High Coordination Numbers
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Coordination numbers from 2 to 6 and the associated coordination complexes are very common, especially in transition metal chemistry. These complexes can take on a variety of geometries and each individual complex will have unique properties based on its substituents and their arrangement spacially. Though there is no set number of ligands, ions, or bound atoms required for a species to be considered "highly coordinated" versus general coordination, when the coordination number exceeds seven it is generally considered a high coordination number. High coordination numbers are of particular interest in solution phase and inorganic chemistry.
The coordination number of a specified atom in a is the "number of other atoms directly linked to that specified atom." Coordination itself is "the formation of a covalent , the two shared electrons of which have come from only one of the two parts of the linked by it, as in the reaction of a and a to form a ; alternatively, the bonding formed in this way." Coordination number is a measure of one of two things:
Lower Coordination Numbers
Following are a few examples of more commonly encountered coordination numbers
Coordination Number 2
Coordination numbers of two are more common in organic molecules, or in non transition-metal containing species and relatively rare in transition metal containing species. Coordination numbers of two result in a linear or collinear spacial arrangement.
Carbon Dioxide, Coordination Number 2 [Ag(NH3)2]+, Coordination Number 2
Coordination Number 6
Coordination number 6 is the most common coordination number, especially among transition metal containing species.
Generally, the octahedron is the prefered coordination geometry.
[Co(H2O)6)]3+, Coordination number 6
Higher coordination numbers; 7,8, 9, and above exist though they are less common.
Coordination Number 7
Seven coordinated atoms or molecules results in three main geometries:
Coordination Number 8
Eight coordinated atoms or molecules results in two main geometries:
Coordination Number 9
The only structure yet identified for coordination number 9 systems is the tricrapt trigonal prism.
Two examples of these structures are [ReH9]2- and [TcH9]2- with the representative geometry shown below.
[TcH9]2-, Coordination number 9
Higher Coordination Numbers
Species exist with 10, 11, and 12 coordination numbers.
Coordination number 12 has a structure that is involved in boron chemistry, the icosahedron.
Coordination number 15 is the highest reported coordination number currently, being described for [PbHe15]2+
International Union of Pure and Applied Chemistry. "Coordination Number". Compendium of Chemical Terminology Internet edition.
1. What is the coordination number of the nitrate ion NO3-?
2. What is the coordination number around Co in this complex? How many different ligands are around Cobalt?
3. What coordination number do you expect [Th(NO3)6]2- to have? (Draw out a rough structure, remember lone pairs are required for coordination. What is the likely geometry for this?
1. Coordination number 3. Pi Bonds do not contribute to coordination number.
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