Unlike components of the 3d series, 4d and 5d elements have little simple aqueous cationic chemistry. The main exceptions are La3+ and Y3+, and Ag+, which builds some soluble salts (AgF, AgNO3). The aqua Ag+ ion produces strong class b complexing behavior, with an affinity for ligands such as I-, NH3 and CN- comparable with Cd2+ in the next group. Some other aqua cations may be build, but they are extensively polymerized and hydrolyzed (e.g. Zr4+, Hf4+), strongly reducing (e.g. Mo3+), or have a very big affinity for other ligands (e.g. Pd2+) and are difficult to ready in uncomplexed form.
Table A selection of oxides and halides of elements from the 4d and 5d series. M describes either of the two elements from the corresponding group, and X any halogen unless exceptions are specified.
Numerous complexes are building, the most stable with early groups being ones with oxygen and F- donor ligands, and in later groups ones with softer ligands such as nitrogen donors and heavy halides. This trend is same to that appear in the 3d series but is more marked. The most commonly encountered solution substance for later elements are chloride complexes such as [PdCl4]2-, [PtCl6]4- and [AuCl4]-.