Question 20.4: Write the formula for each of the following: (a) Potassium t......
Write the formula for each of the following:
(a) Potassium tetracyanonickelate(II)
(b) Aquachlorobis(ethylenediamine)cobalt(III) chloride
(c) Sodium hexafluoroaluminate
(d) Diamminesilver(I) ion
STRATEGY
To find the formula and charge of the complex cation or anion, note the number of ligands of each type, their charge, and the oxidation state of the metal in the name of the compound or ion. If the name refers to an electrically neutral compound, balance the charge of the complex cation or anion with the appropriate number of ions of opposite charge.
(a) The bonding of four CN^{-} ligands to Ni^{2+} gives an [Ni(CN)_{4}]^{2-} anion, which must be balanced by two K^{+} cations. The compound’s formula is therefore K_{2}[Ni(CN)_{4}]. This compound is obtained when excess KCN is added to a solution of a nickel(II) salt.
(b) Because the complex cation contains one H_{2}O, one Cl^{-}, and two neutral ethylenediammine (en) ligands, and because the metal is Co^{3+}, the cation is [Co(en)_{2}(H_{2}O)Cl]^{2+}. The 2+ charge of the cation must be balanced by two Cl^{-} anions, so the formula of the compound is [Co(en)_{2}(H_{2}O)Cl]Cl_{2}. The cation is the first product formed when [Co(en)_{2}Cl_{2}]^{+} reacts with water.
(c) Sodium hexafluoroaluminate, also called cryolite, is used in the electrolytic production of aluminum metal (Section 18.13) and is a coordination compound of a main-group element. The oxidation state of aluminum is omitted from the name because aluminum has only one oxidation state (+3). Because F^{-} has a charge of -1, the anion is [AlF_{6}]^{3-}. The charge of the anion must be balanced by three Na^{+} cations; therefore, the formula for the compound is Na_{3}[AlF_{6}].
(d) Diamminesilver(I) ion, formed when silver chloride dissolves in an excess of aqueous ammonia, has the formula [Ag(NH_{3})_{2}]^{+}.