Problem 25 - Chemistry A Molecular Approach

Question

Write the formula for each complex ion or coordination compound. a. hexaamminechromium(III) b. potassium hexacyanoferrate(III) c. ethylenediaminedithiocyanatocopper(II) d. tetraaquaplatinum(II) hexachloroplatinate(IV)

Step-by-Step Solution

Answer

[Cr(NH3)6]^{3+}, K3[Fe(CN)6], [Cu(en)(S2CN2)], [Pt(H2O)4][PtCl6]

Step 1: Recognize the Ligands and Central Metal

Identify the central metal ion and its oxidation state, as well as the ligands attached to it. Ammine (NH3) is a neutral ligand, cyanide (CN-) is a negatively charged ligand, ethylenediamine (en) is a neutral chelating ligand, dithiocyanato (S2CN2) indicates two SCN- ligands, aqua (H2O) is a neutral ligand and chloro (Cl-) is a negatively charged ligand.

Step 2: Write the Chemical Formula for the Complex Ions

For each complex, combine the central metal with its corresponding ligands taking into account the metal's oxidation state to balance the overall charge.

Step 3: Apply Naming to Coordination Compounds

Start with the cation part followed by the anion part and include the metal's oxidation state in parentheses. Use prefixes 'hexa,' 'tetra,' etc. to denote the number of ligands.

Key Concepts

Complex Ion Nomenclature

Nomenclature for complex ions is a systematic way of naming these structures, which include a central metal atom or ion surrounded by a number of ligands. When writing the formula, the central metal is listed first, followed by its ligands in alphabetical order. The ligands are always named before the metal within the complex. Coordination numbers and oxidation states of the central metal dictate how many ligands are attached and their respective charges, ensuring that the overall charge of the complex is correct.

For example, in hexaamminechromium(III), 'hexa' indicates six ammine (NH₃) ligands while the Roman numeral III indicates a +3 oxidation state for chromium. The entire complex would be written as [Cr(NH₃)₆]³⁺. It is important to note that the ligands such as ammine, aqua, and ethylenediamine retain their neutral charge, while others like cyanide possess negative charges and contribute to balancing the overall charge of the complex ion.

Ligand Identification

Identifying ligands is crucial for correctly writing the formula of coordination compounds. Ligands are ions or molecules that donate a pair of electrons to the central metal atom to form coordinate bonds. They can be neutral, like water (aqua) or ammonia (ammine), or negatively charged like cyanide (CN-) and thiocyanate (SCN-). Neutral ligands are written with their molecular formulae, while negative ligands often have special names ending in 'o', such as 'cyano' for cyanide.

In ethylenediaminedithiocyanatocopper(II), 'ethylenediamine' (en) is a bidentate ligand meaning it binds through two donor atoms, while 'dithiocyanato' suggests two monodentate thiocyanate ligands. 'Copper(II)' signifies that the copper ion has a +2 oxidation state, which influences how many and what charge of ligands will surround it to balance the metal's charge.

Oxidation State Determination

Determining the oxidation state of the central metal in a coordination compound is essential to writing its formula correctly. The oxidation state is the charge that the metal would have if all ligands were removed along with the electrons they shared. You can deduce the oxidation state by balancing the overall charge of the complex ion with the sum of charges from the ligands.

Using tetraaquaplatinum(II) hexachloroplatinate(IV) as an example, the '(II)' and '(IV)' represent the oxidation states of platinum in each part of the compound. Since aqua is a neutral ligand, you know that in the first part of the compound, the charge of the complex is solely due to the metal's oxidation state. In the hexachloroplatinate(IV) ion, however, we need six chloride ions, each with a -1 charge, to balance the +4 oxidation state of platinum resulting in a complex anion with a 2- charge.