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.


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.