What is a bridging carbonyl compound and how is it identified?
In the field of organic chemistry a simple carbonyl compound group is a group which is made up of carbon atom double bonded to an oxygen atom. C=O is the perfect representation of it. This is true to many classes of organic compounds and as part of many bigger carbonyl compounds. A compound which has a carbonyl group is a carbonyl compound. The term carbonyl can also be referred to carbon monoxide as a ligand in an inorganic complex substance. Examples would be metal carbonyl. In carbonyl compounds which are much more acidic than typical C-H bonds (Methane for example) the oxygen has more polarity hence the alpha hydrogen formation.
We can take the example of Amides which are one of the most stable carbonyl couplings due to their high resonance which is due to Nitrogen carbon and carbon oxygen bonds.
The reactivity for carbonyl compounds as seen by experiments: Oxygen is much more electronegative than Carbon since the carbonyl compounds have resonance structures and this affects the reactivity easily. This electronegativity easily draws electrons away from carbon and in turn increases the polarity for it. That is why Carbon is considered as an electrophile. (which means slightly positive). Carbons can be attacked by nucleophiles (negatively charged)ones from another molecule. Hence during this reaction, the Carbon oxygen double bond is thus broken down and the carbonyl group will get much more reactive. This process is condensation or addition -elimination process(because a water molecule is lost). The oxygen which is electronegative can react with an electrophile. This process is seen mostly in cases where elctrophile reactions.
We can reduce carbonyl groups by reduction in the reactions between hydrides like NaBH4 with some yeast or even by catalytic hydrogenation. Ketones also give secondary alcohols while the ones like esters and carboxylic acids will give primary alcohol. We can do carbonyl alkylation in some nucleophilic reactions using some metal compounds such as organolithium reagents. Carbonyls are also in prototypic groups where there is vinylogous reactivity. Carbonyl selectivity for chemicals can be described by saying that when we have many carbonyl types in one molecule then the most electrophilic carbonyl carbon will react first. Acyl chlorides types react first and which is followed by aldehydes and ketones. Esters rect much slowly and amides do not react at all due to resonance of the amide nitrogen towards the carbonyl group.
The bridging carbonyl compound is the ligand that usually holds two or more atoms which are usually metal ions. The ligand is either atomic or polyatomic. All the complex organic compounds works as bridging ligands and the term hence is usually for small ligands such as psedohalides or usually means ligands that attach two metals. The synergistic bonding is more effective and formal one other than the back bonding. A carbonyl group will give its both pair of electrons to the empty metal. There is a sigma bond and involves lone pairs in the orbital of the carbonyl. Here the carbonyl is acting as the Lewis base. However when this happens this is a good overlap between the metals in the orbitals with an empty orbital in the carbonyl group.
The overlap here depicts that metals are capable of donating some of the excess electron density into the carbonyl orbital. This thus form a pi bond. Here also the carbonyl is acting like a Lewis acid.
The synergistic effect of the metals only strengthens it – the metal carbonyl reaction that is. It also will make weak the CO triple bond. Here the electron density is being inserted in to an orbital on CO.
Metal carbonyls: These carbonyls are ones which are complex ones of transition metals with some carbon monoxide ligands. They are used for organic synthesis and they also act as catalysts in catalysis. Eg- nickel tetra carbonyl is used to produce pure nickel. In organometallic chemistry we have metal carbonyls act as precursors for the preparation of other such complexes which have the same qualities of organometallic complexes. The physical characteristics are – colourless or pale yellow coloured which are toxic and flammable. Eg- bimetallic and poly metallics are the ones which are carbonyls and have deep colours. They form deep green crystals. They are subsumable in vacuum and this process is usually followed by degradation. These are soluble in non polar and also polar organic solvents such as Benzene, acetone, glacial acetic acid and Carbon tetrachloride. Some salts of metal carbonyls are soluble in water.
We can see one experiment here – if there is direct reaction with Carbon Monoxide to any metal. Example- Nickel tetra carbonyl can be made by the following equation –
Nickel and 4 CO (carbon monoxide) which makes Ni(CO)4 ( 1 bar and 55 degree C)
Iron and 5 CO which makes Fe(CO)5 that is 100 bars and 175 degree C.