Central Atoms and Central Ions

A coordination compound comes under a class of chemical compounds in which central atoms and central ions are bonded to several ligands simultaneously. 

The central atoms and central ions act as Lewis acid and the ligands as Lewis bases. Ligands are the molecules or atoms that contain a lone pair of electrons which can be donated to the empty orbitals of the metal. 

Central Atoms and Central Ions

As discussed above, the ligands in coordination chemistry have either a lone pair of electrons or a negative charge. Thus, they have an excess of electron density that is available for donation to the vacant orbital of the metal. The metal atom to which the ligands bind is called the central atom. The metal ion to which the ligands bind is called the central ion. Example: in K3[Fe(CN)6], the central ion is Fe3+. These central atoms and central ions act as acceptors of the electron pair, i.e., as Lewis acids. 

To understand this, we have to understand transition metals in detail, as d-block elements act as central atoms and central ions in coordination complexes. Transition metals are called so because they lie in the middle of the periodic table and a transition happens from the s-block elements to p-block elements and the general properties of elements. 

They are d-block elements, i.e., the last electron enters in the d-orbital of the shell. There are ten such elements in each period (from d1 to d10). 

Let’s look at the 3d elements, their electronic configuration, and their common oxidation states :-

As can be seen in the table above, transition metals can exist in multiple oxidation states. This property makes them highly useful in catalysis as catalysts are required to exhibit multiple oxidation states. In coordination compounds, one metal can exist in different oxidation states. For example, in K3[Fe(CN)6], Fe exists in the +3 oxidation state. In K4[Fe(CN)6], Fe exists in the +2 oxidation state. 

These metals are capable of showing paramagnetism. Also, they form coloured complexes (it happens due to the effect of ligands as ligands are capable of splitting the degenerate d-orbitals into two different sets of orbitals).

Finding Charge of Metal Centre

The charge on a particular element is not fixed when it comes to d-block elements. Sodium (Na) would always exist as Na+ (in +1 oxidation state), regardless of what it is bound to. The same, however, cannot be said for transition elements.

But finding their oxidation state is crucial as it decides several things including the geometry, colour, and optical properties of the coordination complex. Iron in a compound in the +3 oxidation state would show paramagnetic behaviour, but in the +2 oxidation state, the compound may be diamagnetic as well. 

Now the question comes of how to find it. For that, the charge on the coordination sphere and the charge contribution of the ligands must be known. Neutral ligands like ammonia, water, carbonyl, and nitrosyl have zero charge contribution. Charge contribution only comes from anionic ligands like cyanide, halides, oxide, acetate, oxalate, EDTA, thiocyanide, isocyanide, etc.

To find the charge, a similar methodology is followed as in simple compounds. Let’s take the example of [CoCl2(NH3)4]Cl. In this, the charge on the coordination sphere = +1 (since there is one chloride to balance the coordination sphere charge).

Now, ammonia is a neutral ligand, so there is zero charge contribution from the four ammonia molecules.

Chloride is an anionic ligand, having the charge of -1. Hence, charge on two chloride ions = -2.

Charge on Co (cobalt) = +1 – (-2) = +3.

Conclusion

The understanding of central atoms and central ions is crucial to study coordination compounds. Transition metals form central ions. The presence of d-orbitals is essential. The colour of coordination complexes, which is a remarkable property, exists because of the d-d transitions that can happen in transition metals. Finding the charge on metal present in a coordination sphere is of utmost importance to determine the properties of the complex. When the coordination sphere is anionic, i.e., when there is a negative charge on the coordination sphere, the metal ion gets the suffix “ate” in the nomenclature.