Going down a group (3d → 4d → 5d): (1) Atomic radii increase from 3d to 4d but 4d ≈ 5d (due to lanthanoid contraction). (2) IE increases (5d > 4d > 3d) — heavier elements less readily oxidised. (3) Crystal field splitting increases (5d > 4d > 3d) — so 4d and 5d complexes are almost always low spin. (4) M-M bond strength increases — cluster compounds more common in 4d, 5d. (5) Maximum oxidation states more easily achieved: RuO4, OsO4 (Os = +8) exist but FeO4 does not. (6) +2 state less stable going down — $Pd^{2+}$, $Pt^{2+}$ prefer +4 or +2 with square planar geometry. (7) Melting points: 5d has highest (W = 3422°C, highest of all metals).