Question

Why do transition elements show a tendency to form a large number of complexes?

Answer 1

In the above question, we are asked why transition elements show a tendency to form a large number of elements. This is because the transition element can have a number of oxidation states which ranges from $+ 2$ to $+ 6$, which helps them to form multiple bonds.

Complete step-by-step answer: Transition elements are also known as transition metals. They are the elements which have partially filled d orbitals. According to IUPAC, transition elements are the elements which have a d subshell which is partially filled with electrons. In other words, they are the elements which have the ability to form stable cations with an incompletely filled d orbital.
In, we can say that the elements belonging to group 3 to group 12 are generally considered as transition elements. Even the elements of f-block which comprises lanthanides and actinides are considered as transition metals.
Since these elements have incomplete d-orbitals, they can have oxidation states ranging from $+ 2$ to $+ 6$ so as to attend half-filled or full-filled d-orbitals and get stabilised.
Hence, transition elements show a tendency to form a large number of complexes.
Additional Information-
The atomic and ionic radii of the transition elements decrease from group 3 to group 6 because of poor shielding effect by the small number of d-electrons. But the elements which are placed between group 7 to group 10 have similar atomic radii and those in group 11 and 12 have comparatively large radii because of electron-electron repulsion.

Note: Mercury, cadmium and zinc are not considered transition elements because they have completely filled d-orbitals. They have a full-fledged d-orbital in their oxidation state and hence, form only $+ 2$oxidation state.