Question

In a formula, what is the order of the metal and ligands?

Short answer

In the chemical formula for a coordination compound, the metal is written first and then the ligands in alphabetical order.

Step-by-step solution

Identify the metal

In a coordination compound, start by identifying the metal ion that is bonded to one or more ligands.

List the ligands

After identifying the metal, list all the ligands that are attached to the metal ion. Ligands are ions or molecules that bind to the central metal ion in a complex.

Determine the order

In a chemical formula for a coordination compound, the metal is written first, followed by the ligands. Ligands are listed in alphabetical order by the ligand name (ignoring any prefixes like 'di-', 'tri-', etc.).

Apply Formatting Conventions

If the complex is an anion, the name ends in 'ate'. Use proper punctuation and formatting such as brackets to denote the coordination sphere.

Key concepts

Metal Ion Identification

Understanding the composition of coordination compounds begins with metal ion identification. This requires a clear recognition of the metal that serves as the centerpiece of the complex. It's typically represented by its symbol from the periodic table and is the reference point from which the structure of the compound is understood. In practice, the metal ion's positive charge is balanced by the ligands, which can be neutral molecules or negatively charged ions.

When looking at a coordination compound's formula, expect to see the metal ion positioned at the very beginning. For example, in the compound \[\text{[Cu(NH_3)_4]^2+}\], copper (Cu) is the central metal ion. This information is critical as it often influences the properties of the compound such as its color, magnetic behavior, and reactivity.

Ligand Listing

Once the metal ion is identified, the next step is ligand listing. Ligands are atoms, ions, or molecules that donate at least one pair of electrons to form a coordinate covalent bond with the metal ion. They are crucial in determining the geometry and the overall chemical behavior of the coordination compound.

To list the ligands, examine the remainder of the formula and note each unique ligand present. One must be mindful that prefixes such as 'di-', 'tri-', or 'tetra-' are not used to determine the alphabetical order in the formula. Ligands must be listed alphabetically based on their chemical name, not the quantity, nor the charge. For clarity, let's illustrate this with the compound \[\text{[Co(NH_3)_5Cl]Cl_2}\]: here 'ammine' (NH_3) and 'chloro' (Cl) are the ligands, with ammine coming first in the listing.

Chemical Formula Conventions

Chemical formula conventions for coordination compounds involve specific rules to accurately represent the structure and composition of these species. After identifying the metal and ligands, it's important to write them according to the prescribed order in the formula. As mentioned, the metal is named first followed by the ligands in alphabetical order, disregarding numerical prefixes.

Moreover, coordination compounds' formulas are often enclosed in square brackets to indicate that the ligands are directly attached to the metal, forming the coordination sphere. In cases where the compound carries a charge, this is indicated outside the brackets, such as in \[\text{[Cu(NH_3)_4]^2+}\]. Any counterions are listed outside the brackets and are not included in the alphabetical ordering of the ligands. Proper punctuation must be used, such as parentheses around polyatomic ligands and subscripts to indicate the number of each ligand. For multi-dentate ligands, or those that attach at multiple binding sites, their names often contain 'bis-', 'tris-', etc., which are considered when naming the compound but not when arranging the formula.

Coordination Complex Nomenclature

The nomenclature of coordination compounds is a standardized way to name these chemicals based on their composition. The coordination complex nomenclature involves many rules, but a few primary ones include naming the metal first, followed by the ligands with appropriate prefixes to indicate their quantities. The names of anionic ligands often end in 'o', while neutral ligand names are normally the molecule's common name.

If the coordination complex forms an anion, the metal's name ends with '-ate', such as in 'cuprate' for a negative copper complex. Greek prefixes are used to describe the number of each type of ligand, and when the entire complex is an anion, it is typically paired with a cation, which is named first. For example, the compound \[\text{hexaamminecobalt(III) chloride}\] or \[\text{[Co(NH_3)_6]Cl_3}\] features 'hexa-' to indicate six ammine ligands and states the oxidation state of cobalt as (III), followed by the counterion 'chloride'. This precise language ensures clear communication among chemists regarding the structure of coordination compounds.