Got a textbook saying that due to the relativistic effect, 5d metals build stronger bonds than 4d and 3d metals, because their d-orbitals don't experience as strong pauli-repulsion to ligands or other metals (nevermind stronger pi-backdonation of the 5d metals). Makes sense and it's also referenced in this paper:
https://onlinelibrary.wiley.com/doi/abs/10.1002/jcc.20522

However, the textbook goes on to say that 5d metals will form bonds more readily with more electronegative elements. Why is that? Does increased electronegativity increase the energy state of the ligand orbitals, thus bringing them closer to the more elevated 5d orbitals? But then, wouldn't that also be the case for 3d and 4d metals?