Why do the metals of the second and third transition series have a greater tendency to form low-spin complexes as compared to the first transition series?

Explanation

Order of crystal field splitting energy `(Delta)` is `3d lt 4d lt 5d.` Hence, value of `Delta` for the elements of second & third transition series is greater than that for the elements of first transition series. So metals of 2nd & 3rd transition series show greater tendency to form low - spin complexes.