Modern inorganic chemistry

364 THE TRANSITION ELEMENTS

carbon monoxide (for example Ni(CO) 4 , Fe(CO) 5 ) and phosphorus

trifluoride, PF 3 (for example Ni(PF 3 ) 4 ).

Some important properties of these coordination complexes will

now be considered.

Shape

The rules governing the shapes of molecules and complex ions

have already been discussed (p. 37,46). The common shapes of com

plexes are octahedral, for coordination number 6, and tetrahedral, for

coordination number; all the 6- and 4-coordinate complexes so far

considered have these shapes. Other coordination numbers (for

example, 2 in Ag(CN)2 (linear) and 5 in Fe(CO) 5 ) (trigonal bipyra-

midal) are less common, and lie outside the scope of this book. Some-

times other shapes are possible; thus, for example, platinum(II) forms

planar 4-coordinate complexes (for example [PtG 4 ]^2 ), and 6

coordinate copper(II) usually forms distorted octahedral complexes

in which two of the ligands are further away from the central copper

ion than the other four. Moreover, the coordination number and

shape of a complex may vary for a given transition ion when com-

plexed with different ligands; thus, cobalt(II) forms 6-eoordinate

octahedral complexes with water or ammonia as ligands,

([Co(H 2 O)J^2 +, [Co(NH3) 6 ]2+) but a tetrahedral 4 coordinate

complex with chloride as ligand ([CoQ 4 ]^2 ~).

Colour

Transition metal compounds are very often coloured; frequently
(but not always) the colour is due to the presence of coordination
complexes. When a cation containing d electrons is surrounded by
other ions or polar molecules, either in a complex ion in solution or
in a solid, a splitting of the energy levels of the five d orbitals (all
originally having the same energy) occurs; when light falls on such
a system, electrons can move between these split levels. The energy
absorbed in this process corresponds to absorption of the light at
certain wavelengths, usually in the visible part of the spectrum,
hence colour is observed. For a given cation the kind of absorption
produced—its intensity and position in the spectrum—depends
very much upon the coordination number and surrounding ligands.
We can illustrate this by reference to the Cu^2 + ion. In solid anhydrous
copper(II) sulphate, the Cu2+ ion is surrounded by ions SO%~ ; in
this environment, the d orbital splitting is such that absorption of