Coordination Number Six

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Coordination Number Six

Octahedral is a very important geometry. It is the starting point for the shapes of most transition metal complexes.

1. Regular Octahedron
all distances are EQUIVALENT

2. Distorted Octahedron
(axial distortion) a ≠ b

3. Distorted Octahedron
(Rhombic distortion)

4. Distorted Octahedron
(trigonal distortion)


The last example, trigonal prismatic is not as stable as the regular octahedron, because L-L distances are not maximized in the trigonally distorted geometry. This is a rare geometry for ML6 complexes

Higher Coordination numbers Seven

Pentagonal bipyramid

Octahedron + an extra ligand

Trigonal prism + an extra ligand

Eight 1. Cube →

2. square antiprism twist one face rotate the corners of once face until it is 45˚ out of phase

1. Cube →

2. dodecahedron pull corners away from each other

(Grab opposite ends & pull up & down)

Nomenclature of Coordination Complexes
Follow IUPAC rules very specific
Examples 1. mer–trichlorotris(triphenylphosphine)rhodium(I)
2. potassium tetrabromocuprate (II)
3. trans–dichlorotetraaquachromium (II) chloride
key aspects of these names
1. prefix cis, trans, mer, fac
1 define
2. Which ligands come first in a mixed – ligand complex
3. Which type of prefix bi, bis, tri, tris, tetra, tetrakis etc.,

4. oxidation state of the metal 5. Special issues such as
• Optical isomers • Bridging vs. non-bridging • Endings for ligands
“ite” “ide” “ate” “o” • Special names for ligands • Charge on the compound
Book gives 11 rules Distill them down
1 No spaces in name except if it is a salt, put a space between cation and anion, and the cation is first.

Name is organized according to:

Anionic Ligands/Neutral Ligands/CationicLigands/ Metal Name/Oxidation state of Metal

Within these categories we must establish rules of:

(a) alphabetical order (b) prefixes and suffixes for ligands (c) suffix or not for the metal
2 Prefixes and Suffixes

Prefixes 2 di 3 Tri 4 Tetra 4 Penta 6 Hexa

bis tris tetrakis pentakis hexakis

use di, tri etc., unless the ligand name already has one of these as part of it’s name
then use bis, tris etc.


Suffixes (a) anionic ligands

end in “o”

ate → ato ide → ido ite → ito

acetate nitride sulfite

CH3CO2- acetato N3- nitrido SO32 sulfito

→ acetato → nitrido → sulfito

(b) neutral ligands are the same as molecule name with a few exceptions:

NH3 → ammonia

M-NH3 ammine

H2O → water

M-OH2 aqua

NO → nitrogen monoxide

M-NO nitrosyl


CO → carbon monoxide

M-CO carbonyl

(c) organic groups (keep same name)

CH3 Methyl

M-CH3 methyl



(d) metal suffix (i) if compound is neutral or cationic [RuL6]3+ no suffix ruthenium → ruthenium
(ii) if compound is anionic, then metal name is
changed to ate ending [RuL6]3- ruthenium → ruthenate

3 Within each category of ligand, alphabetize the ligands within each group if more than one type is present (anions first, neutral second, cationic third) (NOTE: don’t count bi, tri etc., in alphabetizing)

4 Metal oxidation state is written in parentheses in Roman numerals at the end
5 Special Characters and abbreviations (a) Geometrical isomers
cis, trans arrangements for two types of X ligands
fac, mer arrangements for three types of X ligands

(b) optical isomers use symbols ∆ and Λ

(c) bridging ligands - use prefix µ “mu” - if two of the same kind di-µ
M-NO2- nitro M-ONO- nitrite

µ-Cl di-µ-Cl

(a) H2O (hydrate) Water can be in a formula like this MCI6·nH2O (Number of water molecules of hydration)
• H2O monohydrate • 2H2O dihydrate etc.

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Coordination Number Six