theory is the prediction of the number of C-O vibrations in substituted metal carbonyl complexes.  The most common applications of symmetry to spectroscopy involve vibrational and electronic spectra.

As an instructional tool, group theory highlights commonalities and differences in the bonding of otherwise disparate species, such as [[Tungsten(VI) fluoride|WF<sub>6</sub>]] and [[Molybdenum hexacarbonyl|Mo(CO)<sub>6</sub>]] or [[Carbon dioxide|CO<sub>2</sub>]] and [[nitrogen dioxide|NO<sub>2</sub>]].

 


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