Question

Give formulas for the following. a. hexakis(pyridine)cobalt(III) chloride b. pentaammineiodochromium(III) iodide c. tris(ethylenediamine)nickel(II) bromide d. potassium tetracyanonickelate(II) e. tetraamminedichloroplatinum(IV) tetrachloroplatinate(II)

Short answer

\([Co(\text{C}_5\text{H}_5\text{N})_6]Cl_3\), \([Cr(\text{NH}_3)_5 \text{I}]I_2\), \([Ni(\text{en})_3]Br_2\), \(K_2[Ni(\text{CN})_4]\), and \([Pt(\text{NH}_3)_4\text{Cl}_2][PtCl_4]\).

Step-by-step solution

Identify the ligand and the metal

In this complex, the ligand is pyridine and it is present six times, indicated by 'hexakis'. The metal is cobalt and its oxidation state is III. The complex is a cation thus named as it is and the counter anion, chloride, follows.

Write down the formula

Because there are six pyridine ligands, we denote it as \( \text{C}_5\text{H}_5\text{N} \) and since cobalt has an oxidation state of III, the corresponding Roman numeral is included in parentheses. The chloride ion is written as \(\text{Cl}^{-}\). Putting it all together: \([Co(\text{C}_5\text{H}_5\text{N})_6]Cl_3\). b. pentaammineiodochromium(III) iodide

Identify the ligand and the metal

In this complex, ammonia is the ligand, indicated by 'ammine', and it is present five times, indicated by 'penta'. The metal is chromium and its oxidation state is III. The complex is a cation, thus it is named first, and the counter anion, iodide (I-), follows.

Write down the formula

Because there are five ammonia ligands, write it as \( \text{NH}_3 \). As it's 'iodo' it explicitly tells we also have an iodine directly bound to the chromium, so \( \text{I} \) is also a part of the complex. After, the anion iodide is written as \(\text{I}^{-}\). Putting it all together: \([Cr(\text{NH}_3)_5 \text{I}]I_2\). c. tris(ethylenediamine)nickel(II) bromide

Identify the ligand and the metal

In this complex, the ligand is ethylenediamine and it is present three times, indicated by 'tris'. The metal is nickel and its oxidation state is II. The complex is cationic and the counter anion, bromide, follows.

Write down the formula

Because there are three ethylenediamine ligands, we denote it as \( \text{en} \). Putting it all together: \([Ni(\text{en})_3]Br_2\). d. potassium tetracyanonickelate(II)

Identify the ligand and the metal

In this complex, the ligand is cyanide and it is present four times, indicated by 'tetra'. The metal is nickel and its oxidation state is II. The complex is anionic and the counter cation, potassium, is named first.

Write down the formula

Because there are four cyanide ligands, we denote it as \( \text{CN} \). Putting it all together: \(K_2[Ni(\text{CN})_4]\). e. tetraamminedichloroplatinum(IV) tetrachloroplatinate(II)

Identify the ligands and the metals

In this complex, there are two complex ions. In the first one, ammonia is the ligand (indicated by 'ammine') presented four times (indicated by 'tetra') while chloride (indicated by 'dichloro') is present twice. The metal is platinum with oxidation state IV. In the second complex, the ligand is also chloride presented four times (indicated by 'tetrachloro'). The metal is again platinum but with oxidation state II.

Write down the formula

In the first complex, there are four ammonia ligands, denoted as \( \text{NH}_3 \), and two chloride ligands, denoted as \( \text{Cl} \). Thus, the first complex is \([Pt(\text{NH}_3)_4\text{Cl}_2]\). In the second complex, there are four chloride ligands, so this complex is \([PtCl_4]^{2-}\). Putting it all together: \([Pt(\text{NH}_3)_4\text{Cl}_2][PtCl_4]\).

Key concepts

Ligand Naming

Ligand naming is an essential part of understanding coordination complexes. Ligands are ions or molecules that bind to a central metal atom to form coordination compounds. To name ligands within these complexes, it is important to recognize some rules:

• Neutral ligands are usually named as the molecule itself, such as water being "aqua," or ammonia being "ammine."
• Anionic ligands often take the root name of the group and add an "-o" at the end. For example, chloride becomes "chloro," cyanide becomes "cyano."
• If there are multiple identical ligands, prefixes like mono-, di-, tri-, tetra-, penta-, and hexa- are used, but when dealing with polydentate ligands like ethylenediamine, Latin/Greek prefixes like bis-, tris-, and tetrakis- are employed to avoid confusion.

Understanding these naming guidelines makes it much easier to write and recognize chemical formulae of coordination complexes.

Metal Oxidation State

Metal oxidation state in coordination chemistry refers to the charge of the metal atom within the complex. Recognizing and calculating this value is vital because it impacts the properties and reactivity of the chemical complex.

• The oxidation state is often surrounded by parentheses, appearing after the metal's name in Roman numerals, such as cobalt(III) in hexakis(pyridine)cobalt(III) chloride.
• This oxidation state can be calculated by summing the known charges of the ligands and the overall charge of the complex to determine the metal's state. For example, in a neutral complex like \[[Ni(en)_3]^{2+}\], knowing the ethylenediamine ligand is neutral indicates that the nickel must balance with a +2 oxidation state to result in the overall charge indicated.
• Knowing the oxidation state helps predict the stability, color, and structure of the coordination compound.

Chemical Complexes

Chemical complexes, also known as coordination complexes, are structures formed by the attachment of ligands to a central metal atom. Understanding these structures is crucial in inorganic chemistry:

• The central metal atom, usually a transition metal, has empty orbitals that can accept electron pairs from the ligand, forming a coordinate covalent bond.
• Such complexes often possess an electric charge, either positive or negative, that results from the sum of ligand charges and the metal's oxidation state.
• Complexes can exhibit different geometries based on their coordination number, such as square planar, tetrahedral, or octahedral, which impacts their physical properties and reactivity.
• The interaction between the central metal and the ligands influences the color, magnetism, and potential catalytic activity of the complexes.

These complexes are widely studied for their applications in catalysis, materials science, and medicine.

Formula Writing

Writing formulas for coordination compounds can initially seem daunting, but understanding the systematic approach makes it manageable:

• Start by identifying the metal and its oxidation state, followed by the ligands and how many are present. This is reflected in the prefix used in their naming, such as tetra for four or tri for three.
• Write the formula for the entire coordination sphere enclosed in square brackets. For example, the complex \[[Co(C_5H_5N)_6]Cl_3\] shows cobalt complexed with six pyridine molecules.
• Outside the brackets, add any counterions needed to balance the charges of the entire molecule. This might result in the need to multiply different counter ions or metals.
• Ensure the compound reflects the correct stoichiometry implied in its name. Formula writing communicates not just the make-up of a complex but hints at its possible properties.

Mastering this skill is a valuable asset for understanding both the composition and potential reactions of coordination compounds.