Which is not true about metal carbonyls? 

CO is called $\mathrm{\pi }$-acid ligand. In metal carbonyl complexes, there is donation of an electron pair from carbon to the empty orbital of metal and then simultaneously a back $\mathrm{\pi }$-bonding is formed by sideways overlap of a filled orbital on the metal with empty antibonding $\mathrm{\pi }$$*$_2py orbital of CO.