Introduction:
In order to arrange the metal complexes in increasing order of their hydration, we need to understand the concept of hydration and its effect on metal complexes. Hydration refers to the process of water molecules surrounding and binding to a metal ion in a complex. The degree of hydration depends on various factors such as the charge and size of the metal ion, as well as the nature of the ligands present in the complex.

Factors affecting hydration:
1. Charge of the metal ion: Metal ions with higher charges tend to have a higher degree of hydration. This is because the electrostatic attraction between the positively charged metal ion and the negatively charged water molecules is stronger.

2. Size of the metal ion: Smaller metal ions have a higher degree of hydration compared to larger metal ions. This is because a smaller metal ion can accommodate more water molecules around it due to its smaller size.

3. Nature of ligands: The nature of ligands bound to the metal ion also affects the degree of hydration. Ligands that have a higher affinity for water molecules can compete with water molecules for binding to the metal ion, thereby reducing the degree of hydration.

Arranging the metal complexes:
Based on the above factors, we can arrange the metal complexes in increasing order of their hydration as follows:

1. Metal complexes with higher charges: Metal complexes with higher charges will have a higher degree of hydration. Therefore, complexes with metal ions such as Fe3+, Al3+, and Cr3+ will have a higher degree of hydration compared to complexes with metal ions such as Cu2+, Zn2+, and Ag+.

2. Metal complexes with smaller metal ions: Among complexes with similar charges, complexes with smaller metal ions will have a higher degree of hydration. For example, Fe3+ will have a higher degree of hydration compared to Al3+ and Cr3+.

3. Metal complexes with ligands having lower affinity for water: Among complexes with similar charges and metal ion sizes, complexes with ligands that have lower affinity for water will have a higher degree of hydration. For example, complexes with ligands such as chloride (Cl-) and nitrate (NO3-) will have a higher degree of hydration compared to complexes with ligands such as ammonia (NH3) and ethylenediamine (en).

Final arrangement:
Based on the above factors, the metal complexes can be arranged in increasing order of their hydration as follows:

1. Ag(NO3)2 - Silver(II) nitrate
2. ZnCl2 - Zinc(II) chloride
3. Cu(NH3)4SO4 - Tetraamminecopper(II) sulfate
4. FeCl3 - Iron(III) chloride
5. Al(NO3)3 - Aluminum(III) nitrate
6. Cr(NO3)3 - Chromium(III) nitrate

Explanation:
1. Ag(NO3)2 has a +2 charge and a relatively larger Ag2+ ion, resulting in a lower degree of hydration.
2. ZnCl2 has a +2 charge and a smaller Zn2+ ion, resulting in a higher degree of hydration compared to Ag(NO3)2.
3. Cu(NH3)4SO4 has a +2 charge, a smaller Cu2+ ion, and ammonia (NH3) ligands that have a higher