Peter Slater y2

d-block chemistry

oxidation states of transition metals

• early/late transition metals have few oxidation states, those in the center have a wide range

• chem of 1st series TMs dominated by M2+ and M3+ ions

2nd and 3rd series TMs

• lower oxidation states are dominated by M-M interactions

• high oxidation states form molecular species and covalent bonds - high oxidation states are most stable for 2nd and 3rd row

CFSE

• tetrahedral - e (2 orbitals) lower, T2 (three orbitals) higher energy - oct opposite but t2h and eg

• only get high spin for 1st series TMs, 2nd and 3rd always low spin

• jahn teller distortion - occurs to remove degeneracy of electronic states eg. Cu2+ d9 and d4 hs eg. Mn4+ (exaggerated version for d8 square planar complexes)

• explains lattice enthalpies for MCl2 and hydration energies of divalent metal ions

determining the spontaneity of a reaction

∆G = -nFE

• combine any two standard reduction reactions by reversing one of the couples and adding the potentials together

• if E is =ve the reaction as written is spontaneous and ∆G = -ve

• if E is negative then the reverse reaction is spontaneous

• E⁰ : most positive = most oxidising

latimer diagrams

used to monitor relative (thermodynamic) stabilities of all oxidations states of a transition metal

• potentials are given as reduction potentials

• can add branches on to describe a metal in the presence of other species

  

what factors change the redox potentials?

generally anything that alters ∆G0

• concentration

• temperature

• other reagents that arent inert

• pH

in aqueous chemistry the most important is pH change hence it’s conventional to construct latimer diagrams for the two extremes of pH=0 and pH=14 (1 M acid and 1 M base)

frost diagrams

plot nE (V) against oxidation state

• determine oxidation states and plot nE using the equation nE = -∆G/F

interpreting frost diagrams

• if the line connecting higher and lower lying species has a +ve slope then the higher lying species is an oxidising agent - if -ve the higher lying species is a reducing agent

• steeper slope = stronger oxidising/reducing agent

• species at the bottom of the graph have low ∆G⁰ and have little tendency to react

• the lowest species on the graph are the thermodynamic final product

determining disproportionation and comproportionation from frost diagrams

• a species lying above the line connecting its neighbours is thermodynamically unstable to disproportionation and will disproportionate to the more stable species either side

• species likely to undergo comproportionation to a third species are located to the left and right of a point which lies below the line connecting two species

• dis - a single species is unstable and an autoredox reaction can occur at any time

• com - required both reactants to be present - if one is missing the other species remains stable

frost diagrams of the 1st row

• all metals are potent reducing agents except Cu

• reducing strength of M0 decreases from Ca to NI

• early TMs favour the +3 oxidation state

• for late TMs +2 is more stable

• elements in the middle (Mn + Fe) have the largest range of accessible oxidation states

• only Ti in its highest state (+4) has virtually no oxidising power

trends in behaviour down group 6

• highest oxidation state for Cr (+6) is strongly oxidising, Mo (+6) mildly oxidising but completely non-oxidising for W

• most stable oxidation states are +3 for Cr, +4 for Mo and +6 for W

• higher oxidation states become more favourable in 2nd and especially 3rd transition series

• in the +5 state the energy of Mo and W species are the same which leads to very similar behaviour

• behaviour is fairly representative of trends down any d-block group

effects of ligands

• eg. Fe2+ much more stable in presence of CN- - weka field ligand (H2O) subbed for strong field ligand so change in CFSE

stabilisation of high oxidation states

• high oxidation states are very strong oxidising agents and so are highly susceptible to reduction

• in order to stabilise TMs in high oxidation states they must be complexed by other species which are even stronger oxidising agents

• O2- and F- are the most commonly found coordinating species for high oxidaiton state TMs

• most hihg ox states are only accessible as fluorides

• bonding in high ox state halides is prodominantly covalent

• group 8: can only get Fe6+ but can get Ru 8+ and Os 8+

high oxidation states in solution

mononuclear oxo complexes

eg. CrO₄^2- (Cr^VI) - high oxidation states are stabilised by the O2- ligand

• strong polarisation of e- density on water of hydroxyl O by high oxidation state cations - increases bronsted acidity of H₂O and OH-

• oxo complex formation also preferred by high pH

what are the key methods of synthesising mixed metal oxides containing transition metals?

