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Uncovering the Fascinating World of Coordination Complexes: Comparing Carbonyl and Nitrosyl Complexes

Introduction to Coordination Complexes

When studying inorganic chemistry, one of the topics that students often encounter are coordination complexes. These are inorganic compounds formed by a central atom or ion that is surrounded by various ligands.

Coordination complexes play an essential role in the chemical industry, medicine, and biological processes. In this article, we will delve deeper into coordination complexes, discussing their definition, the importance of ligands, and their significance in industrial applications.

Definition of Coordination Complexes

A coordination complex consists of a central atom or ion that is surrounded by ligands. The central atom is usually a transition metal ion, such as cobalt, iron, copper, or nickel.

Ligands are molecules or ions that donate electrons to form a coordinate covalent bond with the central atom. This bond involves the sharing of electrons, with the ligand donating a pair of electrons to the central atom.

The central atom and ligands together form a coordination center. This is where the coordination complex takes its name.

The formation of a coordination complex involves the use of valence electrons at the coordination center. The central atom’s d orbitals can also play a significant role in bonding with ligands, creating a variety of potential shapes and structures.

Importance of Ligands in Coordination Complexes

Ligands are essential in coordination complexes as they facilitate the bonding process between the central atom and the ligands themselves. They determine the types of bond that can form and significantly impact the properties of the complex.

Depending on the ligands used, a coordination complex can be either stable or unstable, soluble or insoluble. The type of ligand used in a coordination complex is often determined by the metal ion’s requirements.

For example, a metal ion with a positive charge, such as Fe2+, requires negatively charged ligands, such as Cl ions, to maintain a neutral charge balance. Conversely, a metal ion with a negative charge, like Cu, would require ligands such as NH3, which have a positive charge to balance the negative charge.

Gaseous substances, such as ammonia, carbon monoxide, and nitrogen, are common ligands used in coordination complexes. They all have unshared electron pairs, which they can donate to the central atom, forming coordinate covalent bonds.

Ligand substitution is a phenomenon where a new ligand replaces one of the existing ligands in a coordination complex, thereby altering its properties.

Carbonyl Complexes

One subtype of coordination complexes is carbonyl complexes. These complexes are formed by the bonding of a metal center with one or more carbonyl ligands (-CO ligands).

The carbonyl ligands are composed of a carbon atom bonded to an oxygen atom and have a lone electron pair. Compared to other ligands, carbonyl ligands are highly polar and can act as strong pi-electron donors.

Definition and Structure of

Carbonyl Complexes

Carbonyl complexes get their name from the carbonyl ligands they contain. In these complexes, the central metal atom is typically a transition metal ion, bonded to one or more carbonyl ligands.

The carbonyl ligands can be either terminal or bridging. Terminal ligands are bonded to the metal directly, while bridging ligands form covalent bonds between two metal atoms.

The structure of carbonyl complexes can be tetrahedral, square planar, or octahedral. In a tetrahedral carbonyl complex, the metal ion is surrounded by four carbonyl ligands.

In a square planar carbonyl complex, there are two terminal and two bridging ligands, with all four ligands lying in the same plane. In an octahedral carbonyl complex, six carbonyl ligands surround the central metal atom.

Types of Bonds in

Carbonyl Complexes

In carbonyl complexes, two types of bonds can form between the metal center and the carbonyl ligands. One is a sigma () bond, which involves overlap between the metal’s d-orbitals and the ligand’s *-orbitals.

The other is a pi () bond, which involves the donation of electrons from the ligand’s lone pair to an empty d-orbital on the metal center. The bridged carbonyl ligand can also form semi-bridging bonds where it donates electrons to both metal ions, creating a partial bond between them.

This type of bond has intermediate properties between a purely bridging and purely terminal ligand. Examples of

Carbonyl Complexes

Some examples of carbonyl complexes include [Co(CO)6] 3+, [Fe(CO)6]2+, [Rh(CO)6]3+, [Mn(CO)6]+ and [V(CO)6].

[Co(CO)6] 3+ is a red compound that dissolves in water and has a tetrahedral carbonyl complex structure. [Fe(CO)6]2+ is a yellow compound that is sensitive to light and air, and its carbonyl complex structure is octahedral.

The other compounds also have unique characteristics.


Coordination complexes are an essential area of study in inorganic chemistry. The bonding of transition metals with ligands creates a variety of shapes and structures that have significant industrial applications.

Carbonyl complexes are one subtype of coordination complexes that are formed using carbon monoxide ligands. These complexes have unique bonding properties and structures that make them useful in various applications.

The importance of understanding coordination complexes, including carbonyl complexes, cannot be overstated. As we continue to explore this topic, it opens our minds to unlimited possibilities in chemistry and beyond.

3. Nitrosyl Complexes

Nitrosyl complexes are another class of coordination compounds.

These complexes consist of a transition metal center bonded to nitric oxide ligands, typically abbreviated as -NO. The nitric oxide ligands are isoelectronic to carbon monoxide ligands and, hence, have similar bonding properties.

Nitrosyl complexes have been used in various applications, including as potential therapeutic agents for chronic diseases.

Definition and Structure of Nitrosyl Complexes

Nitrosyl complexes have a nitrosyl cation, which is NO+, coordinated to a transition metal center. Unlike carbonyl complexes, the nitrosyl ligands can form linear or bent structures.

