Chemistry · Ch 5 — Coordination Chemistry
Solvate Isomers
Solvate Isomers
Solvate isomers arise when a free solvent molecule sitting in the crystal lattice (water, ammonia, alcohol, etc.) is exchanged with a ligand already inside the coordination entity, giving different isomers that share the same overall formula but differ in how many solvent molecules are actually bound to the metal versus sitting free in the lattice. When the solvent involved is specifically water, these solvate isomers are given the more familiar name hydrate isomers. Because moving a water molecule between the coordination sphere and the free lattice also changes how many other ligands (typically halide ions) are pushed out into the ionisable outer sphere, hydrate isomers characteristically differ in colour and in how many ionisable ions they release in solution -- exactly the pattern seen in the three …
Formula | Colour and behaviour in solution
[Cr(H₂O)₆]Cl₃ | Violet; gives three chloride ions in solution
[Cr(H₂O)₅Cl]Cl₂.H₂O | Pale green; gives two chloride ions in solution
[Cr(H₂O)₄Cl₂]Cl.2H₂O | Dark green; gives one chloride ion in solution …
Worked out. A self-check problem: three compounds A, B and C, all with the same empirical formula CrCl₃.6H₂O, are dried over a dehydrating agent until they reach constant weight. Compound A drops from 4 g to 3.46 g, compound B from 0.5 g to 0.466 g, and compound C shows no change at all (3 g to 3 g). Since only FREE (uncoordinated) lattice water is driven off by a dehydrating agent, the compound losing the most water (A, losing about 3 out of its 6 waters' worth by mass) has the most free water and the fewest coordinated waters, while the compound losing none (C) must have all six waters bound inside the coordination sphere -- letting the student match A, B, C to the violet, pale-green and dark- …