In both $[Co(NH_3 )_6 ]^{3+} $ and $[CoF_6 ]^{3-} Co$ is present as $Co^{3+}$ Thus, the electronic configuration of $Co$ is $_{27}Co = [Ar] 3d^7 ,4s^2 $ $_{27} Co^{3+} = [Ar] 3d^6 , 4s^0$ In case of $[Co(NH_3 )_6 ]^{3+} NH_ 3 $ is a strong field ligand, so pairing of electrons in $3d$-orbital takes place. $_{27} Co^{3+} = [Ar] 3d^6 , 4s^0$
In $[CoF_6 ]^{3-}, F$ is a weak field ligand, thus does not cause pairing. Hence, $_{27} Co^{3+} = [Ar] 3d^6 , 4s^0$
The elements, in the columns of the periodic table in which d subshells are being occupied are known as d block elements.
These are the elements that have the capability of forming stable cations with incompletely filled d orbitals. Elements like mercury and Zinc are not considered transition metals because they have electronic configurations: (n-1)d10 ns2. These elements have filled d-orbitals in their ground state and, therefore, even in some of their oxidation states.
General Properties Of d-Block Elements
Multiple oxidation states- The oxidation states of d block elements show very few energy gaps; therefore, they exhibit many oxidation states. Also, the energy difference between s and d orbital is very less. Therefore both the electrons are involved in ionic and covalent bond formation, which ultimately leads to multiple oxidation states.
Formation of complex compounds- Ligands show a binding behaviour and can form so many stable complexes with the help of transition metals. This property is mainly due to:
Availability of vacant d orbitals.
Comparatively small sizes of metals.
Hardness- Transition elements are tough and have high densities because of the presence of unpaired electrons.
Melting and boiling points- Melting and boiling points of transition are very high. This is because of the presence of unpaired electrons and partially filled d orbitals. Because of these two things, they form strong bonds and therefore have high melting and boiling points.
Atomic radii- The atomic and ionic radius of the transition elements decreases as we move from Group 3 to group 6. However, it remains the same between group 7 and group 10, and from group 11 to group 12 increases.
Ionization enthalpy- The ionization enthalpies of the transition elements are generally on the greater side as compared to the S block elements
![1/2 Mn2 (CO)10+Na Na[Mn(CO)5 ] CH3Cl-NaCl [CH3Mn(CO)5](https://images.collegedunia.com/public/qa/images/content/2024_07_31/image_a08f12a11722411541392.png)