Chromium Chemistry, Exercises of Chemistry

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Chromium Chemistry^2262 Cr : 1s^2 s^2 p^3 s^3 p

6154 s^3 d

F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochman,

th^ Advanced Inorganic Chemistry, 6ed., John Wiley, New York.

Elemental Chromium • Chief ore is chromite (FeCrO), which has a spinel structure consisting of Fe(II) [^24

T]d and Cr(III) [O].h • Pure Cr is a white, hard, lustrous, and brittle metal. • First discovered in 1797 in the mineral crocoite (

lead chromate) which is famous for its deep yellow color and is used extensively in paints. • The^ name^ chromium^ is^ derived^ from

the^ Greek^ word^ "chrōma",^ meaning

color,

-^ The^ name^ chromium^ is^ derived

from^ the^ Greek^ word^ "chrōma",^

meaning^ color, because many of its compounds are intensely colored. • Extremely resistant to corrosive reagents, which accounts for its extensive use asan electroplated protective coating. • Readily dissolves in mineral acids (HCl, H

SO) but not oxidizing acids (HNO 243

Cr(0)

-^ This low valent oxidation state of Cr can be stabilized by a highly electronwithdrawing ligand-set by back-donation into a ligand

π*-orbital.

Cr(II)

-^ The most stable and generally most important oxidation states of chromium areCr(II) and Cr(III). •^ Cr(III) is typically green, and Cr(II) is usually blue-violet.

3+^ Crion has the 3 2+^ configuration 3d, and Crion has the configuration 3

(^4) d, so both species are expected to be paramagnetic in an^

Oenvironment.h^ II2+^ • [Cr(OH)]is prepared by the dissolution of elemental Cr in mineral acid^26 resulting in a bright blue solution.II2+^ • Alternatively,^ [Cr(OH)]can^26 be^ prepared^ by^ the^ reduction

of^ Cr(III)^ by electrochemical or chemical (Zn/Hg) methods in the absence of O

II • Even in the absence of O[Cr(OH 2 2+^ )]will oxidize with concurrent H 26 evolution, 2 +^ i.e. Hreduction.+ –^ 2H+ 2^ e→ H^2 o^ E=^ 0.000 V 3+^ −−−−^ 2+^ Cr+ e→ Cr

0 E= - 0.410 V

2+^ +^ 3+^ 2 Cr+ 2H→ H+ 2Cr^2 E= + 0.410 Vcell^

-^ Potassium dichromate, KCrO^227

, is the starting material for today's synthesis.

-^ In basic solution, chromium(VI) exists as the orange-colored tetrahedral CrO

2—ion. 4

-^ In acid solution, this can become HCrO -^ but dimerization to red-orange Cr 4

2–O 27

predominates:–^ +^ HCrO+ HO → HCrO+ HO^24

Potassium Dichromate - Cr(VI)K= 4.1^1 –^ 2–^ +^ –6 HCrO+ HO → CrO+ HOK= 1.3 × 10 42432 2–^ +^ 2–^ –3 2 CrO+ 2 H→ CrO+ HO^ K^ = 6.3 × 10 4272

-^ The configuration of Cr in both chromate and dichromate ions is

(^0) d[Cr(VI)]. The 2–^ 2–intense colors of CrOand CrO 427 are the result of^ ligand-to-metal (O→Cr) charge transfer bands. • Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent,^0 0 which will oxidize 1and 2alcohols to carboxylic acids and ketones respectively,while rarely affecting unsaturated bonds.

-^ The dichromate ion has two tetrahedrally coordinated Cr atoms linked by anoxygen bridge:

Overall Redox Reaction Uncertain • From the electrode potentials, it might be assumed that the reduction of Cr(VI) toCr(II) is accomplished by Zn as the reducing agent, in which case the balancedequation would be2–^ +^ CrO+ 14 H+ 4 Zn → 2 Cr^27

2+^ 2+^ + 4 Zn+ 7 HO^2

-^ But^ in^ this^ preparation,^ Zn^ and^ H

+^ from^ the^ acid^ are^ also^ reacting^ to^

produce^ H^ (g):

-^ But^ in^ this^ preparation,^ Zn^ and^ H

+^ from^ the^ acid^ are^ also^ reacting^ to^

produce^ H(g):^2 +^ –^ o^ 2H+ 2^ e→ HE= 0.000 V^2 2+ –^ o^ Zn+ 2^ e→ Zn^ E= - 0.763 V+ 2+^ 2 H+ Zn → H+ ZnE= + 0.763 V^2 cell^0 (vigorous reaction^ ∆∆G= -147 kJ mol∆∆

-1^ )

-^ Hydrogen gas might be the reducing agent for converting Cr(VI) to Cr(III). •^ If so, the reduction of Cr(III) to Cr(II) would then be effected by Zn as the reducingagent in a second step.2–^ CrO+ 3 H^27

+^ 3+^ + 8 H→ 2 Cr+ 7 HO (^2 2) 3+ 2+^ 2+2 Cr+ Zn → 2 Cr+ Zn

-^ More likely, the production of Cr

2+^ is a mixture of reduction by Hand reduction by^2 Zn. 2– +^ 2+^ 2+^ CrO+ 3 H+ 8 H+ Zn → 2 Cr+ Zn+ 7 HO 272 2

-^ Regardless of the exact stoichiometry, the limiting reagent is Cr

2–^ O, from which 27 2+^ twice as many moles of Cris produced.

2+^ –^ 2 Cr+ 4 CHCO+ 2 HO → Cr^3222

(OCCH)(HO)^23422

-^ When wet, the complex is subject to oxidation to Cr(III) from oxygen:+^ O+ 4 H+ 4^2

−−−−^ o^ e→ 2 HO^ E= + 1.23 V^2 3+ – 2+^ o^ 4Cr+ 4e→ 4CrE= - 0.41 V2+ + 3+^ 4Cr+ O+ 4H→ 4Cr+ 2HO^ E= + 1.64 V 2 2 cell^ Avoid sucking air through the wet productduring the cold water and alcohol washes.

-^ After the final ether wash, allow air to pass through the product on the filter forabout 30 sec., then collect it on a large piece of dry filter paper or a porcelainplate to allow the residual ether to evaporate.

• Chromium(II) Acetate • Chromium(II) acetate dihydrate, Cr(OCCH)(HO)
  • , was discovered in 1844.
    • • It has long been regarded as unusual for a Cr(II) compound because it is brick-redand diamagnetic. • It is the longest-known example of a family of compounds that have the generalformula Cr(OCR)L, which have the following structure:

Cr(I)

-^ In the absence of^ π−back-donation low valent oxidation states of Cr such as Cr(I)can be stabilized by strong^ σ-donor ligands, i.e. strong field ligands:^ N^ N^ N^ N^ Cr^ N^ N^ N