STRUCTURES
The structures of coordination compounds are governed largely by the coordination
number of the metal. Many have structures similar to the simple molecules and ions we
studied in Chapter 8. Unshared pairs of electrons in dorbitals usually have only small
influences on geometry because they are not in the outer shell. Table 25-7 summarizes
the geometries for common coordination numbers.
Both tetrahedral and square planar geometries are common for complexes with coor-
dination number 4. For coordination number 5, the trigonal bipyramidal structure and
the square pyramidal structure are both common. Transition metal complexes with coor-
dination numbers as high as 7, 8, and 9 are known. The geometries tabulated are ideal
geometries. Actual structures are sometimes distorted, especially if the ligands are not all
the same. The distortions are due to compensations for the unequal electric fields gener-
ated by the different ligands.
ISOMERISM IN COORDINATION COMPOUNDS
Isomersare different compounds that have the same molecular formula; they have the
same number and kinds of atoms arranged differently. The term isomerscomes from the
Greek word meaning “equal weights.” Because their structures are different, isomers have
different physical and chemical properties.Here we shall restrict our discussion of isomerism
to that caused by different arrangements of ligands about central metal ions.
There are two major classes of isomers: structural (constitutional) isomers and stereoiso-
mers. For coordination compounds, each can be further subdivided as follows.
Structural Isomers Stereoisomers- ionization isomers 1. geometric (positional) isomers
- hydrate isomers 2. optical isomers
- coordination isomers
- linkage isomers
25-5
25-5 Structures 981Some coordination compounds. Starting at the top
left and moving clockwise:
[Cr(CO) 6 ] (white), CO is the ligand.
K 3 {Fe[(COO) 2 ] 3 } (green), (COO) 22
(oxalate ion) is the ligand.
[Co(H 2 NXCH 2 XCH 2 XNH 2 ) 3 ]I 3 (yellow-
orange), ethylenediamine is the ligand.
[Co(NH 3 ) 5 (OH 2 )]Cl 3 (red), NH 3 and H 2 O
are ligands.
K 3 [Fe(CN) 6 ] (red-orange), CNis the ligand.
(A drop of water fell on this sample.)