Complex Ion Composition Essay, Research Paper

Complex Ion Composition

By Job’s Method

By Kelly Miller Smith, III

Inorganic Chemistry

Dr. Virgil Payne 1:00 lab

Objective

The objective of this experiment is to study the continuous variations method that is used to determine the composition of the solution Ni2+ ethylenediamine complexes.

Background

To examine the interaction between two molecules in solution without isolating the compound Job’s method is used. Although unstable compounds tend to be generated, this is not a reflection of weak interactions. In some cases, the transition metal species cannot be crystallized from the solution and separated from the other species present. Without Job’s Method this composition can be very difficult to deduce.

Job’s Method analyzes the value of n in the equilibrium:

Z + nL →ZLn

Varying the n value carries out the experiment. Absorbencies of each of the ZLn complexes are obtained. The sum of the concentrations of the metal, Z, and the ligand, L, are kept equal. With the ratio of the ligand to the metal in the solution with the maximum absorbance for the ZLn complex, the value of n can be determined as well as the composition of ZLn.

The metal Ni2+ and the ligand ethylenediamine (en) are studied in this experiment. Solutions are prepared with varying compositions of Ni(en)n2+. Using the equilibrium constants, it is possible to identify which species is present. If the constant for the formation of a species where n is 2 is larger than a species whose constant equals 3 then the former species is pre-dominant. Job’s Method is limited in that it will give non-integral values of the n present if a fourth complex, ZLn+1, exists. If there is a large variation between the equilibrium constants then only two complexes will be present in the prepared solutions. The absorbance values are plotted, then the value of n can be calculated.

Procedure

The procedure for this experiment can be found in Inorganic Chemistry Lab Manual prepared by Dr. Virgil Payne.

Data and Observations

530 545 578 622 640

0.3 0.079 0.134 0.277 0.555 0.605

0.4 0.088 0.141 0.333 0.596 0.638

0.5 0.099 0.154 0.35 0.57 0.579

0.6 0.158 0.218 0.414 0.584 0.552

0.7 0.212 0.289 0.436 0.459 0.411

0.8 0.376 0.424 0.396 0.203 0.149

0.9 0.213 0.224 0.168 0.061 0.036

1 0.052 0.067 0.142 0.465 0.647

Results and Discussion

1. Ni(en)32+ Ni(en)2(H2O)22+ Ni(en)1(H2O)42+

2. The higher placement in the electrochemical series of en is evidence that it has a higher energy of transition than H2O in the same complex. This gives en a higher ligand field strength.

5. Job’s Method would be useful in the reaction of Ni2+ and NH3 in H2O.