In all their compounds, cations are surrounded by anions or neutral molecules.
The groups immediately surrounding a cation = LIGANDS
The branch of inorganic chemistry concerned with the combined behaviour of cations and
their ligands = COORDINATION CHEMISTRY
Coordination compound is described in terms of an essentially constant cationic central
species Mn+ around which a great variety of ligands L, L, L, etc. may be placed in an
esentially unlimited number of combinations. The overall charge on the resulting complex
[MLxLyLz] is determined by the charge on M and the sum of the charges on the ligands:
Square planar complexes with Pt(II) in the middle:
[Pt(NH3)4]2+, [Pt(NH3)3Cl]+, [Pt(NH3)2Cl2], [Pt(NH3)Cl3]-, [PtCl4]2-
Inner-Sphere Complex: Immediate vicinity of the metal atom or ion where the ligands are
attached directly to the metal
Primary Coordination Sphere: Region of directly attached ligands
Coordination Number: Number of ligands in the primary coordination sphere
Outer-Sphere Complex: A complex with ligands in the primary coordination sphere as well
as ligand in the next sphere which are held by weaker Van der Waals forces.
Factors determining the C.N. of a complex:
1. Size of the central atom or ion
2. Steric interactions between the ligands
3. Electrostatic interactions
Coordination Numbers and Geometries:
Relatively rare, occurring with +1 ions of Cu, Ag and Au and Hg2+.
Geometry is linear.
Relatively rare coordination number.
Most important geometries are planar and pyramidal.
Examples: planar HgI3- and pyramidal SnCl3- ions.
For compounds such as AlCl3, FeCl3, PtCl2PR3, etc. no 3-coordinate mononuclear species
occur. Instead, there are dinuclear species in which two ligands are shared in order to give
each cation a C.N. of 4.
R3 P Cl Cl
Cl Cl R3 P
Most important coordination number.
Gives two main geometries: tetrahedral and square planar.
Tetrahedral complexes are most common, being formed almost exclusively by nontransition
metal cations as well as by a number of transition metals in the right of the d-block.
Examples to tetrahedral complexes include:
Li(H2O)4+, BeF4+, BF4- AlCl4-, FeCl4-, CoBr42-, etc
Square planar complexes are particularly common for Cu2+, Ni2+, Pd2+, Pt2+, Au3+, Rh+, and
Ir+. The cations that form square planar complexes are those with 8 d-electrons. These 8
electrons form 4 electron pairs that fill all the d orbitals except dx2-y2. This orbital, together
with s and px and py orbitals form a set of hybrids, dsp2, directed toward the vertices of a
Less common than 4 or 6 but still very important.
There are 2 symmetrical geometrical arrangements: Trigonal bipyramid (tbp) and square
pyramid (sp). Although these two may appear quite different, they usually do not differ much
in energy. One can be converted into the other by rather small changes in bond angles. In
reality, the structure is an intermediate of the two. Even if a compound is close to either of the
structures, it would still be stereochemically nonrigid; the ligand do not remain fixed in their
places, interchanging places rapidly.
Nearly all cations form 6-coordinate complexes. They all have octahedral geometry.
Octahedron is an extremely symmetrical figure, with all M-L bonds being equivalent in an
If different kind of ligands are present, full symmetry cannot be retained. Distortions are
often seen. A compression or elongation of L-M-L axis relative to the other two is called a
tetragonal distortion (a and b). A complete breakdown of the equality of the axis gives a
rhombic distortion (c). A third kind of distortion would lead to a trigonal prism (d) by a 600
rotation of of the faces containing the arrows.
Higher Coordination Numbers: C.N.’s 7, 8 and 9 are not infrequent for some larger cations.
These are stereochemically nonrigid.
TYPES OF LIGANDS
Majorityof ligands are anions or neutral molecules that are electron pair donors.
Common examples: F-, Cl-, Br-, CN-, NH3, H2O, CH3OH and OH-.
Monodentate ligands: Ligand donating one pair of electron to one metal ion or atom
Example: Cl-, NH3.
Polydentate ligands: Ligands that contain two or more atoms, each of which can
simultaneously form a 2-electron donor bond to the same metal. They are also called Chelate
ligands (from the Greek for claw).
Bidentate ligands: Most common of polydentate ligands,.
Bidentate ligands forming 5-membered rings:
H2NCH2CH2NH2 ethylenediammine, en
H3CCOCHCOCH3 acetylacetonate, acac
Bidentate ligands forming 4-membered rings:
Polydentate ligands, including tri-, qudri-, penta and hexadentate ligands.
H2NCH2CH2NHCH2CH2NH2 diethylene triamine, dien
a. Cations are named before anions (as in common salts)
[Ag(NH3)2]Cl Diamminesilver(I) chloride
K3[Fe(CN)6] Potasasium hexacyanoferrate(III)
b. The inner coordination sphere is enclosed in brackets in the formula
c. Ligands are named before central metal, but in formula the metal ion is written first
d. The number of ligands of one kind is given by the following prefixes. If the ligand
name includes these prefixes or is complicated, it is set off in parantheses and the
second set of prefixes is used.
2 di bis
3 tri tris
4 tetra tetrakis
5 penta pentakis
6 hexa hexakis
7 hepta heptakis
8 octa octakis
9 nona nonakis
10 deca decakis
e. Neutral liands are named before anionic ones in alphabetical order
f. Anionic ligands are suffixed by –o:
g. Neutral ligands – exceptions
RNH2, R2NH, R3N amine
h. Bridging ligands between two metal ions have the prefix --
NH3 NH3 4+
i. If the charge is negative, –ate is added to the end of the metal name
j. Oxidation state of metal ion is given in paranthesis:
e.g. Pt(II) or Pt(2+)
NH3 NH3 6+
H3N NH2 NH3
-amido--nitro octa ammine dicobalt (III)
h. The prefixes cis- and trans- indicate adjacent and opposite geometrical locations
H3N Cl H3N Cl
H3N Cl Cl NH3
cis-diamminedichloroplatinum (II) trans-diamminedichloroplatinum (II)
i. When a complex is negatively charged, the names of the following metals are
derived from the sources of their symbols, rather than from their English names:
Iron (Fe) Ferrate Tin (Sn) Stannate
Silver (Ag) Argentate Gold (Au) Aurate
Lead (Pb) Plumbate
ISOMERISM AND CHIRALITY
Isomers in coordination chemistry include many types.
I. The following have the same overall formula but have different ligands attached to
the central atom or ion. The names indicate whether solvent, anions or other
coordination compounds form the changeable part of the structure.
a. Hydrate or Solvent Isomers
b. Ionization Isomers
c. Coordination Isomers
II. For cases of bonding through different atoms of the same ligand, the term Linkage
Isomerism or Ambidentate Isomerism is used.
III. Isomers having the same ligands but differing in the geometrical arrangement of
the ligands are called Stereoisomers.
Two or more molecules with identical formulas
Are the bond between the same atoms?
Stereo or configurational isomers Structural or constitutional isomers
Is each identical to its mirror image
Ionization Linkage Coordination Hydrate
isomers isomers isomers isomers
Diastereomers or Enantiomers or
Geometrical optical isomers
May have Chiral, non-
isomers mirror images
(different twists or
bands of bonds)
These include cis and trans isomers, chiral isomers, compounds with different conformations of
chelate rings, and other isomers that differ only in the geometry of attachment to the metal ion.