Chemistry 401

Symmetry and Point Groups

Find the point group for the following species:

H₂O

NH₃

CCl₄

[PtCl₄]^2– (square planar)

PF₅

BrF₃

XeO₃

[Cr(NH₃)₆]³⁺ (ignore H atoms)

CH₃CH₃

Transition Metal Chemistry

d block chemistry is vast, often thought of in terms of Lewis Acid/Base interactions

Metals act as Lewis acids, which react with Lewis bases

The Lewis acid/base product is generally called a coordination complex and the Lewis bases are called ligands

Wide variety of d-block chemistry to be addressed:

geometries: coordination number (CN = the number of Lewis bases bonded to the metal) can vary from 2 to 8; 4 and 6 are most common

oxidation states: from negative to +7 or +8

bonding: nearly purely ionic to nearly purely covalent; intermediate much more common

reactivity: substitution, addition, elimination, redox - and a number of mechanisms

Structures of Coordination Complexes

CN = 2, 3

Rare, follow VSEPR rules CN =2, linear [Ag(NH₃)₂]⁺; CN = 3, trigonal planar

CN = 4

Quite common; found in two geometries-tetrahedral (T_d) and square planar (D_4h) some complexes are known that are between T_d and D_4h (D_2d)

Square planar complexes can exhibit geometric isomerism: ML₂L'₂ cis or trans makes a difference in the chemistry: cis-Pt(NH₃)₂Cl₂ is an anticancer drug, the trans isomer is inert

CN = 5

Not rare, not common; two geometries-trigonal bipyramid (D_3h) and square-based pyramid (C_4v); the two geometries are generally close in energy so ligands can scramble.

Berry pseudorotation is seen in mixed ligand complexes, this scrambles ligand positions:

CN = 6

Most common coordination number, octahedral

Often can get distortions from O_h

Extension or compression along one axis : tetragonal : D_4h

Change along two axes : rhombic : D_2h

Change along the diagonal (the C₃ axis) : D_3d

Substitutions:

disubstituted ML₄L'₂ : cis (C_2v) or trans (D_4h)

trisubstituted ML₃L'₃ : fac (C_3v) or mer (C_2v)

CN = 8

Very rare for first row (steric), found occasionally in 2nd or 3rd row complexes