Year 2 Inorganic 2
Linkage Isomerism of Some Cobalt Complexes
Objectives
Upon completion of this experiment you should understand:
(1)
Techniques of synthesising nitrito-κO and nitrito-κN linkage isomers of cobalt(III), understand their structures, and the mechanisms of the reactions by which they form.
(2)
How to confirm if nitrito-κO or nitrito-κN isomers are present using infra-red spectroscopy.
Aim
This experiment will illustrate linkage isomerism.
Introduction
A common property of coordination compounds is that they often occur in isomeric forms having the same empirical formula, but with a different arrangement of atoms. Many types of isomerism exist, more so than in organic chemistry
e.g. geometrical, optical, ionisation, ligand isomerism etc. (see CH160 lecture notes). Linkage isomers arise because some ligands can bond via one donor atom in one compound, but via a different atom of the same ligand in another compound. Typical examples of linkage isomersism are:
NO2– M-NO2 nitrito-κN (formerly nitro) M-ONO nitrito-κO (formerly nitrito)
CN– M-CN cyanido-κC (formerly cyano) M-NC cyanido-κN (formerly isocyano)
NCS M-SCN thiocyanato-κS (thiocyanato) M-NCS thiocyanato-κN (isothiocyanato)
(Note the old names are given for some of these complexes, which may help with background reading.)
In this experiment, you will prepare two compounds with identical formula [Co(NH3)5X]Cl2 where X=NO2. One form is red and the other yellow, the difference arises depending on if NO2 is N- or O-bonded to the Co(III); see
Equilibrium (1).
(1)
The equilibrium, above, actually lies well to the right and the only reason the κO form can be obtained is that it is the first product when the aqua complex [Co(NH3)5OH2]3+, formed as an intermediate, reacts with a nitrite buffer
[details are in R.G. Pearson,P.M. Henry, J.G. Bergmann and F. Basolo, J. Am. Chem. Soc., 76, 5920, (1954)]. This reaction is interesting because it is a good example of an intramolecular rearrangement (see R.K Murmann and H.
Taube, J. Am. Chem. Soc., 78, 4886, (1954)). It is also possible to reverse this reaction (κN to κO) using UV light.
In these two isomers the ligand possesses different steric requirements in the two forms, the angle (see
Figure 1 below) being greater than the angle . In the nitrito-κO isomer, the O–atom bonded to Co also acts as a π– donor ligand, and since the acceptor orbital on Co is already full (t2g6) this is not the thermodynamically favoured bonding mode. In the nititro-κN isomer, the N–bonded nitro group acts as a π-acceptor ligand, and accepts electron density from the full Co t2g orbital
1
Figure 1: The different steric requirements of nitrito-κN (left) and nitrito-κO (right)
The rate of isomerisation of the nitrito-κO to the nitrito-κN compound can, of course, be determined by UVvisible spectroscopy, since the species are different colours. However in this experiment you will use infra-red spectroscopy to monitor the change.
Safety
Carry out all experimental work in a fume cupboard; wear safety spectacles and lab coats at all times.
Concentrated ammonia (ammonium hydroxide 18.1 M, density = 0.88) Corrosive, vapour intense irritant to eyes and respiratory system, Use in fume cupboard, wear blue nitrile gloves and eye protection.
Metal ions Often toxic, wear gloves. Carefully mop up any spillages, and avoid inhaling any metal containing dust powders. The metal salts you will use are all water soluble and small quantities may be dissolved in water for disposal.
Sodium nitrite Irritant
Concentrated hydrochloric acid (11.65 M HCl) & diluted hydrochloric acid (6 M HCl) Caustic, will burn skin and eyes, irritant vapour. Use in fume cupboard, wear gloves and eye protection, wash away any small spillages with plenty of water.
Experimental Protocol
Notes
You will require a number of ice-cold liquids for these experiments, you will save time if you prepare these solutions first and store on ice.
You use three different concentration of
