Transition metals are the elements in the d and f blocks. They are called transition metals because they make the transition between the main groups, between metals and nonmetals. However, they are metals because they have electropositive character and tend to form ionic compounds.
The most important transition metals are the d-block metals. That is because f-block metals are heavy and usually radioactive.
The outer electronic structure for d-block metals is \(ns^2(n-1)d^x\). New electrons are added to the \((n-1)d\) subshell, but the valence shell is the \(n\) shell. Because of the \(ns^2\) valence shell structure, all d-block metals are found in divalent state.
However, some d-block metals form \(M^{3+}\) or \(M^+\) cations. This happens due to electron transitions to or from the \((n-1)d\) subshell to get to a more stable structure. There is a number of different cases:
Other higher valencies (IV, VI, VII) are possible, but they are explained by more complex covalent structures
Because of their low electropositivity (compared to s-block metals), d-block metals do exhibit some nonmetal characteristics. One of these is the tendency to bond with a high number of oxygen atoms, forming \(MO_n^{x-}\).
Example of such ions are the manganate ion (\(MnO_4^{2-}\)), the permanganate ion (\(MnO_4^-\)), the dichromate ion (\(Cr_2O_7^{2-}\)) or the chromate ion (\(CrO_4^{2-}\)). These are very strong and useful oxidizing agents, but this will be further explained in the electrochemistry chapter.
In intermolecular forces, we learned that cations in solution are surrounded by negative poles of the polar solvent - a purely electrostatic interaction with the solvent molecules. However, that only is the case for main group metals.
Transition metals have partially filled d orbitals which enable the interaction to be not just electrostatic, but also chemical. This creates aqua complexes, \([M(H_2O)_x]^{n+}\). Aqua complexes allow complex short electron transitions to take place. A direct consequence of these transitions is the absorption of different wavelengths of light which leads to colorful compounds. Indeed, d-block metal ions usually form colored solutions.
d-block metals don't have a lot of characteristic reactions. They are involved in many redox reactions, due to their ability to have different valencies. They can react with acids, depending on their position in the activity series. One interesting phenomenon we see when some transition metals react with oxoacids is called passivation. This occurs when the (usually concentrated) acid creates a protective oxide layer on the surface of the metal and the salt forming reaction no longer takes place, even if the activity series suggests it should.
f-block chemistry is very similar to d-block chemistry. By arguments similar to those made for the d-block, f-block metals (lantanides and actinides) are generally divalent and some of them can have monovalent and trivalent forms.
One difference between f-block metals and d-block metals is the fact that f metals form oxocations more often than d metals. One example of an oxocation is the uranyl cation, \([UO_2]^{2+}\).
Written by Alex Jicu