Question

Write chemical formulas for each of the following: (a) tetraaquadicyanoiron(III) ion (b) tetraammineoxalatonickel(II) (c) pentaaquahydroxoaluminum(III) chloride (d) potassium hexathiocyanatomanganate(III) (e) tetrachlorocuprate(II) ion

Short answer

[Fe(H2O)4(CN)2]+, [Ni(NH3)4(C2O4)], [Al(H2O)5(OH)]Cl2, K3[Mn(SCN)6], [CuCl4]2-

Step-by-step solution

Determine the Chemical Formula for Tetraaquadicyanoiron(III) Ion

Tetraaquadicyanoiron(III) ion contains an iron atom in the +3 oxidation state, four water molecules (aqua) attached, and two cyanide ions. Using the formula for water (H2O) and cyanide (CN-), and noting the overall charge, we combine them with the iron (Fe) with its +3 charge to get the formula. The iron ion's charge is balanced by the cyanide ions, leading to the formula \([Fe(H_2O)_4(CN)_2]^{+}\).

Determine the Chemical Formula for Tetraammineoxalatonickel(II)

This complex contains a nickel atom in the +2 oxidation state, four ammonia (ammine) molecules attached, and one oxalate group. Considering the charge of oxalate (C2O4^2-) and ammonia (NH3), the formula is \([Ni(NH_3)_4(C_2O_4)]\) as oxalate is a bidentate ligand and balances the +2 charge of the nickel.

Determine the Chemical Formula for Pentaaquahydroxoaluminum(III) Chloride

The complex contains an aluminum atom in the +3 oxidation state, five water molecules and one hydroxide group. The chloride, which is outside the complex, is a counter ion. The formula for this complex is \([Al(H_2O)_5(OH)]Cl_2\) because the hydroxide ion brings one negative charge, leaving two positive charges to be balanced by two chloride ions.

Determine the Chemical Formula for Potassium Hexathiocyanatomanganate(III)

Here, we have a manganese ion in the +3 oxidation state, six thiocyanate (SCN-) ligands, and the complex is a salt with potassium. Considering the -1 charge of each thiocyanate, the formula is \(K_3[Mn(SCN)_6]\) with three potassium ions required to balance the +3 charge of the manganese ion.

Determine the Chemical Formula for Tetrachlorocuprate(II) Ion

This complex ion includes a copper atom in the +2 oxidation state and four chloride ions. Chloride carries a -1 charge. Thus, the formula for the tetrachlorocuprate(II) ion is \([CuCl_4]^{2-}\), with four chlorides contributing to a total charge of -4 which balances the +2 charge of copper.

Key concepts

Coordination Chemistry

Coordination chemistry involves the study of complex compounds formed when central metal atoms or ions bind with molecules or anions known as ligands. These interactions result in the formation of coordination compounds, which have distinctive structures, colors, and chemical behaviors. A key feature of these compounds is the coordination number, which signifies the number of ligand attachment points to the metal ion.

For example, the compound from the exercise tetraaquadicyanoiron(III) ion, contains an iron ion surrounded by water and cyanide ligands. The water ligands provide a 'sphere of coordination,' which is typical in coordination chemistry and influences the properties of the iron ion.

Oxidation State

The oxidation state or oxidation number is a critical concept in coordination chemistry that indicates the degree of oxidation of an atom within a compound. For metals in coordination compounds, it dictates how many electrons are shared with the ligands.

For instance, in the tetraammineoxalatonickel(II) complex, the nickel ion has an oxidation state of +2. It highlights the number of electrons nickel can share or transfer to ligands. Understanding oxidation states, as in the solved step 2, is vital to correctly write chemical formulas for such complexes.

Ligands

Ligands are ions or molecules that can donate at least one pair of electrons to form coordinate bonds with the central metal ion in a complex. They are broadly classified as monodentate, bidentate, or polydentate, depending on the number of attachment points to the metal.

In the chemical formula for the complex ion tetraammineoxalatonickel(II), ammonia acts as a monodentate ligand bonding through a single pair of electrons, whereas oxalate is a bidentate ligand since it can attach through two sites. These ligands are fundamental in creating the specific geometries and properties of coordination compounds.

Complex Ions

Complex ions consist of a central metal ion surrounded by ligands. These ligands form coordinate covalent bonds with the metal, resulting in a charged entity. The charge on a complex ion is the algebraic sum of the oxidation state of the metal and the charges on the ligands.

For example, the potassium hexathiocyanatomanganate(III) from the exercise results in a complex ion with manganese (III) surrounded by six thiocyanate anions. The central metal's charge is balanced by counter ions, in this case, potassium, outside the complex ion. These ions play a crucial role in the solubility, reactivity, and magnetic properties of the coordination compounds.