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chem214 > Group 6 > Flashcards

Group 6 Flashcards

1
Q

What’re the properties of chromium?

A

Hard but brittle silver-coloured metal

Highly resistant to rust / acids & bases

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2
Q

What does chromium’s electron configuration emphasise?

A

4s1 3d5 emphasises the small energy gap between 4s and 3d orbitals - more stable configuration

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3
Q

What is the spinel structure of chromite (FeCr2O4)?

A

Chromite has a spinel structure, with Cr(III) in octahedral sites and Fe(II) in tetrahedral sites

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4
Q

How is chromite processed?

A

Most is smelted with charcoal and coke in an electric arc furnace at high T

Gives FeCr alloy, used as an additive in specialist steels

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5
Q

How is chromium isolated from chromite?

A

It is chromite (FeCr2O4) is combined with Ca(OH)2 or NaOH at 1000C to produce [CrO4]-

[CrO4]- is water soluble and so is extracted, Fe(OH)3 is filtered off

Cr(IV) is reduced with C or S to give Cr2O3 - GREEN

Cr2O3 is then reduced with Al or Si to Cr

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6
Q

What’re Cr’s most dominant oxidation states?

A

Cr(II) and Cr(III)

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7
Q

Which is more stable Cr(II) or Cr(III)?

A

Cr(III) is more stable and is more common as a result

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8
Q

How are CrX2 halides formed?

A

X = F, Cl, Br

From HX gas + finely-divided Cr at 600-700C

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9
Q

Why can aqueous HCl not be used to create CrCl2?

A

Because Cr is too inert and hence won’t react

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10
Q

How are CrX3 halides produced?

A

X = Cl, Br

From finely-divided Cr and X2 gas in a tube furnace

  • a stronger oxidising agent is needed
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11
Q

What colour is Cr2O3?

A

GREEN

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12
Q

What process is used to reduce Na2[Cr2O7]?

A

Sulfur reduction

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13
Q

What properties does Cr2O3 display?

A

Cr2O3 is amphoteric - it dissolves in acid and in base

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14
Q

What colour is [Cr(H2O)6]2+?

A

Bright blue

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15
Q

What is the problem with [Cr(H2O)6]2+ when making Cr(II) complexes?

A

It oxidises itself

Cr slowly reduces the H2O to H2 to give a violet [Cr(H2O)6]3+ complex

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16
Q

Why are non-aqueous solvents and a N2 atmosphere used to create Cr(II) complexes?

A

Because [Cr(H2O)6]2+ oxidises itself to Cr(III)

Allows for stable Cr(II) complexes to be made

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17
Q

Are most Cr(II) complexes high spin or low spin?

A

Usually high spin - requires very strong field ligands to give low spin Cr(II) complexes.

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18
Q

Are ‘diars’ strong or weak field ligands?

A

Very strong - similar to CO or CN

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19
Q

What occurs in Cr(II)-acetate / carboxylate complexes?

A

M-M bonds are formed

They are diamagnetic

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20
Q

How are dimers formed in Cr(II) complexes?

A

Dz2 orbitals give sigma bond

Dxz and dyz orbitals give two degenerate pi-bonds

The two dxy orbitals overlap to give a fourth bond so called ∂ bond

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21
Q

What’s the significance of the extra bond in Cr(II) acetate complexes?

A

All d-electrons are paired - explains diamagnetic properties

Extra stability

Cr-Cr bond is short = strong

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22
Q

Why are octahedral Cr(III) complexes more common?

A

Because they maximise CFSE

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23
Q

What is the rate of ligand exchange for Cr(III) complexes?

A

Very slow - similar to low spin Co(III) which is d6.

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24
Q

Why does CrCl3 not dissolve readily in water?

A

Because it is Cr(III) - hence ligand exchange is VERY SLOW

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25
Q

How can CrCl3 be encouraged to dissolve in water?

A

Add catalytic amount of reducing agent - such as Sn(II) to generate Cr(II)

Main species in solution are now mixed Cr-Cl hydrates

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26
Q

What’s a good route for making Cr(III) complexes?

A

Start with Cr(II)!!

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27
Q

What ligands does Cr(III) prefer? Hard or soft?

A

Hard ligands

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28
Q

What’s the issue with diars when forming Cr(III) complexes?

A

Diars and phosphines do not form complexes with common Cr(III) starting materials

Too soft

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29
Q

What’s the relevance of Cr in rubies?

A

Chromium is an impurity which is why rubies are red

30
Q

How does Cr cause rubies to be red?

A

Cr(III) replaces some Al(III), in an octahedral hole

Cr(III) is bigger than Al(III). Cr-O are shorter, with the crystal field strength abnormally high - d3 ion.

Therefore the d-d bands for Cr(III) in ruby are at higher than usual energy - causing red colour.

31
Q

Why’re emeralds green?

A

Cr(III) replaces some Al(III) in octahedral holes

In this case, Cr(III)-O distances are close to that seen in Cr2O3 - hence green. (Much longer than bonds in rubies)

[Be3Al2(SiO3)6 becomes Be3Cr2(SiO3)6 in some cases]

32
Q

What colour is Cr2O3?

A

Green

33
Q

Why’re Cr(IV) and Cr(V) uncommon?

