The Organometallic Reader

Dedicated to the teaching and learning of modern organometallic chemistry.

Glossary

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A reader suggested that I create a growing list of common organometallic chemistry terms. This page contains that list. Each definition includes links to some relevant blog posts where the term is defined or applied.

The 18-Electron Rule: an “empirical guideline” reflecting the observation that organometallic complexes containing 18 total electrons or fewer tend to be more stable than those that possess more than 18 total electrons. (Simplifying the Organometallic Complex 3)

Actor: a ligand that engages in chemical change when bound to a metal center. Organic substrates in organometallic reactions include at least one actor ligand.

Backbonding: the donation of electron density from a metal center to a ligand; alternatively, an orbital interaction describing such donation. Ligands containing low-energy π* or σ* orbitals—including phosphines and CO—are susceptible to backbonding from metals possessing two or more d electrons. (Ligand Field Theory and FMO Theory)

Bridging: a ligand that possesses two bonds to two separate metal centers. The bonds need not involve the same ligand atom or be of the same type (i.e., dative or covalent). (Epic Ligand Survey: Carbon Monoxide, Epic Ligand Survey: Metal Hydrides)

Chelation: the association of two or more electronically independent points on a ligand with a single metal center. Chelating ligands bind metals more tightly than monodentate ligands; this phenomenon is known as the “chelate effect.” (Epic Ligand Survey: Phosphines)

Cone angle: the angle of a cone whose points include the metal center (real or hypothetical) and the widest hydrogen atoms of the ligand when it is as tied back as possible without bending bonds. Wider cone angles are associated with sterically hindered phosphine ligands. (Epic Ligand Survey: Phosphines)

Cross-coupling reaction: a nucleophilic substitution reaction accomplished by an organometallic catalyst, resulting in the bonding of two organic fragments (one nucleophilic and the other electrophilic). Characterized by three general steps: oxidative addition, transmetallation, and reductive elimination. (Oxidative Addition: General IdeasReductive Elimination: General Ideas)

Deconstruction: the hypothetical process of separating ligands from the metal center to reveal the oxidation state, number of d electrons, and total electron count of an organometallic complex. Electron movement during deconstruction is governed by formalized rules. (Simplifying the Organometallic Complex 1)

Isolobal analogies: correspondence of the frontier orbitals of two fragments or molecules, typically one organic and one organometallic. Isolobal analogies help us connect the behaviors of familiar organic species to those of organometallic complexes. (Isolobal Analogies)

L-type: a ligand donating a lone pair of electrons in a dative fashion. Formally, L-type ligands possess a neutral charge upon deconstruction. Examples include phosphines, carbon monoxide, and π-system ligands. The term “L-type” may also be used to describe a specific dative binding point within a polydentate ligand. (Simplifying the Organometallic Complex 1)

Ligand: an organic molecule bound to a metal center. Ligands are typically envisioned as Lewis bases, and metals as Lewis acids, in metal-ligand bonds. (Simplifying the Organometallic Complex 1)

Ligand substitution: the replacement of one ligand for another at a metal center, with no change in oxidation state at the metal. (Ligand Substitution: General Ideas)

Microscropic reversibility: the coincidence of reaction coordinate and mechanism for two reactions that are the inverse of one another. The principle of microscopic reversibility states that the lowest energy mechanism that converts A to B is also the lowest energy mechanism that converts B to A, and the two mechanisms are called microscopic reverses of one another. (Reductive Elimination: General Ideas)

Migratory insertion: an elementary step (characterized by a single mechanism) involving the concerted insertion of one or two datively bound ligand atoms into an M–X bond (where X is a covalent ligand). The oxidation state of the metal does not change, but its coordination number decreases by one unit and total electron count decreases by two units.

Number of d electrons: the number of valence electrons possessed by the metal center of a complex following deconstruction. Number of d electrons has implications for the geometry and reactivity of complexes. (Simplifying the Organometallic Complex 2)

Open coordination site: a location that may be occupied by an incoming two-electron ligand. To bear an open coordination site, a complex must have a low energy, empty valence orbital—stated another way, the complex must bear 16 or fewer total electrons and have coordination number less than 6. Complexes bearing an open coordination site are said to be coordinatively unsaturated. (What is an Open Coordination Site?)

Oxidation state: the formal charge on the metal center after deconstruction, including any overall charges of the complex. Oxidation state is equal to the difference between the metal’s group number and its number of d electrons. (Simplifying the Organometallic Complex 2)

Oxidative addition: an organometallic reaction (characterized by multiple mechanisms) involving the formal insertion of a metal center into a covalent bond. The metal is formally oxidized by two units, its coordination number increases by two, and its total electron count increases by two. Oxidative addition is the microscopic reverse of reductive elimination. (Oxidative Addition: General Ideas, Oxidative Addition of Non-polar ReagentsOxidative Addition of Polar Reagents)

Reductive elimination: an organometallic reaction (characterized by multiple mechanisms) involving the formal deinsertion of a metal center from a covalent bond. The metal is formally reduced by two units, its coordination number decreases by two, and its total electron count decreases by two. Reductive elimination is the microscopic reverse of oxidative addition. (Reductive Elimination: General Ideas)

Spectator: a ligand that does not engage in chemical change when bound to a metal center. Spectator ligands are used to modulate the electronic and steric properties of the metal center to achieve a desired chemical outcome.

Total electron count: the sum of the number of d electrons possessed by the metal center and the number of electrons donated by ligands to the metal. (Simplifying the Organometallic Complex 3)

Transition metal: metals found in the d-block of the periodic table, spanning the “transition” from the electropositive alkali earth metals to the main-group p-block elements. (Simplifying the Organometallic Complex 1, Periodic Trends)

X-type: a ligand donating a pair of electrons in a covalent fashion. Formally, X-type ligands possess a negative charge upon deconstruction. Examples include hydrides, alkyl ligands, halides, and alkoxides. (Simplifying the Organometallic Complex 1)

Written by Michael Evans

February 25, 2012 at 11:40 am

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