Coordination compounds are mainly known for transition metals .

Answer

### Step-by-Step Text Solution:

1. **Understanding Coordination Compounds**: Coordination compounds are complex structures formed by a central metal atom or ion bonded to surrounding molecules or ions known as ligands.

2. **Role of Transition Metals**: Transition metals are the primary constituents of coordination compounds. They are found in the d-block of the periodic table and include elements such as scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn).

3. **First Transition Series**: The first transition series consists of the aforementioned metals. These metals exhibit unique properties that make them suitable for forming coordination compounds.

4. **Examples of Coordination Compounds**: Some common examples of coordination compounds include:
- **[Fe(CN)₆]³⁻**: This is the hexacyanoferrate(III) ion, where iron is the central metal atom surrounded by six cyanide ligands.
- **[Fe(CN)₆]⁴⁻**: This is the hexacyanoferrate(II) ion, where again iron is the central atom but in a different oxidation state.
- **Nickel Tetracarbonyl (Ni(CO)₄)**: In this compound, nickel is the central atom bonded to four carbon monoxide ligands.

5. **Importance of d-Orbitals**: The ability of transition metals to form coordination compounds is largely due to the presence of d-orbitals. These orbitals can accommodate multiple electrons, allowing for various bonding configurations with ligands.

6. **Variable Valency**: Transition metals can exhibit multiple oxidation states (variable valency), which further enhances their ability to form diverse coordination compounds. This property allows them to bond with different types of ligands and form stable complexes.