What is synergic effect?

An effect arising between two or more agents, entities, factors, or substances that produces an effect greater than the sum of their individual effects.

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.