Geometry of Molecules & Ions

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Complexes

A central metal atom bonded to a group of molecules or ions is a metal complex.If the complex bears a charge, it is a complex ion.Compounds containing complexes are coordination compounds.

The molecules or ions coordinating to the metal are the ligands.They are usually anions or polar molecules.

Complexes

Structure & isomerization

Three categories of isomerization1. Structural isomers: atoms connected in

different ways1. Coordination isomers2. Linkage isomers

2. Geometric isomers: ligands have different spatial arrangement1. Cis-trans isomers2. Octahedral complex isomers

3. Optical isomers: nonsuperimposable mirror-images (enantiomers)

Structural isomerism: coordination isomers

Coordination isomersCoordination ligand exchanges places w/uncoordinated counter-ion

Ex: [Co(NH3)5Br]Cl vs. [Co(NH3)5Cl]Br

Structural isomerism: linkage isomers

Either one of atoms in NO2- can bond to

metalWhen O, nitrito: ONO-

When N, nitro: NO2-

Different color compounds

Geometric isomerism: cis-trans isomers

Occurs in sq-planar: MA2B2

And octahedral complexes: MA4B2

8

Geometric isomerism: octahedral complex isomers

MX3Y3

Fac (facial) isomerThree identical ligands at corners of a triangular face of octahedron

Mer (meridian) isomerThree identical ligands at corners of a triangular meridian (inside octahedron)

complex Ion colorAbsorbs all colors-but- the one you see orReflects most colors but absorbs the complimentary

Complex Ion Color and Crystal Field Strength

The colors of complex ions are due to electronic transitions between the split d sublevel orbitals

The wavelength of maximum absorbance can be used to determine the size of the energy gap between the split d sublevel orbitals

Ephoton = hn = hc/l = D

Ligand and Crystal Field Strength

The strength of the crystal field depends in large part on the ligands

strong field ligands include: CN─ > NO2─ > en

> NH3

weak field ligands include: H2O > OH─ > F─ > Cl─ > Br─ > I─

crystal field strength increases as the charge on the metal cation increases

Magnetic Properties and Crystal Field Strength

The electron configuration of the metal ion with split d orbitals depends on the strength of the crystal fieldThe 4th and 5th electrons will go into the higher energy dx2-y2 and dz2 if the field is weak and the energy gap is small – leading to unpaired electrons and a paramagnetic complexThe 4th thru 6th electrons will pair the electrons in the dxy, dyz and dxz if the field is strong and the energy gap is large – leading to paired electrons and a diamagnetic complex

Low Spin & High Spin Complexes

paramagnetic

high-spin complex

diamagnetic

low-spin complex

Only electron configurations d4, d5, d6, or d7 can have low or high spin

Tetrahedral Geometry &Crystal Field Splitting

Because the ligand approach interacts more strongly with the planar orbitals in the tetrahedral geometry, their energies are raisedMost high-spin complexes

Square Planar Geometry & Crystal Field Splitting

d8 metalsThe most complex splitting patternMost are low-spin complexes

Occurs when polar molecules are attracted to each other.

Dipole interaction happens in waterpositive region of one molecule attracts the negative region of another molecule.

Dipole interactions

Dipole interactions

Occur when polar molecules are attracted to each other.Slightly stronger than dispersion forces.Opposites attract, but not completely hooked like in ionic solids.

H Fd+ d-

H Fd+ d-

d+

d-

d+ d-

d+ d -

d+ d-

d+ d -

d+

d-

d + d

-d+

d-

Dipole interactions

The End

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