Question

Write the formulas for each of the following ions and compounds: (a) dichlorobis(ethylenediamine) chromium(III), (b) pentacarbonyliron(0), (c) potassium tetracyanocuprate(II), (d) tetraammineaquachlorocobalt(III) chloride.

Short answer

(a) CrCl2(en)2, (b) Fe(CO)5, (c) K2[Cu(CN)4], (d) [Co(NH3)4(H2O)Cl]Cl2.

Step-by-step solution

Identifying Ions and Ligands

First, identify the metal center, ligands, their charges, and oxidation states for each compound. (a) Dichlorobis(ethylenediamine)chromium(III) involves chromium with ethylenediamine and chloride ligands. (b) Pentacarbonyliron(0) has iron bound to carbon monoxide ligands. (c) Potassium tetracyanocuprate(II) includes copper with cyanide ligands and potassium as the counterion. (d) Tetraammineaquachlorocobalt(III) chloride consists of cobalt, ammonia, water, and chloride ligands with an additional chloride as a counterion.

Writing the Formulas

Next, write out the chemical formulas. (a) For dichlorobis(ethylenediamine)chromium(III), it's CrCl2(en)2, where 'en' stands for ethylenediamine. (b) Pentacarbonyliron(0) is written as Fe(CO)5. (c) Potassium tetracyanocuprate(II) has the formula K2[Cu(CN)4]. (d) Tetraammineaquachlorocobalt(III) chloride is [Co(NH3)4(H2O)Cl]Cl2.

Verifying Oxidation States and Charge Balance

Verify the oxidation states and overall charge balance in each compound formula. (a) Chromium(III) indicates Cr is in a +3 oxidation state; thus, the formula CrCl2(en)2 balances with the charges of the ligands. (b) Iron in Fe(CO)5 is in the zero oxidation state, consistent with the neutral CO ligands. (c) Copper in K2[Cu(CN)4] is in a +2 state, balanced by the two cyanide ions. The potassium ions are necessary to balance the charge electrically. (d) Cobalt in [Co(NH3)4(H2O)Cl]Cl2 has a +3 oxidation state; the formula balances with the ammonia, water, and chloride ligands and counter-ions.

Key concepts

Coordination Compounds

Coordination compounds are fascinating structures composed of a central metal atom or ion surrounded by a group of molecules or anions called ligands. These compounds are crucial in various fields like biochemistry, medicine, and industrial chemistry.
In our examples, these compounds feature different metal centers: chromium, iron, copper, and cobalt. Each metal ion forms a coordination complex by establishing coordinate covalent bonds with ligands known for donating electron pairs.
Understanding coordination compounds involves knowing the type and number of ligands attached to the metal center, which can greatly influence the properties and reactivity of the complex. The arrangement of these ligands around the metal ion, known as the coordination sphere, can vary, leading to diverse geometric structures, such as octahedral or tetrahedral, which are common in coordination chemistry.

Oxidation States

Oxidation states are a central concept in understanding the properties of coordination compounds. They represent the degree of oxidation of an atom within a compound, reflecting the number of electrons lost or gained relative to the elemental state.
To find the oxidation state of a metal in a coordination compound, consider the known charges of the ligands and the overall charge of the complex. For instance, in dichlorobis(ethylenediamine) chromium(III), the '-bis(ethylenediamine)' and 'dichloro' parts suggest the presence of chloride ions with a -1 charge each and a neutral ethylenediamine ligand, indicating the chromium ion is in the +3 oxidation state.
Determining the correct oxidation state is vital because it affects the compound's color, magnetic properties, and reactivity. It's worth noting that ligand type can also influence the oxidation state, offering insights into how these compounds might interact or bond with other substances in chemical reactions.

Ligands

Ligands are molecules or ions that donate electron pairs to form coordinate bonds with the central metal atom in a coordination compound. Understanding ligands is crucial for grasping how coordination complexes are structured and interact.
There are various types of ligands:

• Monodentate ligands: Donate one pair of electrons (e.g., chloride ion, water molecule).
• Bidentate ligands: Donate two pairs of electrons, as seen in ethylenediamine (en) in the complex CrCl2(en)2.
• Polydentate ligands: Can donate multiple electron pairs, often forming more stable complexes due to the chelate effect.

Ligands determine the shape, stability, and reactivity of coordination compounds. They can significantly impact a complex's electronic environment, ultimately influencing its color and how it engages in chemical reactions. Understanding the nature of ligands allows chemists to predict and modulate the properties of coordination compounds for desired applications.

Charge Balance

In coordination chemistry, maintaining charge balance in complexes is fundamental. Charge balance means ensuring that the sum of the charges from the metal center, ligands, and any counterions results in a neutral or specified charge for the compound.
This involves:

• Summing the charges of the ligands and the metal to ensure the internal charge within the complex is what’s expected.
• Adding counterions if necessary, to achieve the overall charge neutrality or desired charge of the entire compound. For example, potassium ions are needed in K2[Cu(CN)4] to balance the charge of the overall complex.

Always check charge balance when writing formulas of coordination compounds. It's an automatic verification step to confirm the validity of complex structures. Correct charge balancing is crucial for compound stability and reactivity, as any imbalance can lead to unexpected chemical behavior.