The first ionization enthalpies of d-block elements are higher than those of s-block elements and are lesser than those of p-block elements. The ionization enthalpies of 3d and 4d-series are irregular but increase across the series while those of 5d-series are much higher than 3d and 4d-elements. This is because of the weak shielding of nucleus by 4f electrons in 5d-transition series which results in greater effective nuclear charge acting on the outer valence electrons.
