Question 22.2: Name the coordination compounds (a) Na4[Co(CN)6] and (b) [Co...
Name the coordination compounds (a) Na_{4}[Co(CN)_{6}] and (b) [Co(NH_{3})_{5}Cl](NO_{3})_{2}.
Collect and Organize We are asked to write a name for each compound that unambiguously identifies its composition. The formulas of the complex ions appear in brackets in both compounds. Because cobalt, the central metal ion in both, is a transition metal, we express its oxidation state by using Roman numerals. The names of common ligands are given in Table 22.3.
| TABLE 22.3 Names and Structures of Common Ligands | ||||
| Ligand | Name within Complex Ion | Structure | Charge | Number of Donor Groups |
| Iodide | Iodo |  |
1 – | 1 |
| Bromide | Bromo |  |
1 – | 1 |
| Chloride | Chloro |  |
1 – | 1 |
| Fluoride | Fluoro |  |
1 – | 1 |
| Nitrite | Nitro |  |
1 – | 1 |
| Hydroxide | Hydroxo |  |
1 – | 1 |
| Water | Aqua |  |
0 | 1 |
| Pyridine (py) | Pyridyl |  |
0 | 1 |
| Ammonia | Ammine | NH_{3} | 0 | 1 |
| Ethylenediamine (en) | (same)^{a} |  |
0 | 2 |
| 2,2′-Bipyridine (bipy) | Bipyridyl |  |
0 | 2 |
| 1,10-Phenanthroline (phen) | (same)^{a} |  |
0 | 2 |
| Cyanide^{b} | Cyano |  |
1 – | 1 |
| Carbon monoxide^{b} | Carbonyl |  |
0 | 1 |
Analyze It is useful to take an inventory of the ligands and counterions:
| LIGAND | |||||
| Compound | Counterion | Formula | Name | Number | Prefix |
| Na_{4}[Co(CN)_{6}] | Na^{+} | CN^{-} | Cyano | 6 | Hexa- |
| [Co(NH_{3})_{5}Cl](NO_{3})_{2} | NO_{3}^{-} | NH_{3} | Ammine | 5 | Penta- |
| Cl^{-} | Chloro | 1 | — | ||
The oxidation state of each cobalt ion can be calculated by setting the sum of the charges on all the ions in both compounds equal to zero:
\begin{matrix} a. & Ions: & (4 Na^{+} ions) & + & (1 Co^{x} ion) & + & (6 CN^{-} ions) \\ & Charges: &4+ & + & x & + & 6- & & =0 \\ & & & & & & & & x=2+ \end{matrix}
\begin{matrix} b. & Ions: & (1 Co^{x} ion) & + & (1 Cl^{-} ion) & + & (2 NO_{3}^{-} ions) \\ & Charges: & x & + & 1- & + & 2- & & =0 \\ & & & & & & & & x=3+ \end{matrix}
Solve
a. Because the counterion, sodium, is a cation, its name comes first. The complex ion is an anion. To name it, we begin with the ligand cyano, to which we add the prefix hexa- and write hexacyano. This is followed by the name of the transition metal ion: hexacyanocobalt. We add -ate to the ending of the name of the complex ion because it is an anion: hexacyanocobaltate. We then add a Roman numeral to indicate the oxidation state of the cobalt: hexacyanocobaltate(II). Putting it all together, we get sodium hexacyanocobaltate(II).
b. The complex ion is the cation in this compound, and we begin by naming the ligands directly attached to the metal ion in alphabetical order: ammine and chloro. We indicate the number (5) of NH_{3} ligands with the appropriate prefix: pentaamminechloro. We name the metal next and indicate its oxidation state with a Roman numeral: pentaamminechlorocobalt(III). Finally we name the anionic counterion: nitrate. Putting it all together, we obtain the name: pentaamminechlorocobalt(III) nitrate.
Think About It Naming coordination compounds requires us to (1) distinguish between ligands and counterions and (2) recall which ligands are electrically neutral and which are anions. The structures of the complex ions in the named coordination compounds are shown in Figure 22.5.
