Transition-metal atoms often function as a Lewis acid in a chemical reaction. Remember that a Lewis acid accepts electron pairs from molecules or ions.
Example:
Complex ions are not held together by ionic bonds, but coordinate covalent bonds. Remember that coordinate covalent bonds are bonds between atoms where both electrons involved in the bond come from only one of the atoms.
In water, the Fe2+ ion ultimately bonds to six H2O, forming the [Fe(H2O)6]2+ ion. The final charge on the ion can be found by adding up all the charges of its components.
Example:
Fe = +2
H2O = 0
2 + (0 x 6) = 2
Charge on the ion = +2
Definitions:
Complex Ion: A metal ion with Lewis bases attached to it through coordinate covalent bonds.
Complex/Coordination Compound: a compound consisting either of complex ions and other ions of opposite charge or a neutral complex species (such as the anti-cancer drug in the chapter opening). Has a cation and an anion.
Example:
[FeOH2]Cl2 ← means 2 Cl–, not Cl2 gas. Two negatively charged chlorine ions neutralize the +2 charge on the complex ion. Everything in the brackets in front of the Cl2 is the cation.
Ligands: the Lewis bases attached to the metal atom in a complex. Electron pair donors. Can be neutral (such as H2O or NH3) or anions (such as Cl–).
Coordination Number: the total number of bonds the metal atom forms with ligands. The number of lone pairs that are attached to the metal via the coordinate covalent bond.
Example:
In [Fe(H2O)6]2+, the metal atom, Fe, bonds to each oxygen atom in six water molecules.
Coordination Number (C.N.) = 6
Example:
[Ag(NH3)2]+ → [H3N꞉Ag꞉NH3]+
One coordinate covalent bond between the Ag and each N of the two NH3 molecules.
C.N. = 2
Polydentate Ligands: ligands that can bind with two or more atoms to a metal atom. Think of the word “bite” when you hear dentate. Monodentate means it forms one bond with its atoms (one bite), bidentate means it forms two bonds with its atoms (two bites), etc.
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