Problem 9 - General Chemistry: Principles and Modern Applications

Question

Draw plausible structures corresponding to each of the following names. (a) pentamminesulfatochromium(III) ion (b) trioxalatocobaltate(III) ion (c) triamminedichloronitrito-O-cobalt(III)

Step-by-Step Solution

Answer

The detailed structures of the complex ions named are pentamminesulfatochromium(III) ion, trioxalatocobaltate(III) ion and Triamminedichloronitrito-O-cobalt(III) respectively. All structures ensure correct accounting of ionic charges, correct representation of ligands and appropriate labelling of central metal ions.

Step 1: Draw Pentamminesulfatochromium(III) Ion

The name tells us that the metal is chromium (III) meaning it has a 3+ charge. The ligands are ammonia (NH3 - known as ammine in coordination complexes) and sulfate (SO4 2-). There are 5 ammine and 1 sulfato ligands. Since sulfate has a 2- charge, the five neutral ammine ligands and 3+ Chromium form a complex ion with a net charge of 1+.

Step 2: Draw Trioxalatocobaltate(III) Ion

The metal is cobalt (III) meaning 3+ charge. The ligand is oxalate (C2O4 2-). There are 3 oxalate ligands. The total negative charge from the oxalate ligands is 2- x 3 = -6. Combined with the 3+ charge on cobalt, the overall charge on the entire ion is 3-, so it’s a complex anion.

Step 3: Draw Triamminedichloronitrito-O-cobalt(III)

The metal ion is cobalt (III) meaning 3+ charge. The ligands are ammonia (NH3 - known as ammine in coordination complexes), chloride (Cl-), and nitrite (NO2 -). The nitrito has two different modes of bonding: it can be either nitrito-N or nitrito-O. Here it's nitrito-O as aroused from the name, which means the Oxygen in nitrite bonds to the central cobalt. The entire complex ion has a neutral charge, because the charges of the ligands and the central metal ion balance each other out.

Key Concepts

Ligand

In the realm of coordination chemistry, ligands play a starring role. A ligand is a molecule or ion that binds to a central metal atom to form a complex. This bonding is usually achieved through a coordinate or dative covalent bond, where the ligand donates a pair of electrons to the metal. Ligands are versatile and can be a wide range of substances, including ions, atoms, and molecules.

Their nature can significantly influence the properties and characteristics of the complex ion they help form. Ligands are classified based on their denticity, which refers to how many donor atoms they have:

• Unidentate ligands: They have one donor atom that binds to the metal atom. Examples include the ammonia (NH3) as seen in pentamminesulfatochromium(III) ion.
• Bidentate ligands: These can bind through two sites. An example is the oxalate ion (C2O4 2-) in trioxalatocobaltate(III) ion.
• Polydentate ligands: Ligands that bind through multiple atoms, forming rings (chelates) with the central metal atom, increase the stability of the complex.

As they come in various shapes and sizes, ligands can greatly impact the geometry, reactivity, and even color of coordination compounds.

Complex Ion

Complex ions are fascinating entities in coordination chemistry, consisting of a metal ion at its center bound to various ligands. These ions can carry a positive, negative, or neutral charge depending on the combined charges of the metal and the ligands. The overall charge and geometry of complex ions influence their role in chemical reactions and their unique properties.

For example, pentamminesulfatochromium(III) ion forms a positive complex due to the combination of chromium's 3+ charge with neutrally charged ammonia and sulfate ligands. Trioxalatocobaltate(III) ion, however, forms a negatively charged complex due to its cobalt 3+ center ion paired with multiple negatively charged oxalate ions. Understanding the balance between the metal ion and its ligands is essential to grasping the nature of complex ions. They are structurally diverse:

• Octahedral: Typically observed when a central metal ion is surrounded by six ligands, such as in many cobalt(III) complexes.
• Square planar: Frequently seen in certain nickel and platinum complexes.
• Tetrahedral: Common when four ligands bond to a central metal ion, often seen in zinc complexes.

Exploring complex ions reveals much about their role in biological processes, industrial applications, and materials science.

Charge Balance

Charge balance is a crucial concept in understanding coordination compounds and involves ensuring that the total charges of all components in a complex add up to the net charge of the ion. In coordination chemistry, the central metal ion and its surrounding ligands must cooperate to result in a balanced charge for the entire complex.

Consider the triamminedichloronitrito-O-cobalt(III) complex. Here, a cobalt(III) center which holds a 3+ charge is balanced by three ammonia molecules (neutral), two chloride ions (each with a charge of -1), and one nitrito ligand (also neutral when bonding through oxygen). A key rule is:

• The sum of all ligand and metal ion charges must equal the overall charge of the complex ion. If the charges perfectly balance, the ion is neutral, as in the triamminedichloronitrito-O-cobalt(III) complex.
• When total ligand charges exceed the metal's positive charge, the complex carries a negative net charge, like in trioxalatocobaltate(III).
• Conversely, if the metal's positive charge is not fully offset, a positive complex ion forms, as seen in pentamminesulfatochromium(III).

By mastering charge balance in coordination compounds, one can predict and design complexes with desired properties and reactivities.