Theoretical basics of Inorganic chemistry

Q1. Define the term coordination compound.

Answer: In chemistry, a coordination complex consists of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents.  Many metal-containing compounds, especially those of transition metals, are coordination complexes. A coordination complex whose centre is a metal atom is called a metal complex.

 

Q2. The structure of coordination compounds. Denticity.

Answer: The ions or molecules surrounding the central atom are called ligands. Ligands are generally bound to the central atom by a coordinate covalent bond (donating electrons from a lone electron pair into an empty metal orbital), and are said to be coordinated to the atom. There are also organic ligands such as alkenes whose pi bonds can coordinate to empty metal orbitals. An example is ethene in the complex known as Zeise's salt, K⁺[PtCl₃(C₂H₄)]⁻.

 

Q3. Classification and nomenclature of complex compounds.

Answer:

·         The name of the positive ion is written before the name of the negative ion.

·         The name of the ligand is written before the name of the metal to which it is coordinated.

·         The Greek prefixes mono-, di-, tri-, tetra-, penta-, hexa-, and so on are used to indicate the number of ligands when these ligands are relatively simple. The Greek prefixes bis-, tris-, and tetrakis- are used with more complicated ligands.

·         The names of negative ligands always end in o, as in fluoro (F-), chloro (Cl-), bromo (Br-), iodo (I-), oxo (O2-), hydroxo (OH-), and cyano (CN-).

·         A handful of neutral ligands are given common names, such as aquo (H2O), ammine (NH3), and carbonyl (CO).

·         Ligands are listed in the following order: negative ions, neutral molecules, and positive ions. Ligands with the same charge are listed in alphabetical order.

·         The oxidation number of the metal atom is indicated by a Roman numeral in parentheses after the name of the metal atom.

·         The names of complexes with a net negative charge end in -ate. Co(SCN)42-, for example, is the tetrathiocyanatocobaltate(II) ion. When the symbol for the metal is derived from its Latin name, -ate is added to the Latin name of the metal. Thus, negatively charged iron complexes are ferrates and negatively charged copper complexes are cuprates.

 

Q4. Mention the main postulates of Werner’s Theory.

Answer: Inspite of the capacity to explain the formation of complex compounds, Werner's theory is having many defects. Some of the important drawbacks are enumerated here.

·         Werner's theory does not correlate electronic configuration of the central metal with the formation of the complex compounds. Now it is known that the central metals try to acquire the next inert gas structure through coordinate bond formation. Hence complexes are formed.

·         The postulates of Werner do not offer any explanation to the colour of the complex compounds. It is appropriate at this juncture, to know how the d - d transition take place and result in the colour to the complex.

·         Werner's theory is incapable of explaining the magnetic behaviour. We know that the magnetic property depends on the number of unpaired electrons present in the metal ion; Werner's theory is not related to electronic configuration and so this property could not be explained.

 

Q5. Write the postulates of Werner’s theory of coordination compounds.

Answer: Postulates of Werner’s theory

·         Every metal atom has two types of valencies

ü  Primary valency or ionisable valency

ü  Secondary valency or non ionisable valency

·         The primary valency corresponds to the oxidation state of the metal ion. The primary valency of the metal ion is always satisfied by negative ions.

·         Secondary valency corresponds to the coordination number of the metal ion or atom. The secondary valencies may be satisfied by either negative ions or neutral molecules.

·         The molecules or ion that satisfy secondary valencies are called ligands.

·         The ligands which satisfy secondary valencies must project in definite directions in space. So the secondary valencies are directional in nature whereas the primary valencies are non-directional in nature.

·         The ligands have unshared pair of electrons. These unshared pair of electrons are donated to central metal ion or atom in a compound. Such compounds are called coordination compounds.

 

Q6. Explain primary valence and secondary valence.

Answer: Secondary valency corresponds to the coordination number of the metal ion or atom. The secondary valencies may be satisfied by either negative ions or neutral molecules. ... However, it does not explain the magnetic and spectral properties.

Valence bond theory, primarily the work of Linus Pauling regarded bonding as characterized by the overlap of atomic or hybrid orbitals of individual atoms. The postulates of valence bond theory. The central metal atom/ion makes available a number of vacant orbitals equal to its coordination number.

 

Q7. Define the term ligands.

Answer: In chemistry, ligands often involve the sharing of electron pairs to form the complexes. Many ligands contain extra lone pairs of electrons that they use to distribute among the other atoms in the complex. As a result, many of these are themselves Lewis bases, or electron pair donors.

 

Q8. Give one example of each of monodentate, bidentate and polydentate ligands.

Answer: Monodentate ligands: The term "monodentate" can be translated as "one tooth," referring to the ligand binding to the center through only one atom. Some examples of monodentate ligands are: chloride ions (referred to as chloro when it is a ligand), water (referred to as aqua when it is a ligand), hydroxide ions (referred to as hydroxo when it is a ligand), and ammonia (referred to as ammine when it is a ligand).

Bidentate ligands have two donor atoms which allow them to bind to a central metal atom or ion at two points. Common examples of bidentate ligands are ethylenediamine (en), and the oxalate ion (ox). Shown below is a diagram of ethylenediamine: the nitrogen (blue) atoms on the edges each have two free electrons that can be used to bond to a central metal atom or ion.

Polydentate ligands range in the number of atoms used to bond to a central metal atom or ion. EDTA, a hexadentate ligand, is an example of a polydentate ligand that has six donor atoms with electron pairs that can be used to bond to a central metal atom or ion.