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Key differences emerge when comparing the three d-block series:

Atomic radii: 3d < 4d, but 4d approximately equals 5d (due to lanthanoid contraction filling 4f between them). This makes 4d/5d pairs nearly identical in properties (chemical twins).

Crystal field splitting: 3d < 4d < 5d. Larger, more diffuse 4d/5d orbitals overlap better with ligand orbitals. Consequence: 4d and 5d complexes are almost always low spin.

Oxidation states: Higher OS are more accessible going down. OsO4 ($Os^{8+}$) and RuO4 ($Ru^{8+}$) exist; FeO4 does not. 5d metals more readily achieve high OS.

Metal-metal bonding: Stronger going down due to better d-orbital overlap. Cluster compounds (Nb6Cl14, Mo6Cl12) are common in 4d and 5d but rare in 3d.

Stability of +2 state: Decreases going down. $Fe^{2+}$ is common; $Os^{2+}$ is not.

Melting points: 5d has the highest values (W = 3422 degrees C).

Density: 5d metals are exceptionally dense (Os = 22.59 g/$cm^3$) because lanthanoid contraction gives them similar radii to 4d but much greater atomic mass.