Appearance and visual differences
One of the most noticeable differences between d-elements and s-block representatives (alkali and alkaline earth metals, as well as magnesium and aluminum) is the color of their compounds.
While s-metals are white in the crystalline state and colorless in solutions, transition metals are characterized by bright coloration:
- Copper salts ($Cu^{2+}$) are blue.
- Nickel salts ($Ni^{2+}$) are colored green.
- Salts of some other transition metals can acquire purple hues.
Redox properties
The most characteristic chemical property of d-block metals is the presence of variable oxidation states in complex substances. This leads to their ability to participate in a huge number of redox reactions. Such reactions can occur in any aggregation state: in gases, solutions, and solids.
The nature of the redox properties of a particular compound directly depends on two factors:
- The current oxidation state of the element.
- The pH of the medium.
Typical representatives demonstrating a wide palette of such reactions are manganese ($Mn$), chromium ($Cr$), and iron ($Fe$).
Characteristics of 4th period d-elements:
| Element | Valence shell | Main oxidation states (bold — most stable) |
|---|---|---|
| Sc (Scandium) | $s^2d^1$ | 0, +3 |
| Ti (Titanium) | $s^2d^2$ | 0, +2, +3, +4 |
| V (Vanadium) | $s^2d^3$ | 0, +2, +3, +4, +5 |
| Cr (Chromium) | $s^1d^5$ | 0, +2, +3, +6 |
| Mn (Manganese) | $s^2d^5$ | 0, +2, +3, +4, +6, +7 |
| Fe (Iron) | $s^2d^6$ | 0, +2, +3, +6, +8(?) |
| Co (Cobalt) | $s^2d^7$ | 0, +1, +2, +3, (+5) |
| Ni (Nickel) | $s^2d^8$ | 0, +2, +3, +4 |
| Cu (Copper) | $d^{10}s^1$ | 0, +1, +2, (+3) |
| Zn (Zinc) | $d^{10}s^2$ | 0, +2 |
Complex compounds of d-elements
In complexation reactions, atoms or ions of d-elements always act as acceptors of electron pairs. The coordination numbers of the central atom depend on how many free valence orbitals are available for bond formation.
Ligands can be not only simple inorganic molecules and ions (e.g., ammonia $NH_3$), but also complex organic compounds containing nitrogen, oxygen, and sulfur atoms. For example, in zinc and copper complexes, the ligands can be organic amines: ethylamine ($C_2H_5NH_2$) or dimethylamine ($NH(CH_3)_2$).
Reaction examples:
- Chromium salts are characterized by a coordination number of 6:
$Cr^{3+} + 6NH_3 = [Cr(NH_3)_6]^{3+}$
- Zinc salts are characterized by a coordination number of 4:
$Zn^{2+} + 4NH_3 = [Zn(NH_3)_4]^{2+}$
Energy diagrams and bond nature
Coordination numbers strictly correspond to the number of free orbitals ready to accept electrons from ligands:
- The $Cr^{3+}$ ion has 3 unpaired electrons in the 3d sublevel. It provides 6 free orbitals for acceptance (two 3d, one 4s, and three 4p).
- The $Zn^{2+}$ ion has a fully filled 3d sublevel (10 electrons). It leaves 4 free orbitals (one 4s and three 4p).
The bond between the central metal atom and the ligands is of a donor-acceptor nature ($\sigma$-bonds). However, in some cases, additional $\pi$-bonds also arise (e.g., multiple bonds between iron and carbon). Their presence sharply increases the overall stability of the complex, a striking example of which is potassium ferricyanide — $K_3[Fe(CN)_6]$.
Class and exam questions
Why are s-element compounds colorless, while d-element compounds are brightly colored?
The bright coloration of transition metal salts (blue for copper, green for nickel) is a specific characteristic of the d-block and is associated with the structural features of their electron shells, whereas s-metal crystals are usually white and their solutions are colorless.
What does the coordination number of a d-metal in a complex depend on?
The coordination number is determined by the number of free valence orbitals of the central atom (ion) that can act as electron pair acceptors from ligands. For example, the chromium(III) ion has six, and the zinc(II) ion has four.
What gives potassium ferricyanide its high stability?
The high stability of the $K_3[Fe(CN)_6]$ complex is explained by the emergence of additional multiple π-bonds between the central iron atom and the carbon atoms in the ligands.
What else to study on this topic
- The effect of medium pH on redox reactions involving d-elements
- The mechanism of catalytic activity of metals in protein structures
- The specifics of π-bond formation in coordination compounds
- The role of organic amines as ligands in biochemical processes
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