A

Because they disproportionate to Cr(III) + Cr(VI)

34
Q

How is CrO2 made?

A

Partial decomposition of CrO3 at 800C under pressure

35
Q

What’re the properties of CrO2?

A

Dark brown - d2

Ferromagnetic

Metallic

36
Q

What was CrO2 used in?

A

Used extensively in magnetic data recording - tapes

37
Q

What’s the highest oxidation state of Cr?

A

6+

38
Q

What’s interesting about chromate?

A

It’s colour depends on pH

Yellow at 11

Bright red at 2

Deep red at 0

39
Q

What causes the intense colours seen in chromate complexes?

A

Charge transfers - too intense for d-d transitions

40
Q

What is Jones’ reagent? Why isn’t is used nowadays?

A

CrO3 in pyridine oxidises secondary -OH to ketones

It is a known carcinogen

41
Q

How can Cr(VI) imide formation be controlled?

A

Through steric or electronic control

42
Q

What does ‘M=NR unit being isoelectronic with M=O’ mean?

A

Has the same number of electrons / same electronic structure

43
Q

What drives Cr(VI) formation?

A

Elimination of 2HCl

Formation of -OSiR3 from -NSiR3

Product kinetically tabulated by steric protection - bulky t-Bu and Me3Si groups surround Cr(VI) centre

44
Q

How is Molybdenum obtained from naturally occurring MoS2?

A

MoS2 is oxidised to give MoO3 + SO2

MoO3 is then reduced using H2 gas to give Mo

45
Q

What’s the problem with purifying MoS2 - most common mineral of Mo?

A

It is contaminated with sulphur - process gives SO2

46
Q

How is Tungsten isolated?

A

Generally converted to H2WO4, then to WO3

Then reduced using H2 gas to give W metal

(Similar to Mo isolation)

47
Q

What tungsten alloy is commonly used in steelmaking?

A

Tungsten carbide - WC - almost as hard as diamond, used in cutting tools

48
Q

What is Molybdenum useful for? Why’s this important?

A

Important component of hydrodesulfurisation catalysts - removing S from crude oil

H2SO4 makes holes in engines

S compounds are converted to molecular S - relatively inert

49
Q

How does Cr(VI) compare to Mo(VI) and W(VI)?

A

Cr(VI) is highly oxidising

M(VI) oxides are fairly stable towards reduction - 4d/5d high oxi states more stable

50
Q

What’s significant about molybdenum and tungsten in relation to 4d/5d metals?

A

They’re among those that can form polyoxometallates

51
Q

What are polyoxometallates?

A

They’re large, discrete anions that have a huge surface area

52
Q

What significant about the [Mo7O24]6- POM anion?

A

It is an oxidising agent

53
Q

How are polyoxometallates made?

A

MO3 with strong aqueous base generates monomeric, tetrahedral anions - [MO4]2-

Careful acidification condenses these together to form POM anions - crystal clusters

54
Q

How can Mo and W halide complexes be made?

A

M + 3F2 gives MF6 at room temperature - both stable and colourless

55
Q

Why is WF6 colourless but WBr6 and WCl6 coloured?

A

W(VI) is d0 so cannot be d-d - must be charge transfer

56
Q

What’s the relative stability of M(VI)?

A

5d > 4d&raquo_space;> 3d

57
Q

How do all Mo/W halides react with water?

A

Vigorously

They form halide-carbonyl metal complexes + HX

58
Q

What type of ligands do Mo(VI) and W(VI) prefer?

A

Any hard ligands - such as acac

59
Q

How can M(V) halides be made?

A

M + Cl2 (Br2) —> MX5

60
Q

Why is [Mo2O4(H2O)6]2+ diamagnetic?

A

Has Mo-Mo bond despite being d1 metal ion

61
Q

How are M(IV) oxides formed?

A

MO3 with H2 flow at 720K gives MO2 + H2O

62
Q

Why are Cr(IV) complexes rare?

A

Because Cr(IV) is an oxidising oxidation state

Wants to be Cr(III)

63
Q

What’s significant about Mo(IV) cyanide complexes?

A

It was the first known 8-coordinate complex - 1939

Usually square antiprismatic structure

64
Q

Are M(III) oxidisation states common or rare for Mo/W?

A

Quite rare - easily oxidised to M(IV) or M(V)

65
Q

Is the chemistry of M(II) / M(III) oxidation states similar or very different for Mo/W?

A

These oxidation states are closely inter-related for Mo and W.

66
Q

How is MoCl3 made?

A

By halide abstraction using SnCl2

MoCl5 is reduced to give MoCl3 and SnCl4

67
Q

How is WCl3 made?

A

WCl2 is oxidised with Cl2

WCl2 is made from 6 WCl6 + 8 Bi

This gives W6Cl12 + 8 BiCl3

68
Q

What’s significant about M(III) halides?

A

They are molecular clusters with strong M-M interactions

69
Q

Why do M(III) halides have strong M-M interactions?

A

The discrete molecular clusters show significant d-d bonding - much more common with more diffuse 4d/5d orbitals

70
Q

What’s significant about the Mo(II) complex [Mo2Cl8]4-?

A

Has Mo-Mo quadruple bond - 273pm is VERY SHORT

Is diamagnetic

chem214 (7 decks)

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