I) Transition metals exist in hcc geometry in which there is vacant spaces. In these vacant spaces the elements like C, H & O forms compounds called interstitial compounds.
II) Due to lanthanide contraction, the
metallic radii of the third (5d) series of transition metals are
virtually the same as those of the corresponding group members of the
seconds (4d) series.
"The steady decrease in the values of the radii of M3+ cations in the lanthanide series is called as Lanthanide contraction."
III) This is because of the compareble energies of 5f, 6d and 7s orbitals so all of them can participate.