Problem 78 - General Chemistry: Principles and Modern Applications

Question

Write appropriate formulas for the following species. (a) dicyanoargentate(I) ion (b) triamminenitrito-N-platinum(II) ion (c) aquachlorobis(ethylenediamine)cobalt(III) ion (d) potassium hexacyanochromate(II)

Step-by-Step Solution

Answer

(a) \(Ag(CN)_2^-\) \n (b) \([Pt(NH_3)_3(NO_2)]^{2+}\) \n (c) \([Co(C_2H_8N_2)_2(H_2O)Cl]^{3+}\) \n (d) \(K_2[Cr(CN)_6] \)

Step 1: Formulate Dicyanoargentate(I) Ion

Dicyanoargentate(I) ion consists of cyanide group (CN-) and Argentum (Ag+) with -ate indicating it is a complex ion. Therefore, its formula is \(Ag(CN)_2^-\)

Step 2: Formulate Triamminenitrito-N-platinum(II) Ion

Triamminenitrito-N-platinum(II) ion has three ammine (NH3) ligands, one nitrite ligand (NO2-) and Platinum in a +2 oxidation state. Thus, its formula is \([Pt(NH_3)_3(NO_2)]^{2+}\)

Step 3: Formulate Aquachlorobis(ethylenediamine)cobalt(III) Ion

Aquachlorobis(ethylenediamine)cobalt(III) ion has an aqua (water) ligand, a chloro (Cl-) ligand, two ethylenediamine ligands, and a Cobalt atom in a +3 oxidation state. Thus, its formula is \([Co(C_2H_8N_2)_2(H_2O)Cl]^{3+}\)

Step 4: Formulate Potassium Hexacyanochromate(II)

Potassium hexacyanochromate(II) contains one Chromium (Cr) atom, six cyanide (CN-) ligands and potassium (K+) ions. Since Cr holds a charge of 2, two potassium ions are required for charge balance. Its formula is \(K_2[Cr(CN)_6]\)

Key Concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry that deals with the study of complex ions or coordination compounds. These compounds consist of a central metal atom or ion bonded to a group of surrounding molecules or ions known as ligands. The central atom or ion, also referred to as the coordination center, often forms coordinate covalent bonds with the ligands. The number of ligands attached to the center is called the coordination number, which can vary depending on the size, charge, and electron configuration of the metal atom.

The bonds in a coordination compound are unique because the ligands provide both of the electrons for the bond. Ligands can be neutral molecules like ammonia (NH₃) or water (H₂O), or anions like chloride (Cl⁻) or cyanide (CN⁻). The specific arrangement of ligands around the central atom in a complex can lead to different geometric structures, like tetrahedral or octahedral shapes, which significantly influence the properties of the compound.

The importance of coordination chemistry extends to many fields including biochemistry, medicinal chemistry, and materials science, as the principles are fundamental to the function of many biological systems and the synthesis of new materials.

Naming Coordination Compounds

Naming coordination compounds follows specific rules set by the International Union of Pure and Applied Chemistry (IUPAC). The overall goal is to convey the composition of the complex ion in the name itself. A coordination compound name firstly lists the ligands in alphabetical order, regardless of their charge. Neutral ligands are generally called by their normal names, such as water being called 'aqua', whereas negatively charged ligands often have names ending in 'o', such as 'chloro' for chloride or 'cyano' for cyanide.

After the ligands, the central metal's name is stated. If the complex ion is negatively charged, the metal's name ends in '-ate', like in 'dicyanoargentate(I)'. The oxidation state of the metal in the complex is indicated in parentheses using Roman numerals after the metal name. Additionally, if there are multiple of the same ligand, prefixes such as di-, tri-, or hexa- are used to indicate the quantity.

For example:

The compound \[Pt(NH_3)_3(NO_2)\]^{2+} is named triamminenitrito-N-platinum(II), indicating it has three ammine ligands, one nitrito ligand coordinating through nitrogen, and platinum in the +2 oxidation state.

Oxidation State

The oxidation state, or oxidation number, represents the total number of electrons an atom either gains or loses to form a chemical bond with another atom. In coordination chemistry, determining the oxidation state of the metal ion is crucial as it provides information about the possible number of ligands, the geometry, and the reactivity of the complex ion.

When calculating the oxidation state of the central atom in a complex, sum the charges of all the ligands and any counter-ions present to neutralize the overall charge of the compound. The oxidation state of the central metal is then the charge it would have if all ligands and paired electrons were removed.

For example, in the complex ion \[Co(C_2H_8N_2)_2(H_2O)Cl\]^{3+}, cobalt has an oxidation state of +3. This can be determined by analyzing the charges of the individual ligands and applying the fact that the overall charge of the complex ion is +3. Understanding oxidation states is essential because it helps predict the physical and chemical properties of the coordination compound.