Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct IUPAC name for the complex ion [CoCl \(\left.(\mathrm{ONO})(\mathrm{en})_{2}\right]^{+}\)is (a) bis(ethylenediamine)chloronitrito-O-cobalt (III) ion (b) bis(ethylenediamine)chloronitrito-O-cobaltate (III) ion (c) chlorodiethyldiamine nitrite-O-cobalt(III) ion (d) chlorodiethyl diamine nitrocobalt(III) ion

Step-by-Step Solution

Answer

The correct IUPAC name is (a) bis(ethylenediamine)chloronitrito-O-cobalt(III) ion.

Step 1: Identify Ligands and Metal

In the complex ion \([\text{CoCl} (\mathrm{ONO})(\mathrm{en})_{2}]^{+}\), the ligands are \(\text{Cl}^{-}\) (chlorido), \(\text{ONO}\) (nitrito-O), and \(\mathrm{en}\) (ethylenediamine). The central metal is cobalt (Co).

Step 2: Order the Ligands Alphabetically

In naming the complex, the ligands should be ordered alphabetically irrespective of their charge. Therefore, the order will be chlorido, ethylenediamine (abbreviated as 'en'), and nitrito-O.

Step 3: Count the Number of Each Ligand

There are two ethylenediamine ligands and one each of chlorido and nitrito-O ligands. These quantities are expressed as bis for ethylenediamine, while no prefix is needed for mono-ligands.

Step 4: Determine the Oxidation State of the Metal

Since the overall charge on the complex is \(+1\) and the ligands contribute the following charges: \(\text{Cl}^{-}\) has a \(-1\) charge, and each ethylenediamine \(\text{en}\) contributes no charge, the oxidation state of cobalt must be \(+3\) to balance the charge: \[\text{Co} + (-1) + 0 = +1\].

Step 5: Combine the Name

Combine the information from the previous steps: the ligands' names and amounts are bis(ethylenediamine), chlorido, and nitrito-O. Add the metal's name and oxidation state in Roman numerals. Thus, the complete IUPAC name is: bis(ethylenediamine)chloronitrito-O-cobalt(III) ion.

Key Concepts

Coordination Chemistry

Coordination chemistry revolves around the study of complex compounds that consist of a central atom, typically a metal, surrounded by molecules or ions known as ligands. These compounds are fascinating as they illustrate the intricate way metals interact with various groups to form stable structures. The metal, acting as the heart of the complex, binds with ligands which donate electrons to form coordinate bonds. In coordination compounds, understanding this interaction is key to deciphering their chemical behavior and properties.
Specifying the arrangement and type of ligands around the central metal is essential in identifying the nature and function of the compound. The concept of coordination number, which is the number of ligand donor atoms bonded to the metal, gives insights into the geometry and electronic configuration of these complexes.

Ligand Naming Rules

In coordination chemistry, the naming of ligands is a pivotal step in constructing the complete name of a compound. Ligands can be ions or molecules attached to the central metal, and they are named first in the nomenclature process. Here are some basic rules to follow:

• Negative ion ligands generally end with an 'o', such as "chlorido" for ^-
• Neutral molecules have either their regular name or specific trivial names used in complex naming, like "aqua" for water.
• Polydentate ligands, those which can attach through multiple donor atoms, like ethylenediamine, are represented using prefixes such as "bis" if two are present.

Familiarity with these rules ensures that ligands are properly identified, categorized, and appropriately placed in the compound name. This clarity aids in the accurate communication and recording of compounds for scientific analysis and research.

Oxidation States

Determining the oxidation state of the metal in a coordination compound is crucial for both naming the compound and understanding its chemical behavior. The oxidation state reflects how many electrons the metal would "give up" if the complex were to dissociate into its components. To find it, consider the charges of the ligands and the overall charge of the complex.
For instance, in the complex [CoCl(ONO)(en)₂]⁺, the ^- ligand contributes a charge, and the "en" ligands contribute no charge. The overall charge of the complex ion is +1. Given these factors, the oxidation state of cobalt needs to balance these charges. If cobalt has an oxidation state of +3, the summed charge matches the observed +1 overall, validating this assignment. Properly determining oxidation states is vital to avoid errors in chemical reactions and nomenclature.

Alphabetical Order of Ligands

Alphabetization of ligands in the naming process is an aspect of coordination chemistry that ensures consistency and clarity when writing IUPAC names. The sequence of ligands in a compound name doesn't follow the order of their charge or quantity but is strictly alphabetical.
This applies regardless of the charge or physical structure of the ligand. Therefore, in the complex ion [CoCl(ONO)(en)₂]⁺, the order is chlorido, ethylenediamine (abbreviated "en"), and nitrito-O. This systematic approach avoids ambiguity and ensures that chemists across the world can understand the compound's structure simply by reading its name. It might seem trivial, but following this rule is a cornerstone for clear scientific communication.