Coordinate Covalent Bonds and Complexes - The WikiPremed MCAT Course

In a coordinate covalent bond, a pair of electrons from one atom is shared by two atoms. The distinction is one of accounting. It is not really meaningful in a physical sense, and at high levels in chemistry, the nomenclature is not much utilized. Reactions between Lewis bases and Lewis acids result in coordinate covalent bonds. When a bond is forming, if one of the atoms brought both electrons to the bond, then people will not disagree when you call it a coordinate covalent bond, especially within the context of coordination complexes. Complexes are formed when the Lewis acid is a metal. There is an especially large and important subgroup when the metal is a transition metal.

Coordination Chemistry

Proteins

Bioenergetics and Cellular Respiration

Blood

The Digestive System and Nutrition

Coordination chemistry is of paramount importance in biochemistry because many biologically important substances possess organic functional groups that form coordinate covalent bonds with transition metal ions. One prominent example is the oxygen transport protein in the blood, hemoglobin, or myoglobin in the muscles, in which iron is bound to the porphyrin ring, the iron and porphyrin together constituting heme. In the oxygen binding site of hemoglobin and myoglobin, four of six of iron's coordination positions are occupied by bonds to the porphyrin ring, a fifth is involved in a bond to a histidine side chain of the protein leaving the sixth available for the formation of a coordinate covalent bond with oxygen. Heme is also present in the electron carrier cyctochrome c, as well as in cytochrome oxidase, which has two heme groups as well as two copper ions. Carbonic anhydrase and carboxypeptidase A contain coordinated zinc, where it performs a catalytic function within the active site. In the enzyme, ATCase, zinc plays a structural roll. Further examples of coordination complexes in biology include vitamin B-12, which is a large complex of cobalt. Additionally, iron sulfur clusters within proteins, in which groups of iron atoms are coordinated with inorganic sulfides are very important in electron transfer reactions in the citric acid cycle, oxidative phosphorylation, and photosynthesis. Coordination chemistry is a major weakness among premedical students, so you do not need much more than a basic orientation to get beyond your competition.

Must know for the MCAT
Complex	A complex is a molecule or ensemble formed by the combination of ligands and metal ions.
Coordinate covalent bond	A coordinate covalent bond is a type of covalent bonding between two atoms in which both electrons shared in the bond come from the same atom.
Lewis base	A Lewis base is any molecule or ion that can form a new coordinate covalent bond, by donating a pair of electrons.
Ligand	A ligand is an atom, ion, or molecule donating one or more of its electrons through a coordinate covalent bond to one or more central atoms or ions
Coordination number	Coordination number is defined as the total number of neighbors of a central atom in a chemical compound.
Inorganic chemistry	The field of inorganic chemistry covers all chemical compounds except the myriad organic compounds, which are the subjects of organic chemistry.
Organometallic chemistry	Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal.
Chelation	Chelation is the binding or complexation of a bi- or multidentate ligand.
18-Electron rule	Arising from the size of the valance shell of a transition metal, the 18-electron rule is a rule of thumb used primarily in transition metal chemistry for characterizing and predicting the stability of metal complexes.
Bridging ligand	A bridging ligand is an atom or a polyatomic entity that connects two or more metal centres in a complex.
Should know for the MCAT
Ligand field theory	Applying molecular orbital theory to transition metal complexes, ligand field theory describes the bonding in coordination complexes.
Crystal field theory	Though not describing bonding, crystal field theory represents a reasonanbly successful model for describing the electronic structure of transition metal compounds which are coordination complexes.
Trigonal bipyramid molecular geometry	A trigonal bipyramid is a molecular geometry with one atom at the center and 5 more at the corners of a triangular dipyramid.
Inclusion compound	In host-guest chemistry an inclusion compound is a complex in which one chemical compound the host forms a cavity which molecules of a second compound the guest are located.
Clathrate compound	A clathrate or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule.
Sandwich compound	A sandwich compound in organometallic chemistry is any chemical compound containing a metal atom oriented between two arene units.
May appear in context in MCAT passages - advanced terminology
Octahedral molecular geometry	Octahedral molecular geometry describes a molecular geometry in which 6 ligands are symmetrically arranged around a central atom in an arrangement having the form of an octahedron.
Coordination polymer	Coordination polymer is the term given to a metal coordination compound where a ligands bridge between metal centres to create an infinite array of metal centres.
Transition metal carbene complex	A transition metal carbene complex in organometallic chemistry is a compound bearing a formal carbon-metal bond.
Spinel	The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit.

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Must know for the MCAT

Should know for the MCAT

May appear in context in MCAT passages - advanced terminology