Just like it has been mentioned by Ian and the rest, Synergistic bond stems from the fact that some bonds are self-strengthening in nature.In the chemistry of organo-metallics, the effect is observed when a pi-back donation of electron density occurs from an atomic orbital of one atom to the anti-bonding pi-orbital of another.The anti-bonding pi-orbital must be pi-acceptor ligand.
For instance, in the formation of metal-carbonyl bond, M-C-O, two different interactions occur:
1. The carbonyl possesses a full sp orbital that is electron rich.The lone pair from the carbon which are mainly p-character are weakly bonded as compared to the s-character electrons in oxygen.It becomes easier for carbon to donate its electrons to the empty orbitals of the metal.i.e donation
2.Having accepted the electrons, the metal( mostly a transition element) becomes so negative and will attempt to stabilize by donating its nd electrons(present in the d-orbitals which are at the right symmetry with to the anti-bonding orbitals of the ligand. This result to a mutual sharing of electrons where the ligand donates its electrons to the empty orbitals of the metal while the metal upon becoming negative back-donates its d-electrons to the anti-bonding of the ligands.
The effect of synergistic bonds are as follows:
- It strengthens the M-C bond while weakening the C-O bond.The phenomenon can clearly be observed by the increase in vibrational frequency of M-C bond in infra red to a point that is outside the normal range for IR spectrophotometer.
- it also leads to shortening of the M-C-O bond length.
Synergistic bonds in organ-metallic chemistry are affected by various factors which have the effect of tampering with the back-bonding phenomenon.They include:
- Charge on the metal-If the metal possess a charge, it becomes difficult for the back-donation process to happen thus weakening the synergistic bonds.To increase the strength of the synergistic bonds, one has to use a metal with a center that is electron rich.
- Contribution of other ligands on the metal center-electron donating ligands increase the back-bonding process while the electron withdrawing reduce the back-bonding process.
Applications
Synergistic bonds find wide applications in chemistry especially in catalysis,metathesis and infra-red analysis.
a + a = 2a ....normal situation
a + a > 2a (or different than 2a) ....synergy
There is a lot of examples in organic chemistry, check out in catalysis there is a bunch a synergetic catalysts. Look at MacMillan work.
https://www.princeton.edu/chemistry/macmillan/publications/Synergistic-catalysis.pdf
The typical example given for synergy in chemistry is the synergic bonding seen in transition metal carbonyl complexes.
CO (the carbonyl ligand) can interact with a transition metal in two ways:
1) It possesses a full sp-orbital on carbon, which can donate electron density to a transition metal and thereby form a bond (much like any other two electron donor, e.g. amines, pyridine, water etc.)
2) CO also possesses an empty p-orbital on carbon at 90 degrees to the sp-orbital. This empty p-orbital can accept electron density from a transition metal, again forming a bonding interaction.
These two opposing effects happen at the same time, enhancing the strength of each interaction: As the CO ligand donates more electron density to the metal (from the sp-orbital) the metal becomes more electron-rich and more able to donate electron density to the empty p-orbital on the carbon, i.e the strength of each bonding interaction is in part dependant on the other.
In the style of George's reply, a (the bond strength) is not a constant, so a+a doesn't necessarily equal 2a.
Just like it has been mentioned by Ian and the rest, Synergistic bond stems from the fact that some bonds are self-strengthening in nature.In the chemistry of organo-metallics, the effect is observed when a pi-back donation of electron density occurs from an atomic orbital of one atom to the anti-bonding pi-orbital of another.The anti-bonding pi-orbital must be pi-acceptor ligand.
For instance, in the formation of metal-carbonyl bond, M-C-O, two different interactions occur:
1. The carbonyl possesses a full sp orbital that is electron rich.The lone pair from the carbon which are mainly p-character are weakly bonded as compared to the s-character electrons in oxygen.It becomes easier for carbon to donate its electrons to the empty orbitals of the metal.i.e donation
2.Having accepted the electrons, the metal( mostly a transition element) becomes so negative and will attempt to stabilize by donating its nd electrons(present in the d-orbitals which are at the right symmetry with to the anti-bonding orbitals of the ligand. This result to a mutual sharing of electrons where the ligand donates its electrons to the empty orbitals of the metal while the metal upon becoming negative back-donates its d-electrons to the anti-bonding of the ligands.
The effect of synergistic bonds are as follows:
- It strengthens the M-C bond while weakening the C-O bond.The phenomenon can clearly be observed by the increase in vibrational frequency of M-C bond in infra red to a point that is outside the normal range for IR spectrophotometer.
- it also leads to shortening of the M-C-O bond length.
Synergistic bonds in organ-metallic chemistry are affected by various factors which have the effect of tampering with the back-bonding phenomenon.They include:
- Charge on the metal-If the metal possess a charge, it becomes difficult for the back-donation process to happen thus weakening the synergistic bonds.To increase the strength of the synergistic bonds, one has to use a metal with a center that is electron rich.
- Contribution of other ligands on the metal center-electron donating ligands increase the back-bonding process while the electron withdrawing reduce the back-bonding process.
Applications
Synergistic bonds find wide applications in chemistry especially in catalysis,metathesis and infra-red analysis.
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