The structure of a linear nitrosyl complex has a bond angle of 180 degrees between the nitrogen and oxygen atoms, which is the same as the bond angle in carbon monoxide complexes. Conversely, a bent nitrosyl ligand has a bond angle of less than 180 degrees due to lone electron pairs on the nitrogen atom.

Types of Nitrosyl Ligands

Nitrosyl ligands can exist in two forms: linear and bent. Linear nitrosyl ligands have an NO bond that is perpendicular to the metal-ligand bond, which results in them having covalent bonds.

In contrast, bent nitrosyl ligands have an NO bond that is angled towards the metal ligand bond, resulting in them possessing both covalent and ionic character. In both forms, nitrosyl ligands can participate in a back-bonding process, where electrons from the metal center are donated to the vacant *-orbital of the nitrosyl ligand.

Comparison with

Carbonyl Complexes

Carbonyl and nitrosyl complexes have numerous similarities. They both consist of a metal center bonded to a gaseous ligand, have similar electron-counting rules, and form similar structures depending on the metal ion and ligand used.

However, some differences between them exist. Nitric oxide is a better electron donor than carbon monoxide due to nitrogen’s low electronegativity compared to carbon.

Consequently, nitrosyl complexes are generally more covalent and less ionic than carbonyl complexes. Also, the bent and linear nitrosyl ligands exhibit different redox activity, with bent nitrosyls being more prone to reduction, while linear nitrosyls are resistant.

Comparison of Carbonyl and Nitrosyl Complexes

When comparing carbonyl and nitrosyl complexes, the primary difference lies in their ligands. Carbonyl complexes use carbon monoxide ligands (-CO), while nitrosyl complexes use nitric oxide ligands (-NO).

This difference translates to changes in the ligand bond strength, with NO being a more potent electron donor and hence resulting in different chemical properties. However, despite this difference, both complexes have similar bonding properties, involving the donation of electron density from the ligands to the metal center through covalent bonding.

This similarity makes them essential in various applications, including catalysis and pharmacology.


Coordination complexes play an essential role in inorganic chemistry, with carbonyl and nitrosyl complexes being two classes of coordination compounds. Both classes feature transition metal ions bonded to gaseous ligands, with unique bonding properties that have enabled various industrial and medical applications.

While they have differences in terms of ligand structure and electron donation, their similar bonding properties make them an exciting area of study, with numerous potential applications yet to be discovered. 5.


Inorganic chemistry is a vast field that covers a myriad of topics, including coordination complexes. Coordination complexes are inorganic compounds that contain a central metal ion or atom bonded to various ligands.

In this article, we focused on two types of coordination complexes: carbonyl complexes and nitrosyl complexes, as well as their differences and similarities.

Recap of Main Topics

Carbonyl complexes consist of a metal center bonded to one or more carbonyl ligands (-CO), while nitrosyl complexes consist of a metal center bonded to one or more nitric oxide ligands (-NO). Despite their different ligands, both carbonyl and nitrosyl complexes exhibit similar bonding properties, resulting in useful applications in various fields, including industrial processes, chemical reactions, and biological systems.

The bonding process between the transition metal ions and ligands in both complexes is critical, as it determines the stability, reactivity, and properties of the resulting compound. For carbonyl complexes, it involves overlap between the metal’s d-orbitals and the ligand’s *-orbitals, while in nitrosyl complexes, it involves donation of electrons from the ligand’s lone pair to an empty d-orbital on the metal center.

Significance of Coordination Complexes

Coordination complexes play a vital role in multiple fields, ranging from industrial processes to biological systems. In the chemical industry, coordination complexes are used as catalysts for various chemical reactions.

For example, the Rh catalyst in the Monsanto Acetic Acid Synthesis process uses a rhodium coordination complex as a catalyst to convert methane into acetic acid. Similarly, metal catalysts like Pt-based coordination complexes are used to catalyze the oxidation of hydrocarbons in fuel cells.

In biological systems, coordination complexes have been studied for their potential use in treating various chronic diseases such as cancer, diabetes, and Alzheimer’s. The use of metal-based coordination complexes, such as cisplatin, has been significantly advanced in cancer chemotherapy, where the metal center kills cancer cells through DNA cross-linking.

In conclusion, coordination complexes play a critical role in inorganic chemistry. The study of carbonyl and nitrosyl complexes allows us to understand their unique properties, structures, and applications.

Their significance in various fields continues to unveil novel uses, leading to future applications and discoveries. By delving further into the study of coordination complexes, we continue to expand our understanding of the world around us and unlock unlimited possibilities in chemistry and beyond.

In conclusion, coordination complexes, such as carbonyl and nitrosyl complexes, are fascinating areas of study in inorganic chemistry. These complexes, formed by the bonding of transition metal centers with ligands, play a crucial role in various industrial processes, catalysis, and biological systems.

The comparison of carbonyl and nitrosyl complexes highlights their similarities in bonding properties and structures, despite their different ligands. As we delve deeper into the study of coordination complexes, we uncover their significant contributions to chemistry and their potential applications in various fields.

By understanding and harnessing the properties of coordination complexes, we open the door to innovative solutions and advancements that have the potential to impact society and further our understanding of the chemical world.

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