Question

Draw the Lewis diagrams for each ligand. Indicate the lone pair(s) that may be donated to the metal. Indicate any you expect to be bidentate or polydentate. $$ { a. } \ \mathrm{NH}_{3} \quad \text { b. SCN }^{-} \quad \text { c. } \mathrm{H}_{2} \mathrm{O} $$

Short answer

NH3 has one nitrogen with one lone pair acting as monodentate ligand. SCN- can be bidentate, with lone pairs on S and N available to donate. H2O has one oxygen with two lone pairs acting as a monodentate ligand.

Step-by-step solution

Drawing the Lewis Structure for NH3

Ammonia (NH3) has one nitrogen (N) atom bonded to three hydrogen (H) atoms. Nitrogen has 5 valence electrons and each hydrogen has 1 valence electron, for a total of 8 valence electrons. Each hydrogen shares one electron with nitrogen to form a covalent bond. Nitrogen has one lone pair of electrons remaining after forming the bonds. In the Lewis structure, the nitrogen atom is at the center surrounded by three hydrogen atoms, and there is one pair of non-bonding electrons (lone pair) on the nitrogen.

Drawing the Lewis Structure for SCN-

The thiocyanate anion (SCN-) has a sulfur (S) atom, a carbon (C) atom, and a nitrogen (N) atom. The S will typically act as the central atom, with C bound to it and N bound to C. S contributes 6 valence electrons, C contributes 4, and N adds 5, plus 1 extra electron for the negative charge, totaling 16 valence electrons. C and N form triple bonds with each other, and each will have a lone pair. S and C share a single bond, with S having two lone pairs. SCN- can act as a bidentate ligand, coordinating through the S and N atoms.

Drawing the Lewis Structure for H2O

Water (H2O) has one oxygen (O) atom bonded to two hydrogen (H) atoms. Oxygen has 6 valence electrons and each hydrogen has 1 valence electron, for a total of 8 valence electrons. Each hydrogen shares one electron with oxygen to form a covalent bond, which leaves oxygen with two lone pairs of electrons. In the Lewis structure, the oxygen atom is at the center with two hydrogen atoms attached, and there are two pairs of non-bonding electrons (lone pairs) on the oxygen.

Key concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules and compounds. It is fundamental to our understanding of chemistry and essential for life as we know it. Bonds between atoms are created through the sharing or transfer of valence electrons, which are the outermost electrons of an atom. There are several types of chemical bonds, with covalent and ionic bonds being the most prevalent.

Covalent bonds involve the sharing of electron pairs between atoms, allowing each atom to achieve a full valence electron shell, resembling that of the noble gases. In the exercise, the molecules of ammonia (NH₃) and water (H₂O) both demonstrate covalent bonding. Ammonia displays a classic trifecta of bonds where nitrogen shares its electrons with three hydrogen atoms. Similarly, in water, each hydrogen atom shares its electrons with oxygen, resulting in two covalent bonds.

Ionic bonds, on the other hand, involve the transfer of electrons from one atom to another, resulting in the formation of positively and negatively charged ions. These ions are held together by the strong electrostatic forces between oppositely charged particles. Thiocyanate (SCN⁻) is an example of a molecule that can engage in ionic bonding due to its negative charge, allowing it to bond with positively charged metal ions.

Valence Electrons

Valence electrons are the electrons located in the outermost electron shell of an atom, and they play a pivotal role in chemical bonding. These electrons can be shared with other atoms to form covalent bonds, donated or received in ionic bonds, or used to form metallic bonds. The number of valence electrons an element has determines its chemical reactivity and bonding potential.

In the provided exercise, the valence electrons are used to determine how atoms will interact with each other. Nitrogen, for instance, has 5 valence electrons, which allows it to form three covalent bonds with hydrogen atoms in ammonia and keep one lone pair. Oxygen, with 6 valence electrons, forms two bonds with hydrogen in water and retains two lone pairs.

Understanding the concept of valence electrons is crucial when predicting molecule formation, bond types, and chemical reactivity. The ability to draw Lewis diagrams is directly tied to knowing the number of valence electrons for each element involved, as seen in the step-by-step solution for NH₃, SCN⁻, and H₂O.

Ligands

Ligands are ions or molecules that can donate a pair of electrons to a central metal ion to form a coordination complex. The ligands act like 'glue' that can bind to metals, forming coordination compounds with diverse properties and structures. In the context of the exercise, the ligands considered are NH₃, SCN⁻, and H₂O.

Ligands can be classified based on their binding sites. Monodentate ligands, such as NH₃ and H₂O, bind through a single atom, donating a lone pair of electrons to the metal. Bidentate or polydentate ligands, like SCN⁻, have multiple binding sites; SCN⁻ can coordinate through both the sulfur and nitrogen atoms.

Bidentate Ligands

The versatility of ligands like SCN⁻ allows them to form more complex and stable interactions with metals. Their ability to attach at two points can create ring structures, enhancing the stability of the metal complex. This characteristic is called chelation, and such ligands are referred to as chelating agents. Understanding the different types of ligands and their binding modes is crucial for the synthesis of new compounds in inorganic chemistry.