The d Block Elements The d block elements fall between the s block and the p block. They share the common characteristics in that the d sublevel of the atom is being filled. The d block elements include the transition metals. The transition metals are those d block elements with a partially filled d sublevel in one of its oxidations states. Since the s and d sublevels are very close in energy, the d block elements show certain special characteristics including Multiple oxidation states The ability to form complex ions Colored compounds Catalytic behavior Magnetic properties Variable Oxidation States in the d Block The s block metals such as Na and Ca have s electrons that are easily lost, but the ionization energies of the inner electrons are so high that they are never lost in chemical reactions. Therefore the oxidation number of sodium is always +1 and calcium is always +2 The transition metals have slightly higher effective nuclear charges (more protons). Their first ionization energies are slightly higher than those of the s block. Therefore they are generally less reactive than the alkali metals or the alkaline earth metals There is no sudden sharp increase in ionization energy as one proceed through the d electrons as there would be with the s block. As a result the d block elements can lose or share d electrons as well as s electrons, allowing for multiple oxidation states. The d Block elements usually have a +2 oxidation number which corresponds to the loss of the two s electrons. This is especially true on the right side of the d block, but less true on the left. For example Sc+2 does not exist, and Ti+2 is unstable oxidizing in the presence of any water to the +4 state. Common Oxidation States of the d Block Elements Sc Ti V Cr Mn Fe Co Ni Cu Zn 7 X 6 X X 5 X 4 X X X 3 X X X X X X 2 X X X X X X X X 1 X Colored ions In an isolated atom all of the d sublevel electrons have the same energy. If the atom is surrounded by charged ions or polar molecules, the effect of the electric field from these ions or molecules has a slightly different effect on the energies of the various d orbitals and d electrons. The colors of the ions and complex ions of d block elements depends on a variety of factors including: The particular element The oxidation state The kind of ligands bound to the element The presence of a partially filled d subshell in the element is usually necessary for the ion or complex to show color Most transition metals that have partially filled d subshells have colored compounds. Those with completely full or completely empty subshells do not, For example Zinc which has a full d subshell. Its compounds are white A transition metal ion exhibits color when it absorbs visible light with wavelengths ranging from 400-700 nanometers. If the compound absorbs particular wavelengths of light its color is the composite of those wavelengths that it does not absorb. The d orbitals are usually split up into two groups such as one of the three orbitals at a low energy and one of two orbitals at a higher energy. The difference in energy of these orbitals varies slightly with the nature of the ligand or ion surrounding the metal ion The frequency of the light corresponding to this difference ∆E =h occurs in the visible region. When white light passes through a compound of a transition metal, light of a particular frequency is absorbed and an electron is promoted from a lower energy d orbital to a higher one. The light that is reflected appears colored because some of the frequencies of otherwise white light have been absorbed. For example, in copper (II) compounds most of the red and yellow light is absorbed so the compounds have a blue to green color. If there are no electrons in the d orbitals as in then case of Sc3+ or Ti4+ There are no d electrons to move and the compounds are colorless. If the d orbitals are completely full as in the case of Zn2+ there is no space for electrons to move and the compounds are again colorless. Complex ions The ions of the d block elements and those of the lower p block have unfilled d or p orbitals. These orbitals can accept electrons from another species, either an ion or polar molecule, to form a dative bond. This attraction results in the formation of a complex ion. A complex ion is made up of two or more ions or polar molecules joined together. It should not be confused with a polyatomic ion.The molecules or ions that surround the metal ion donating the electrons to form the complex ion are called complexing agents or ligands. Compounds that are formed with complex ions are called coordination compounds Common ligands Complex ions usually have either 4 or 6 ligands. The formation of complex ions stabilizes the oxidations states of the metal ion and they also affect the solubility of the complex ion. The formation of a complex ion can also have a major effect on the color of the solution of a metal ion. For example aqueous cobalt salts have a pink color. If concentrated hydrochloric acid is added to the solution, the solution turns blue due to the formation of the tetrachlorocobalt ion CoCl42- . Catalytic Behavior Many transition metals and their ions are used for catalysts for other chemical reactions. A catalyst speeds up the rate of a chemical reaction with out itself being consumed. The transition metals form complex ions with species that can donate lone pairs of electrons. This results in close contact between the metal ion and the ligand. Transition metals also have a wide variety of oxidation states so they gain and lose electrons in redox reactions The d block elements may either heterogeneous or homogeneous catalysts. In a a heterogeneous catalyst the surface of the metal or compound provides an active surface on which the reaction can occur with a reduced activation energy. For example, Manganese (IV) oxide, MnO2, catalyzes the decomposition of hydrogen peroxide in this manner. Nickel and platinum, iron, and vanadium (V) oxide are all important catalysts for industrial processes. In homogeneous catalysis the catalyst is in the same phase as the reactants. In these reactions a particular metal ion is oxidized in one stage of the reaction and reduced (reformed ) in another stage of the reaction. A good example of this kind of catalysis is the role of Fe2+ and Fe3+ in the reaction of hydrogen peroxide with iodide ion. The iron (II) is oxidized by the peroxide to ion (III). The Iron (III) is then reduced by the iodide ions to reform iron (II). Magnetic Properties Molecules with one or more unpaired electrons are attracted into a magnetic field. The more unpaired electrons in the molecule the stronger the attraction. This type of behavior is called paramagnetism. Substances with no unpaired electrons are weakly repelled by a magnetic field. This property is called diamagnetism. Many transition metal complexes exhibit simple paramagnetism. In such compounds the individual metal ions possess some number of unpaired electrons. It is possible to determine the number of unpaired electrons per metal ion by looking at the degree of paramagnetism.