Question

Write the formula for each complex ion or coordination compound. \begin{equation}\begin{array}{l}{\text { a. hexamminechromium (III) }} \\\ {\text { b. potassium hexacyanoferrate }(\mathrm{III})} \\ {\text { c. ethylenediaminedithiocyanatocopper }(\mathrm{II})} \\ {\text { d. tetraaquaplatinum (II) hexachloroplatinate (IV) }}\end{array}\end{equation}

Short answer

[Cr(NH3)6]^{3+}, K3[Fe(CN)6], [Cu(en)2(SCN)2], [Pt(H2O)4][PtCl6]

Step-by-step solution

Identifying the Central Atom and Oxidation State

Begin by identifying the central metal atom and its oxidation state for each coordination compound. For example, hexamminechromium (III) contains chromium with an oxidation state of +3.

Writing the Ligands

Next, write down the ligands, making sure to use the correct prefixes for their quantity (e.g., hexa for six, tetra for four) and use the appropriate endings for anionic ligands (e.g., ammine for NH3, aqua for H2O, cyano for CN-).

Assembling the Formula

Combine the central metal atom, its oxidation state, and the ligands to form the chemical formula. Place the ligands first, in alphabetical order, followed by the metal atom and its oxidation state in Roman numerals if necessary.

Key concepts

Complex Ion Formula

Understanding the formula of a complex ion or a coordination compound is essential for students venturing into the realm of inorganic chemistry. To put it simply, the formula represents the combination of a central metal atom and the ligands attached to it through coordinate bonds.

A complex ion formula is broken down into two parts: the ligands surrounding the metal, and the metal atom itself with its associated oxidation state. When crafting such formulas, the ligands are listed first, usually in alphabetical order, followed by the metal atom. Let's illustrate with an example from our exercise: ethylenediaminedithiocyanatocopper (II) — here, the central metal is copper (Cu) with an oxidation state of +2. The ligands – ethylenediamine (en) and dithiocyanato (shown as S,C,N or dtn) – are placed before the copper. The complete formula would be \[ \text{[Cu(en)(S,C,N)_2]^{2+}} \] where 'en' represents two ethylenediamine molecules and 'S,C,N' represents two dithiocyanate ions.

Understanding this basic structure is critical for students to correctly interpret and construct formulas, aiding in visualizing the arrangement around the metal center.

Ligands in Coordination Chemistry

Ligands are molecules or ions that donate a pair of electrons to a metal atom or ion, forming a coordination complex. They are pivotal players in coordination chemistry. Ligands range from simple ions like chloride (Cl-) to complex organic molecules like ethylenediamine (en). The nature and number of ligands around the central metal determine the properties and reactivity of the coordination complex.

In our examples, the term 'hexammine' refers to six ammonia (NH3) ligands bound to the central chromium atom, resulting in \[ \text{[Cr(NH_3)_6]^{3+}} \] Similarly, 'hexacyano' means six cyanide (CN-) ions are coordinated to the central metal as seen in potassium hexacyanoferrate(III), with the formula \[ \text{[Fe(CN)_6]^{3-}} \] followed by three potassium ions to balance the charge. Each ligand type has its convention in naming, and it's necessary to learn these to construct formulas correctly.

Oxidation States

Oxidation states, also called oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound. Simply put, it's the hypothetical charge an atom would have if the compound were composed of ions. They are fundamental in determining the structure of the coordination compounds.

Using roman numerals to denote the oxidation state in the names of coordination complexes is a standard practice. For instance, chromium (III) indicates that chromium has an oxidation state of +3. Correctly identifying the oxidation state is vital when it comes to writing the formula for a coordination compound. In tetraaquaplatinum(II) hexachloroplatinate(IV), for example, we see platinum with two different oxidation states: +2 and +4. Hence, the formula is constructed as \[ \text{[Pt(H_2O)_4]^{2+}} \text{[PtCl_6]^{2-}} \] As students practice more exercises, they become adept at identifying oxidation states, which aids in conducting redox reactions and understanding the electronics of metal complexes.