• standard solid state reaction

• coprecipitation

• sol-gel synthesis

• hydrothermal synthesis

• mechanochemical synthesis

standard solid state reaction

• direct reaction of a mixture of solid starting materials (use carbonates for group 1/2 and oxides for other groups)

• most widely used synthesis method

• needs high temperatures, typically >900ºC and long reaction times - several hours/days with intermediate grinding

• need to have materials in correct ratios otherwise cannot remove any impurities

why are high temperatures and long synthesis times needed for standard solid state reactions?

1. formation of product nuclei is difficult

• usually large distances between reactants and products

• large amount of structural reorganisation (bond breaking and reforming) involved in forming the product

2. growth of product layer may be even more difficult

• need counter-diffusion of cations through existing product layer to the new reaction interfaces (basically if a lump of product of formed in the middle, the cations left to react are now even further away)

kinetic and thermodynamic considerations of standard solid state reactions

what is the rate determining step?

• transport of ions to the reaction interface?

• reaction at the surface/interface?

• transfer of matter away from reaction interface (couples to 1)?

slow kinetics can lead to metastable products

• not fast enough to form product

phase of interest may not be thermodynamically stable (relative to other possible products) at the temperature required for reaction

• even slight differences can alter stability

disadvantages of standard solid state reaction method

• high temp and long reaction times required - hence lots of energy and so not ideal for industrial synthesis

• inhomogeneity of mixture: can lead to impurities or variations in composition (local excess of reagents even it total ratios are right) - regrind and reheat to help rectify

• volatile products (eg. alkali metal oxides) means the high temperatures lead to a loss of reactant - add excess reactant and/or use seal tube to rectify

• some phases only stable at low temperatures - use low temp synthesis route

coprecipitation

• for a lower reaction temperature we need a high degree of homogenisation and a small particle size - this can be achieved using coprecipitation

• eg. mix the correct molar ratios of aqueous strontium nitrate and iron nitrate. adding NaOH will then cause it to precipitate- precipitate can then be ground with Li2CO3 or LiOH and heated to 700-800ºC for 12 hours in air to give final product

• problems arise if the reactants have different solubilities or do not precipitate at the same rate - means that you don’t get a homogenous mix and can end up with the incorrect reactant ratio

sol-gel synthesis

sol-gel method provides homogenous, small particle size mixtures and so lower reaction temperatures can be employed

• eg.

• dissolve metal salts eg nitrates in water

• add a dicarboxylic acid and a dialcohol

• heat → gel formation (metal ions dispersed in a poly-ester matrix)

• further heat treatment (typically 200-300ºC lower than standard solid state synthesis temperatures) → product (ester network burns off)

• alternative routes use alkoxides

evaluation of sol-gel synthesis

advantages

• lower temperature

• synthesis on new phases possible (including metastable phases)

• capacity to form films or fibres and to control particle sizes and shapes

disadvantages

• can be high cost - especially if using alkoxides

• long processing times sometimes required

• longer than standard solid state synthesis - can sometimes take weeks to form a gel

• can be problems with use of alkoxides if different hydrolysis rates - issues with achieving a homogenous gel

hydrothermal synthesis

• utilises water under pressure and at temperatures above its boiling point to speed up solid state reactions (usually 100-200ºC)

roles of water:

1. serves as a pressure transmitting medium

2. some or all of the reactants are partially soluble in H2O under pressure, and this enables reactions to take place in, or with the aid or liquid and/or vapour phases

• reactions are normally carried out in a sealed teflon lined stainless steel pressure vessel connected to an external pressure control often known as an autoclave

• allows reactions to occur that would normally required very high temperatures

• particularly suited method to phases that are unstable at high temperatures eg. zeolites and MOFs

• sample must be stable in water so cant use to prepare YBa₂Cu₃O_7-x

mechanochemical synthesis

• some compounds can be prepared by simply ball-milling the precursor powders (ball milling causes local heating of the powders

• powders are milled in a ball mill for several hours- not all compounds can be prepared by this routec