A = e bC or A = abc

A = absorbance, unitless number

A = log10[100/%T]

b = path length, 1cm

C = concentration, M

e = extinction coefficient, L/mol cm

a = molar absorptivity, L/mol cm

Dt ~ 4/9 Do. Do tends to be larger for fifth period transition metals than four second period metals, and for fourth period than for third period metals.

d-d transitions in a rigorously octahedral geometry are "forbidden." This is a result of the very high symmetry of an octahedral complex, particularly the presence of the center of symmetry, i. However, small vibrational changes distort real octahedral complexes and thus some d-d transitions do occur. The value of a or e is typically 1 - 100 L/mol cm.

Tetrahedral complexes do not have a center of symmetry and typically have d-d transitions with a or e values of 100 to 1000 L/mol cm.

A transition that results in a change in total spin, S, is forbidden. Thus d0, d10 and high-spin d5 species have forbidden d-d transitions.

Lower symmetry results in higher molar absorptivities.

Very large (103 to 104) molar absorptivities can result from "Charge Transfer". This happens with metals in very high oxidation states and/or with very easily oxidized ligands. For example, CrO42- is bright yellow. The oxidation number of chromimium here is +7, and it is formally a d0 ion. The O2- ligand is electron rich, thus there is some transfer of the charge from the ligand to the metal. You may recall that PbI2 was yellow in color, where as PbCl2 is colorless. The iodide ion, I-, is more easily oxidized than is a chloride ion, Cl-.

Modification of Part I of Experiment 8. Carefully weigh out to the tenth of a milligram approximately 0.06 grams of your compound. Add 10.0 mL deionized water to dissolve the compound. Use this solution to measure the spectrum. In your report, in addition to finding lmax, calculate e (or a). Also, as part of your

Lab Reports and Experiments 7,8 and 9 will be due by 5 PM on November 23. They should include the visible spectra for BOTH complexes (with apprpriate credit given the experimentalist who took the "other" spectrum). For Experiment 8, Part II, 3, you should include a statement about how the above generalities are reflected in the lmax and e of the five spectra shown.