The chemical and physical properties of the first-row transition elements (3d metals) are often substantially different from those of the second and third-row transition elements (4d and 5d metals), which show great similarity to each other. The presence of d electrons in the valence shell is responsible for the colour, electronic conduction, magnetism, and rich organometallic chemistry of the d metals and their compounds.
Transition Elements Properties:
1. Size of the transition elements:
The first-row transition elements (3d metals) are significantly smaller than their second and third-row transition elements (4d and 5d congeners); the similarity in size of the 4d and 5d metals is a manifestation of the lanthanoid contraction. Coordination numbers are typically greater for the larger 4d and 5d metals, with less common geometries, such as square antiprism, being found. Electronegativities generally increase across the d block, resulting in the increasing soft Lewis acid behaviour noted above.
2. Oxidation States:
The table shows both the common, and maximum, oxidation states of nonorganometallic complexes of the d metals, and we can easily see the trends of increasing the stability of high oxidation states down a group and the maximum state peaking in the middle of the series. The chemistries of the d metals also reflect their hard/soft nature.
The harder first-row transition metals and the early 4d and 5d elements have extensive chemistry in combination with oxygen, giving simple and complex oxides which form many functional solid-state materials such as heterogeneous catalysts and electronic and optical materials. The later elements have more extensive chemistries with soft ligands such as sulfide. We can contrast the chemical properties of the d metals with those of the s and p blocks, but the differences all ultimately come down to the presence of d electrons.
3. Colour and Magnetism:
Electronic transitions are possible between the orbitals, with the energy required corresponding to visible light, resulting in coloured complexes for those with configurations d1–d9. Unpaired electrons in these d orbitals are responsible for electrical conductivity of some compounds and also magnetism; there are configurations where the number of unpaired electrons varies. High- and low-spin complexes are possible, though with the increased size of the second- and third-row d metals we see a preponderance for low-spin complexes of these metals.
4. Organometallic Chemistry:
The organometallic chemistry of the d metals is thus substantially richer than of any of the other metals. The physical properties of the elemental d metals are, once again, intimately related to the presence and number of d electrons. Metallic bonding becomes stronger across the rows, peaking in the middle, with a concomitant increase in melting point, density, and enthalpy of atomization.
5. Applications Of Transition Elements In Today’s World:
Nowadays, probably every metal in the d block has at least a small-scale specialist use, whether that be in performance alloys, electronics, or as a component in high-temperature superconductors. Biology has exploited many of the d-block elements as the active sites of enzymes catalysing a wide range of reactions.