Key Structures and Bonding in Metal Carbonyl Complexes
What are Metal Carbonyls?
Metal Carbonyls can be defined as compounds that are volatile and have low melting points. They are made from the compound of Mx(Co)y that decomposes into carbon monoxide and metal on heating. They can be toxic when in contact with skin. They can also be toxic if inhaled or ingested due to their property of carbonylate haemoglobin which converts it into carboxyhemoglobin that further prevents the binding of oxygen in the blood cells. Furthermore, in a metal carbonyl, both characters, σ, and π, are possessed by the metal-carbon bond. This bond is further strengthened by the synergic effect produced by the metal-ligand bond.
Metal Carbonyls and Their Structure
When learning about metal carbonyls, it is important to study their structure as well. One of the unique properties of metal carbonyls is that they exist in two types of bonding. The first kind exists when there is a donation of electrons by the carbonyl molecules to the vacant orbitals of the metal, thereby forming a metal-carbon σ bond.
The second type is when there is a donation of a pair of electrons from a filled d orbital metal into the vacant anti bonding π* orbital of a carbonyl ligand. This way a metal-carbon π bond is formed. The 18-electron rule is generally used in predicting the stability of metal carbonyls. According to this rule, the electrons are gained from the ligands by the metal atom to reach the nearest noble gas configuration.
What Are Metal Carbonyls Organometallics?
Metal Carbonyl Organometallics can be defined as compounds that consist of at least one metal-to-carbon bond. Moreover, the carbon in the metal-to-carbon bond is part of an organic group. The organometallic compounds play a major role in the development of the science of chemistry.
An example of an organometallic compound is ferrocene in which an iron atom is in between two hydrocarbon rings. There are great variations amongst the physical and chemical properties of organometallic compounds. Most of them are solid, specifically those with ring-shaped hydrocarbon groups.
Some organometallic compounds are present both in liquid and gaseous states. They can also be flammable, particularly the compounds of electropositive elements such as lithium, aluminium, and sodium. Major organometallic compounds are highly toxic and volatile. Some common examples of organometallics are Grignard Reagent - RMgX, Gilman Reagent - R2CuLi, Dimethylmagnesium - Me2Mg, Triethylborane - Et3B, Ferrocene, Cobaltocene.
Properties of Metal Carbonyls Organometallics
The 18-electron rule followed by metal carbonyl is surprisingly not followed by metal carbonyls organometallics. Some other basic properties of metal carbonyl organometallics are specified below:
Organometallics are not soluble in water
Instead, they are soluble in ether
Metal Carbonyls Organometallics has a relatively low melting point
Another interesting property of organometallics is their electronegativity. While metals have a lower electronegativity of 20, the organometallic carbon compound has an electronegativity of 2.5.
Organometallic compounds are also highly reactive. That is the reason why they are generally kept in organic solvents.
Uses of Metal Carbonyl Organometallic
The discovery of organometallic compounds has led to their application in various fields. Some of the popular uses of organometallics are specified below:
The most common use of organometallic compounds is as a reagent.
Organoarsenic compounds are also used in the treatment of a common sexually transmitted disease called syphilis.
Grignard reagent, which is a popular organometallic compound is used for various purposes such as in the synthesis of a secondary alcohol, aldehydes, etc.
Another use of organometallic is as an additive
They are also useful for various industrial purposes.
Cis-plastin, an organometallic compound, is used as an anticancer drug
For hydrogenation alkenes, Wilkison’s catalyst is used
FAQs on Metal Carbonyls Organometallics Explained
1. What are organometallic compounds?
Organometallic compounds are chemical compounds that contain at least one direct bond between a carbon atom of an organic group and a metal. This metal can be an alkali, alkaline earth, transition, or main group metal. A key characteristic is the metal-carbon (M-C) bond, which defines their unique chemical reactivity and properties, setting them apart from other coordination complexes.
2. Are all metal carbonyls considered organometallic compounds? Explain with an example.
Yes, all metal carbonyls are classified as organometallic compounds. This is because they feature a direct bond between a metal atom and the carbon atom of a carbonyl ligand (CO). For example, in Tetracarbonylnickel(0), or Ni(CO)₄, the central nickel atom is directly bonded to the carbon atoms of four carbonyl groups, fulfilling the primary definition of an organometallic compound.
3. How are organometallic compounds broadly classified?
Organometallic compounds are broadly classified into three main categories based on the type of metal involved. This classification helps in studying their properties systematically as per the CBSE Class 12 syllabus. The categories are:
Main Group Organometallic Compounds: These involve metals from the s-block and p-block of the periodic table. A common example is the Grignard reagent (R-Mg-X).
Transition Metal Organometallic Compounds: These contain metals from the d-block. Metal carbonyls like Fe(CO)₅ and catalysts like Wilkinson's catalyst are prominent examples.
Lanthanide and Actinide Organometallic Compounds: These are less common and involve metals from the f-block, such as Uranocene.
4. What is the difference between homoleptic and heteroleptic carbonyls?
The distinction lies in the types of ligands attached to the central metal atom. Homoleptic carbonyls are metal complexes where carbon monoxide (CO) is the only type of ligand present, for example, Cr(CO)₆. In contrast, heteroleptic carbonyls contain carbon monoxide along with other types of ligands. An example is [Mn(CO)₅Cl], which has both carbonyl and chloride ligands bonded to the manganese atom.
5. How does the concept of synergic bonding explain the stability of metal carbonyls?
Synergic bonding is a crucial concept that explains the strong and stable metal-carbon bond in metal carbonyls. It involves a two-way process that mutually strengthens the bond. First, the carbonyl ligand donates a lone pair of electrons from its carbon atom to a vacant orbital of the metal, forming a sigma (σ) bond. Simultaneously, the metal atom donates a pair of electrons from a filled d-orbital into a vacant antibonding π* orbital of the carbonyl ligand, forming a pi (π) bond. This back-donation strengthens the M-C bond and is why this mechanism is called synergic, or mutually reinforcing.
6. Why are compounds like metal cyanides (e.g., K₄[Fe(CN)₆]) not typically classified as organometallics, while metal carbonyls are?
This distinction arises from the nature of the bonding and the properties of the ligand. While both cyanides (CN⁻) and carbonyls (CO) bond to metals via carbon, the synergic effect in metal carbonyls is much more pronounced. The π-back bonding from the metal to the CO ligand is significant and imparts properties (like low oxidation states of the metal) that are characteristic of organometallic chemistry. In metal cyanides, the M-C bond has significantly more ionic character and lacks this strong back-donation, causing them to behave more like typical coordination compounds rather than true organometallics.
7. What is the significance of the structure of Ni(CO)₄ in its industrial application?
The structure of Tetracarbonylnickel(0), Ni(CO)₄, is tetrahedral. This structure, combined with its unique bonding, results in a highly volatile and colourless liquid with a boiling point of just 43°C. This volatility is the cornerstone of the Mond process for refining nickel. Impure nickel is heated with carbon monoxide to form gaseous Ni(CO)₄, leaving solid impurities behind. This gas is then transported and heated to a higher temperature (around 220°C), causing it to decompose back into highly pure nickel metal and carbon monoxide, which is then recycled.
