Group 6 Flashcards by Barney Burden

What’re the properties of chromium?

Hard but brittle silver-coloured metal

Highly resistant to rust / acids & bases

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What does chromium’s electron configuration emphasise?

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

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What is the spinel structure of chromite (FeCr2O4)?

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

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How is chromite processed?

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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How is chromium isolated from chromite?

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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What’re Cr’s most dominant oxidation states?

Cr(II) and Cr(III)

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

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

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How are CrX2 halides formed?

X = F, Cl, Br

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

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Why can aqueous HCl not be used to create CrCl2?

Because Cr is too inert and hence won’t react

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How are CrX3 halides produced?

X = Cl, Br

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

a stronger oxidising agent is needed

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What colour is Cr2O3?

GREEN

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What process is used to reduce Na2[Cr2O7]?

Sulfur reduction

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What properties does Cr2O3 display?

Cr2O3 is amphoteric - it dissolves in acid and in base

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

Bright blue

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

It oxidises itself

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

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

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

Allows for stable Cr(II) complexes to be made

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

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

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Are ‘diars’ strong or weak field ligands?

Very strong - similar to CO or CN

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

M-M bonds are formed

They are diamagnetic

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How are dimers formed in Cr(II) complexes?

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

All d-electrons are paired - explains diamagnetic properties

Extra stability

Cr-Cr bond is short = strong

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Why are octahedral Cr(III) complexes more common?

Because they maximise CFSE

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

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

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Why does CrCl3 not dissolve readily in water?

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

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How can CrCl3 be encouraged to dissolve in water?

Add catalytic amount of reducing agent - such as Sn(II) to generate Cr(II) Main species in solution are now mixed Cr-Cl hydrates

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

Start with Cr(II)!!

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

Hard ligands

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

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

What’s the relevance of Cr in rubies?

Chromium is an impurity which is why rubies are red

How does Cr cause rubies to be red?

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.

Why’re emeralds green?

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]

What colour is Cr2O3?

Green

Why’re Cr(IV) and Cr(V) uncommon?

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

How is CrO2 made?

Partial decomposition of CrO3 at 800C under pressure

What’re the properties of CrO2?

Dark brown - d2 Ferromagnetic Metallic

What was CrO2 used in?

Used extensively in magnetic data recording - tapes

What’s the highest oxidation state of Cr?

What’s interesting about chromate?

It’s colour depends on pH Yellow at 11 Bright red at 2 Deep red at 0

What causes the intense colours seen in chromate complexes?

Charge transfers - too intense for d-d transitions

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

CrO3 in pyridine oxidises secondary -OH to ketones It is a known carcinogen

How can Cr(VI) imide formation be controlled?

Through steric or electronic control

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

Has the same number of electrons / same electronic structure

What drives Cr(VI) formation?

Elimination of 2HCl Formation of -OSiR3 from -NSiR3 Product kinetically tabulated by steric protection - bulky t-Bu and Me3Si groups surround Cr(VI) centre

How is Molybdenum obtained from naturally occurring MoS2?

MoS2 is oxidised to give MoO3 + SO2 MoO3 is then reduced using H2 gas to give Mo

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

It is contaminated with sulphur - process gives SO2

How is Tungsten isolated?

Generally converted to H2WO4, then to WO3 Then reduced using H2 gas to give W metal (Similar to Mo isolation)

What tungsten alloy is commonly used in steelmaking?

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

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

Important component of hydrodesulfurisation catalysts - removing S from crude oil H2SO4 makes holes in engines S compounds are converted to molecular S - relatively inert

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

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

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

They’re among those that can form polyoxometallates

What are polyoxometallates?

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

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

It is an oxidising agent

How are polyoxometallates made?

MO3 with strong aqueous base generates monomeric, tetrahedral anions - [MO4]2- Careful acidification condenses these together to form POM anions - crystal clusters

How can Mo and W halide complexes be made?

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

Why is WF6 colourless but WBr6 and WCl6 coloured?

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

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

5d > 4d >>> 3d

How do all Mo/W halides react with water?

Vigorously They form halide-carbonyl metal complexes + HX

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

Any hard ligands - such as acac

How can M(V) halides be made?

M + Cl2 (Br2) —> MX5

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

Has Mo-Mo bond despite being d1 metal ion

How are M(IV) oxides formed?

MO3 with H2 flow at 720K gives MO2 + H2O

Why are Cr(IV) complexes rare?

Because Cr(IV) is an oxidising oxidation state Wants to be Cr(III)

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

It was the first known 8-coordinate complex - 1939 Usually square antiprismatic structure

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

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

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

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

How is MoCl3 made?

By halide abstraction using SnCl2 MoCl5 is reduced to give MoCl3 and SnCl4

How is WCl3 made?

WCl2 is oxidised with Cl2 WCl2 is made from 6 WCl6 + 8 Bi This gives W6Cl12 + 8 BiCl3

What’s significant about M(III) halides?

They are molecular clusters with strong M-M interactions

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

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

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

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