Bonding in Complex Salts

 Hello! yesterday, we started looking at the complex salt which we called tetramine copper II chloride with the formula; [Cu(NH3)4]Cl2. We did say that this is a complex salt because of the presence to the complex cation [Cu(NH3)4]^2+. I want to state that if we had a complex anion, the salt would also be a complex salt. A typical example of this is; K4 [Fe(CN)6]. The complex ion is  [Fe(CN)6]4- which is anionic and the counter ions in this case are potassium cations.

I told you that we will look at the bonding in the complex salt [Cu(NH3)4]Cl2 and why it does not produce simple ions in water. I will try to explain the basics of the concept today as we would still look at coordination chemistry in more detail sometime later. 

Now, the complex salt [Cu(NH3)4]Cl2 has both ionic and covalent bonds. The four ammonia molecules are attached to the central copper II ion by a coordinate covalent bond(see the page timeline for an explanation of what a coordinate covalent bond is). We showed the electron donation of the lone pairs of electrons on the nitrogen central atoms of ammonia molecules into the appropriate dsp2 hybrid orbitals of the copper II ion in yesterday’s lesson.

Thus, the bonding between the copper II ion and the ammonia molecules in the complex cation[Cu(NH3)4]^2+ is covalent and directional. It is oriented in a given direction in space. This is why we said that the complex cation has a square planar geometry. Fixed molecular geometries are only assigned to covalently bonded species based on the arrangement of the atoms or groups in the specie. In complexes, we follow the Kepert model rather than the valence shell electron pair repulsion model (VSEPR). The Kepert model considers the number of ligands (species bound covelently to the central metal atom or ion in the complex) and ignores the electrons present in the metal atom/ion. The valance shell electron pair repulsion model considers the electron pairs on the central atom of the  molecule as well as the atoms or groups covalently bonded to the central atom in a molecule.

If a central metal atom or ion is bonded to four ligands as in [Cu(NH3)4]^2+, a tetrahedral or square planar geometry is possible. The actual geometry that is followed by the complex would depend on symmetry, steric or electronic effects or a combination of all these.

The four ammonia molecules that are covalently bonded to the central copper II ion are said to constitute the COORDINATION SPHERE of the complex and can only be exchanged with other species in a ligand substitution reaction. The number of ligands in the coordination sphere of the complex ion is called the coordination number of the complex. In [Cu(NH3)4]^2+, there are four ammonia molecules in the coordination sphere thus the coordination number of the complex ion is four.

On the other hand, the bonding between the complex ion [Cu(NH3)4]^2+ and the two chloride counter ions is ionic. This is what makes the complex a neutral specie. When we dissolve the complex in water, the chloride ions gets into solution as free Cl^- while the complex ion [Cu(NH3)4]^2+ remains intact. It does not break up to release the copper II ion and ammonia molecules. Conductivity measurements on the solution would confirm the presence of three ionic species in solution which are;

- Two chloride ions

- One tetrammine copper II ion

Hence, we see clearly why we do not obtain simple ions when we dissolve complex salts in solution. The complex ions remain intact and would not break up because the bonding within the complex ion moiety is purely covalent. However, a double salt is purely an ionic bonded specie thus when dissolved in water, all the ions get back into solution.

Hope you enjoyed our lesson today?

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