Question

What are the systematic names for the following ions and compounds? (a) ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{+}$ (c) ${[\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Br}}_2]}^{+}$ (b) $\left[\mathrm{Cr}{\left({\mathrm{NH}}_3\right)}_3{\mathrm{Cl}}_3\right]$ (d) $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_3$

Short answer

(a) Tetraamminechloridocobalt(III) ion, (b) Tris(ammine)trichloridochromium(III), (c) Bis(ethylenediamine)bromidocobalt(III) ion, (d) Hexamminecobalt(III) chloride.

Step-by-step solution

Identify the central metal and its oxidation state

For each compound, determine the central metal ion. Use the standard charge of ligands and the overall charge of the compound to find the metal ion's oxidation state. For compounds (a) and (d), cobalt (Co) is the central metal. For (b), chromium (Cr) is the central metal. For (c), cobalt (Co) is again the central metal.

Determine the ligands and their prefixes

Identify each ligand and determine the appropriate prefix for the number of each type of ligand. Use standard prefixes like "tetra" for four, "hexa" for six, and "bis" for two when the ligand name has numbers or is complex, like 'en' (ethylenediamine).

Name the ligands

For compound (a), the ligands are four ammonia (ammine) and two chloride ions. For compound (b), there are three ammines and three chlorides. For compound (c), there are two ethylenediamine and two bromides. For compound (d), there are six ammines.

Formulate the full systematic name

Combine the prefix and ligand names, followed by the central metal and its oxidation state in Roman numerals. For example, in (a), it's 'tetraammine' followed by 'dichlorido', then 'cobalt(III)'. Complete this for all compounds.

Full answer listing

Write down the names combining all parts from Steps 1-4. For compound (a), it is 'tetraamminechloridocobalt(III) ion'. For compound (b), 'tris(ammine)trichloridochromium(III)'. For compound (c), 'bis(ethylenediamine)bromidocobalt(III) ion'. For compound (d), it's 'hexamminecobalt(III) chloride'.

Key concepts

Oxidation States

Understanding oxidation states is crucial in coordination chemistry as they help identify the charge of the central metal ion in a compound. The oxidation state, sometimes called the oxidation number, indicates the number of electrons lost or gained by the metal when forming the complex. It is an essential part of naming coordination compounds. To determine the oxidation state:

• Identify all ligands and their individual charges. Common ligands like ammonia (NH₃) have a neutral charge, while chloride (Cl^-) carries a -1 charge.
• Use the overall charge of the complex to calculate the metal's oxidation state.

For example, in the compound $[{\text{Co(NH}}_3{\text{)}}_4{\text{Cl}}_2{]}^{+}$, the metal cobalt (Co) can be calculated as follows:

• Assume Co's oxidation state is x: $4imes0+2\times (-1)+x=+1$.
• Solve for x: $x-2=+1$, so $x=+3$.

This results in cobalt having an oxidation state of +3 in this complex.

Ligand Prefixes

Ligand prefixes indicate the number of each type of ligand in a coordination complex. These prefixes help in systematically naming the compounds by describing how many of each ligand are present:

• "Mono-" is often omitted, but means one.
• "Di-" indicates two, "tri-" indicates three.
• "Tetra-", "penta-", "hexa-" progress upwards for four, five, and six.

Complex ligands or ligands which are themselves polydentate (bond to the metal in multiple places) use alternate prefixes:

• "Bis-", "tris-", "tetrakis-" are used to avoid confusion from similar sounding ligand names or those already using numbers, like ethylenediamine ('en').

In the complex ${\text{[Co(en)}}_2{\text{Br}}_2{]}^{+}$, the prefix "bis-" is used for two ethylenediamine ligands, as en is a bidentate ligand.

Naming Coordination Complexes

Correct naming of coordination complexes follows a specific order, ensuring clarity and uniformity. The process begins by stating the prefixes and ligands in alphabetical order before the central metal and its oxidation state.

• List all ligands alphabetically, considering only the name, not the prefix (e.g., "ammine" before "chlorido").
• Use prefixes ("di-", "tris-", etc.) to state the quantity of each ligand.
• The central metal comes last, followed by its oxidation state in Roman numerals enclosed in parentheses.

For instance, in ${\text{[Cr(NH}}_3{\text{)}}_3{\text{Cl}}_3]$, the correct name is "tris(ammine)trichloridochromium(III)." Ammonia and chloride appear as ligands, 'tris' and 'trichlorido' for their numbers, and the oxidation state of chromium is explicitly stated.

Coordination Chemistry

Coordination chemistry delves into the interactions between central metal atoms and their surrounding ligands in coordination complexes. These complexes play a significant role in various fields such as biochemistry, catalysis, and materials science. The central metal is often a transition metal, because these have the ability to form various coordination numbers and geometries. Ligands can range from simple ions like Cl^- to complex organic molecules like ethylenediamine (en).

• The coordination number indicates how many coordinate bonds are formed with the metal, impacting the geometry and overall stability of the compound.
• Ligands can be neutral or charged, donating electron pairs to the metal and stabilizing it.

This study of coordination compounds provides insights into the chemical behavior of metals and supports the development of new materials and medicinal compounds.