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                         COORDINATION CHEMISTRY

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

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
[MLxLyLz] 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-

[Co(NH3)6]3+          Co(III)
[Co(H2O)6]2+          Co(II)

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:

C.N. 2:
Relatively rare, occurring with +1 ions of Cu, Ag and Au and Hg2+.
Geometry is linear.

C.N. 3:
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

       Pt              Pt

Cl             Cl              R3 P

C.N. 4:

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

C.N. 5:

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.

Example: Ni(CN)53-.

C.N. 6:

Enormously important.
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
ML6 complex.

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
               (C6H5)2PCH2CH2P(C6H5)2 diphos
               CH3OCH2CH2OCH3            glyme
               H3CCOCHCOCH3              acetylacetonate, acac

Bidentate ligands forming 4-membered rings:
               [RCO2]-                   Carboxylates
               [R2NCS2]-                 dithiocarbamates
               NO3-                      nitrate
               SO4                       sulphate

Polydentate ligands, including tri-, qudri-, penta and hexadentate ligands.

Tridentate ligands
               H2NCH2CH2NHCH2CH2NH2                 diethylene triamine, dien

Systematic Nomenclature:

   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

      Pt(NH3)BrCl(CH3NH2)            amminebromochloromethylamineplatinum(II)

   f. Anionic ligands are suffixed by –o:
      Cl-   chloro
      CN- cyano
      OH- hydroxo
      NH2- amido
      SCN- thiocyano

   g. Neutral ligands – exceptions
      NH3 ammine
      RNH2, R2NH, R3N amine
      MeNH2 methylamine
      CO     carbonyl
      CS     thiocarbonyl
      N2     dinitrogen
      O2     dioxygen
      PR3 phosphine

   h. Bridging ligands between two metal ions have the prefix --


                                          NH3                 NH3               4+
                                H3N                   H2
                                                      N                   NH3
                                           Co                 Co

                                H3N                   O
                                                      H        NH3

   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+)

                      [Cr(H2O)6]2+                  hexaaquochromium(II)

                                         NH3                NH3                 6+
                             H3N                    NH2               NH3
                                         Co                  Co

                                                    NO2               NH3
                                         NH3                 NH3

                      -amido--nitro octa ammine dicobalt (III)

h. The prefixes cis- and trans- indicate adjacent and opposite geometrical locations

               H3N             Cl                                    H3N                   Cl

                        Pt                                                           Pt

              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?

                 Yes                                                             No

Stereo or configurational isomers                        Structural or constitutional isomers

Is each identical to its mirror image

                                                Ionization       Linkage     Coordination     Hydrate
                                                isomers          isomers     isomers          isomers
      Yes                   No

Diastereomers or         Enantiomers or
Geometrical              optical isomers
May have                 Chiral, non-
conformational           superimposable
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.

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