High school10 min

Complexes: ligands, formation constant and redissolving a precipitate

Write the formula and charge of a complex, set up its formation constant, and explain why a precipitate vanishes in excess ligand.

The essentials

1

Complex and ligand

A complex is a central metal ion surrounded by ligands, molecules or ions that donate an electron pair to it (Lewis bases): HX2O\ce{H2O}, NHX3\ce{NH3}, ClX−\ce{Cl-}, CNX−\ce{CN-}, SCNX−\ce{SCN-}. The coordination number is the number of bonds around the metal: most often 22, 44 or 66.
2

Formula and charge

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3

Formation constant

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4

Two tests to recognise

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5

Complexation versus precipitation

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ƒWrite my own sheetOr start from a blank page: rewriting a concept in your own words is how it sticks.