Final answer:
The ionisation enthalpy trend in transition elements is irregular due to the involvement of d subshell electrons, which have unique electron configurations that do not adhere to the octet rule. Transition metals can form ions with unfilled inner d shells, adding complexity and causing irregular trends in ionization energies.
Step-by-step explanation:
The ionisation enthalpy trend is irregular in transition elements because these elements have electrons in d subshell which do not follow the octet rule. As the ions of transition metals have unfilled inner d shells, their electron configurations do not always adhere to expected patterns. Unlike main group elements which have a predictable ionisation trend, transition metals display complex behaviors due to the involvement of d electrons.
To achieve a noble gas configuration, a transition metal like iron would have to lose multiple electrons from both the 4s and 3d subshells, which is not energetically favorable, making high positive charges unlikely. Moreover, the first ionisation energies of transition metals are somewhat similar, with discrepancies arising at particular electronic configurations, causing the deviations from expected trends.
Overall, the ionization energy trend for transition metals does not exhibit the same level of periodicity seen with s and p block elements because of the complex electron interactions within the d subshell, leading to irregularities in ionisation enthalpy trends.