Write the formulas for the following coordination compounds:
Potassium tetracyanonickelate(II)

Hint: The roman number after the central metal atom name in the coordination compound shows the oxidation state of the metal. The word ‘tetra’ is used to show four of such species in chemistry.

Complete step by step solution:
We will find the chemical formula of the given compound from the name of the complex.
- Here, potassium is written before the name of ligands. That means potassium ions and tetracyanonickelate ions are bonded with ionic bonds. Potassium ions are positively charged and tetracyanonickelate ions are negatively charged. We will not write the potassium ions in the square bracket as they are not part of the metal complex. Note that the chemical symbol of potassium is K.
- Now, we will need to figure out how to write tetracyanonickelate(II) in the form of a chemical formula. Here, (II) shows that the nickel atom is charged and the charge on the nickel atom is +2. As the suffix –ate is used behind the name of the metal, it indicates that the complex is charged.
- The prefix tetra- before the word cyano suggests that there are four cyanide ligands present in the complex. We know that cyanide groups have an overall charge of -1.
- Thus, we can write tetracyanonickelate(II) as ${[Ni{(CN)_4}]^{2 - }}$. Now, if we also add potassium ions to this complex, then we will get the given coordination compound. Here, we will add two potassium ions as they will balance the charge on the complex.

So, the chemical formula for Potassium tetracyanonickelate(II) is ${K_2}[Ni{(CN)_4}]$.

Note: Remember that as potassium ions and tetracyanonickelate(II) ions are bonded with ionic bonds, we do not include potassium ions in the square bracket. We always write the central metal atom before the ligands in square brackets and ligands are arranged in alphabetical order of more than one are present.