Transition Metals

Introduction

Transition elements are the elements in which the last electron enters a d-subshell of the penultimate (second-to-last) energy level, that is, elements with a partly filled d-subshell in the atom or in a common ion. They lie in the middle of the periodic table, between the strongly electropositive s-block metals and the p-block elements, and their properties (ionic, coloured, variable valency) are transitional between the two, which is why they are called transition metals.

The d-block is arranged into three main transition series, plus the f-block (lanthanides and actinides) sometimes treated alongside them:

• 3d series (first transition series): Sc to Zn (Z = 21 to 30)
• 4d series (second transition series): Y to Cd (Z = 39 to 48)
• 5d series (third transition series): La, Hf to Hg (Z = 57, 72 to 80)

Characteristics of Transition Metals

General electronic configuration: $(n-1)d^{1-10}\,ns^{1-2}$. Because the d-orbitals of the inner shell and the s-orbital of the outer shell are close in energy, both sets of electrons take part in bonding, which is responsible for most of the characteristic behaviour of these elements:

• High melting and boiling points and high density, due to strong metallic bonding involving both s and d electrons.
• Good conductors of heat and electricity.
• Variable oxidation states (see below).
• Formation of coloured ions and compounds.
• Formation of complex ions with ligands such as $NH_3$, $CN^-$, $H_2O$ and $Cl^-$.
• Catalytic activity, both as the metal and as compounds.
• Paramagnetism, due to unpaired d electrons in many of their ions.
• Alloy formation, since transition metal atoms are similar in size and can substitute for one another in a metallic lattice.

Variable Oxidation States

Because both the outer $ns$ electrons and the inner $(n-1)d$ electrons are close enough in energy to be lost or shared in bonding, transition metals commonly show several oxidation states in their compounds, unlike the fixed oxidation state typical of s-block metals.

• The +2 state is very common across the first transition series, corresponding to loss of the two $4s$ electrons (e.g. $Fe^{2+}$, $Cu^{2+}$, $Zn^{2+}$).
• Oxidation states from 0 up to +8 are known among the transition metals as a whole; the +8 state is the highest observed, shown by ruthenium (in $RuO_4$) and osmium (in $OsO_4$).
• Common examples: iron shows +2 and +3 ($Fe^{2+}$, $Fe^{3+}$); copper shows +1 and +2 ($Cu^{+}$, $Cu^{2+}$); manganese shows a wide range from +2 to +7 (as in $MnO_4^-$).