ELEMENTS OF THE FIRST TRANSITION SERIES 209Cordination numbers two, four and six are found in transition metal complexes,
with coordination number six commonly found.
Complexes with coordination number two tend to be linear, for example
[Ag(NH 3 ) 2 ] [H 3 NAgNH 3 ]Those with coordination number four tend to be square planarortetrahedral, for
example
Complexes with coordination number six tend to be octahedral, for example
The ligands shown in the previous examples only donate one electron pair to the
central ion. They are monodentate(or ‘one tooth’) ligands. Ligands that can donate
more than one electron pair to the central metal ion are polydentate(‘many
toothed’) ligands. Thus a ligand that donates two electron pairs is bidentate, whereas
one that donates three electron pairs is tridentateand so on. A ligand which has the
common name ethylenediamine (abbreviation ‘en’) has the structure
This has two electron pairs to donate to the metal and acts as a bidentate ligand, for
example in the octahedral complex [Co(en) 3 ]^3 :
Because the ligand gives both electron pairs to the same metal, it is also known as a
chelate(‘claw-like’) or chelating ligand. Notice that rings are formed that include
the metal and the ligand and the coordination number of the metal is six.
NH 2CH 2CH 2NH 2Coen3+=Co CoenenenNH 2 CH 2 CH 2 NH 2H 2 O^2+H 3 NH 3 N
H 2 OOctahedralNH 3NH 3CuClCl ClClFeTetrahedralCN2–CNNCNCNiSquare planar