Group 6 Flashcards
What’re the properties of chromium?
Hard but brittle silver-coloured metal
Highly resistant to rust / acids & bases
What does chromium’s electron configuration emphasise?
4s1 3d5 emphasises the small energy gap between 4s and 3d orbitals - more stable configuration
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
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
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
What’re Cr’s most dominant oxidation states?
Cr(II) and Cr(III)
Which is more stable Cr(II) or Cr(III)?
Cr(III) is more stable and is more common as a result
How are CrX2 halides formed?
X = F, Cl, Br
From HX gas + finely-divided Cr at 600-700C
Why can aqueous HCl not be used to create CrCl2?
Because Cr is too inert and hence won’t react
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
What colour is Cr2O3?
GREEN
What process is used to reduce Na2[Cr2O7]?
Sulfur reduction
What properties does Cr2O3 display?
Cr2O3 is amphoteric - it dissolves in acid and in base
What colour is [Cr(H2O)6]2+?
Bright blue
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
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
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.
Are ‘diars’ strong or weak field ligands?
Very strong - similar to CO or CN
What occurs in Cr(II)-acetate / carboxylate complexes?
M-M bonds are formed
They are diamagnetic
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
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
Why are octahedral Cr(III) complexes more common?
Because they maximise CFSE
What is the rate of ligand exchange for Cr(III) complexes?
Very slow - similar to low spin Co(III) which is d6.
Why does CrCl3 not dissolve readily in water?
Because it is Cr(III) - hence ligand exchange is VERY SLOW
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?
6+
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»_space;> 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
