A. Colours of Transition Metal Complexes

• Why are most transition metal complexes brightly coloured but some aren't?
• Why do the colours change as the ligand changes? As a typical example, consider three complexes of the nickel(II) ion:

[Ni(H2O)6]2+ green

[Ni(NH3)6]2+ deep blue

[Ni(CN)4]2- yellow

• Why do the colours change as the oxidation state of the metal changes, even for complexes of the same ligand? Here are some examples of this phenomenon:

[Cr(H2O)6]2+ pale blue	[Cr(H2O)6]3+ violet
[Co(NH3)6]2+ red	[Co(NH3)6]3+ yellow-brown
[NiF6]3- violet	[NiF6]2- red

Before we can pose our questions, we need to know how the number of unpaired electrons can be determined, and how it is related to the magnetic moment of a complex.

Recall that a Gouy balance is used to measure the mass of a sample first in the absence of a magnetic field, and then when the magnetic field is switched on. The difference in mass can be used to calculate the magnetic susceptibility of the sample, and from the magnetic susceptibility the magnetic moment can be obtained.

The magnetic susceptibility and thus the magnetic moment are due to moving charges. In an atom, the moving charge is an electron:

For the 3d transition metals, the orbital moment is not very important, and the measured magnetic moment can be directly related to the number of unpaired electrons in the ion. This value is called the spin-only magnetic moment, and its units are Bohr Magnetons (B.M.).

Number of unpaired electrons	Spin-only magnetic moment, B.M.
1	1.7
2	2.8
3	3.9
4	4.9
5	5.9

Now to our questions:

• Why do different complexes of the same metal ion in the same oxidation state have different numbers of unpaired electrons? Some examples follow for Fe³⁺, Co³⁺, and Ni²⁺:

FeCl3.6H2O mu = 5.9 B.M.; 5 unpaired electrons	K3[Fe(CN)6] mu = 1.7 B.M.; 1 unpaired electron
K3[CoF6] mu = 4.9 B.M.; 4 unpaired electrons	[Co(NH3)6]Cl3 mu = 0; no unpaired electrons
[Ni(NH3)6]Cl2 mu = 2.8 B.M.; 2 unpaired electrons	K2[Ni(CN)4] mu = 0; no unpaired electrons

• Why are there only certain values of the number of unpaired electrons for a given metal ion? For example, complexes of Fe(II) and Co(III) can only have zero or 4 unpaired electrons, never two. Complexes of Fe(III) can only have 5 unpaired electrons or 1 unpaired electron.
• Why is it that for Ni²⁺ complexes, all octahedral complexes have 2 unpaired electrons but square planar complexes are diamagnetic (no unpaired electrons)?

C. Coordination Numbers and Geometries

• Why do some transition metal ions seem to have a fixed coordination number and geometry, but other metal ions are quite variable? Examples:

Cr3+ practically always 6-coordinate, octahedral
Co3+ practically always 6-coordinate, octahedral	Co2+ 6-coordinate octahedral and 4-coordinate tetrahedral complexes known
Ni2+ octahedral and square planar complexes common; some tetrahedral complexes known	Ni4+ only octahedral complexes known
Pt2+ practically always square planar	Pt4+ always octahedral

• Why do some metal complexes undergo ligand-exchange reactions very rapidly and other similar complexes react very slowly, even when reaction is thermodynamically favourable?


As an example, consider the reaction between hexaamminecobalt(III) ion and hydronium ion:

[Co(NH₃)₆]³⁺ + 6 H₃O⁺ ---> [Co(H₂O)₆]³⁺ + 6 NH₄⁺

The equilibrium constant for this reaction is approximately 10²⁵, and yet an acidic solution of the hexamminecobalt(III) ion requires several days before noticeable change occurs.

Contrast this reaction to the reaction of the corresponding copper(II) complex:

[Cu(NH₃)₆]²⁺ + 6 H₃O⁺ ---> [Cu(H₂O)₆]²⁺ + 6 NH₄⁺

In this case, acidification of the hexamminecopper(II) complex results in practically instantaneous reaction.

• Why are there so many examples of known, characterized, structural and geometric isomers for Co³⁺, Pt²⁺, Cr³⁺, and Pt⁴⁺ but not for very many other transition metal ions? Some examples:
• There are three isomers of CrCl₃.6H₂O that have been isolated and characterized. They are [Cr(H₂O)₆]Cl₃, [Cr(H₂O)₅Cl]Cl₂.H₂O, and [Cr(H₂O)₄Cl₂]Cl.2H₂O. Why don't two of them convert to the most stable third one (whichever it is), or else form an equilibrium mixture of all three?
• Why doesn't FeCl₃.6H₂O, have any isomers even though they are found for the similar compound, CrCl₃.6H₂O?
• Why doesn't cis-[Pt(NH₃)₂Cl₂] convert readily to trans-[Pt(NH₃)₂Cl₂] (or vice versa?)

We will find the answers to these questions as we study the simplest bonding theory for transition metal complexes, called crystal field theory, which is the subject of the next lecture.
 
 

This page is http://chemiris.labs.brocku.ca/~chemweb/courses/chem232/CHEM2P32_Lecture_8.html
Created January 22, 2001 by M. F. Richardson
© Brock University, 2001