Classification of Coordination Compounds

Since the number of known complexes is so large, we need to classify and categorise them  in order to study their chemistry systematically.

The first and the most obvious choice is to classify them on the basis of the central metal, ion. There are over eighty elements which can act as acceptor atoms in a complex. Most of: these elements like transition metals can exist in several different oxidation states. Each of these states behaves as a different kind of acceptor and forms complexes having different formulas and properties. Hence, such a classification cannot be very useful. 

A second way of classification is based on the coordination number of the metal ion. It is useful to the extent that the coordination number determines the geometry of the complex which can be correlated with properties like isomerism. However, majority of the complexes formed are either four coordinated or six-coordinated. Four-coordinated complexes commonly show square planner and less frequently tetrahedral geometry. On the other hand all six coordinated complexes invariable exist in octahedral geometry. Thus, a further sub-classification would become necessary.

There is yet another use fill way for the classification and that is on the basis of ligands. Ligands of one kind generally tend to give complexes with similar structure and properties. Most of the ligands are either simple anion like CI-. Br^-,Nm etc. or are neutral molecules containing at least one donor atom. Let us look at the most commonly countered elements who 'se atoms, either in simple ionic form or as a part of a molecule, can act as a ligand.  These are:

                                           N                          O                                  F

                                          P                            S                                   Cl

                                          As                          Se                                 Br

                                                                                                               L

Halogens are always attached to the metal ion in a simple anionic form whereas all others commonly form a part of a molecule or ion. Examples are: NH₃ H₂0, SCN, CH3COCH2COCH3 etc.

It is an observed fact that some metals form their most stable complexes with ligands having

N,0 and F as donor atoms. These have been grouped together and are called class 'a' elements or 'hard acids'. These include elements of Groups 1.2 and 13 or transition elements with one, two or three d-electrons. Some metals form their most stable complexes with ligands where the coordinating atom is any element (P, S, CI etc.) other than N, 0, F in the above group. These metals are referred to as class 'b' elements or 'soft acids'. These include a few transition elements such as Pt, Pd, Hg etc. A large number of elements, however, cannot be clearly classified under either of the above two categories. These include elements from Mn to Cu amongst the first transition series, and Mo, 0s. Cd etc.  These differences in behaviour of the metal ions arise due to many factors such as charg size, electronegativity, etc.