Transition metal chemistry Flashcards
oxidation states
[earlier transition metals]
+3 = more common than +2
= strongly reducing
[later transition metals]
+2 = more common than +3
+3 oxn state = strongly oxidising
trend in oxidation states
increasing 3rd and higher I.E. across series
d-orbitals become more core-like (closer to nucleus)
halides - stability of oxn states
decreases in order of F- > Cl- > Br- > I-
Ti(IV)
high covalent character
high charge density on metal (v. polarising)
TiCl4 = covalently bonded liquid + soluble in benzene
TiCl2 = ionic solid
thermodynamically unfavourable oxidation states
[intermediate = below line]
can still be observed (disproportionation reaction may be slow)
= kinetically stable
scandium
Sc3+ (d0)
colourless
strong Lewis acid
titanium
+4 (d0) - TiO3 2-, TiO 2+
not Ti4+ -> charge = so high (would pull hydrogens from water ligand - TiO2+)
strong Lewi acid
Ti3+ = strong reducing agent
No Ti2+ aq chem - TiCl2 reacts violently with water, reducing it to H2
vandium
VO4 3- at pH 14
VO2 + at pH 0
reduced to VO2+ (blue), V3+ (green) and V2+ (violet) - all stable rwt disproportionation
VO2+ = square planar
V2+ = strongly reducing and oxidised by air (needs to be kept in inert atmosphere)
chromium
Cr(IV) = powerful oxidising agent
Cr2O7 2- (orange) in acidic solutions
CrO4 2- (green) in alkaline solutions
Cr2O7 2- + 3H2O -> CrO4 2- + 2H3O+
Cr(III) d3 - high CFSE; kinetically inert
Cr(II) = strongly reducing
manganese
Mn(VII) = powerful oxidising agent
MnO4- = tetrahedral anion
intense colour due to M->L transfer band
Mn(III) = distorted octahedral due to J-T distortion
Mn(II) = v. pale (d->d = forbidden by all selection rules)
iron
Fe3+ + e- -> Fe2+
position of equilibrium + stability of oxn state determined by ligands
Fe3+ = acidic solutions - high charge on metal
equilibrium affects by pH
-Fe3+ stable at pH <2
Fe2+ + O2 + 4H3O+ -> 4Fe3+ + 6H2O (stable but slowly oxidises due to presence of dissolved oxygen)
strongly coloured - M->M transfer bands
cobalt
Co(III) = strong oxidising agent
LS complex - kinetically inert despite thermodynamic driving force
N-donor ligands (e.g. NH3) greatly stabilise Co3+ rwt reduction
Co(II) - colour depends on geometry
octahedral = pink
tetrahedral = intense blue
pale -> intense colour
no longer have centre of symmetry
pink -> blue
Δoct = smaller for tetrahedral compared to octahedral
nickel
Ni(II) = stable to oxidation + reduction
range of geometries (depends on counter ion)
copper
Cu2+ (cupric) and Cu+ (cuprous)
octahedral = distorted due to J-T effect
zinc
Zn2+
colourless (no d-d e- transition possible)
wide range of geometries
not really a transition metal - neither metal nor compound has partially filled d orbitals
transition metal triads - chromium, molybdenum + tungsten
CrO3 + [CrO4]2- = strong oxidising agents
WO and [WO4]2- = not readily reduced
high oxn states: 1st row < 2nd row < 3rd row
Mo(IV) and W(IV) = common
Mo(III) and W(III) = sparse
high coordination numbers possible for larger metals (1st row - ions not big enough)
transition metal triads - nickel, palladium + platinum
Ni and Pd = +2
Pt = +4
Pd + Pt = square planar (high Δoct)
M-M bonds and low oxn states (+1,0) = more important down group
why don’t t2g orbitals interact with any ligand orbitals?
don’t have correct symmetry
directed between axis
evidence for covalency
pairing energies have been shown to be lower in metal complexes than in gaseous Mn+ ions
indicates inter-electronic repulsion is less in complexes so effective size of metal orbitals has increased
= nephelauxetic effect
Lenz’s law (MD)
in absence of any magnetic moment (i.e. unpaired e-)
= induced magnetic field that opposes main field
=diamagnetism (MD)
*repelled by magnetic field
MP
if there’s a magnetic moment (unpaired e-) and moments don’t interact with each other, they align to give overall magnetic moment (MP)
*attracted to magnetic field
effect of metal on Δ - charge
as charge on M increases, Δ increases
[reason]
ionic radius decreases
∴ M-L decreases
greater interaction between M and L orbitals
increases energy of antibonding eg orbitals ∴ t2g-eg gap increases
effect of metal on Δ - going down group
Δ increases
[reason]
size of orbital increases
greater interaction between M and L orbitals
increases energy of antibonding eg orbitals ∴ t2g-eg gap increases
change in orbital size from 3d to 4d compared to 4d to 5d
bigger
due to lanthanoid contraction
why are 4d and 5d complexes low spin?
due to increasing orbital size and decrease in pairing energy
dipole moment of CO
0.40 Debye
how does CO coordinate to a metal centre ?
via δ+ carbon atom
σ-donation from filled orbital on CO to empty M d-orbital
π-donation from filled d-orbital on M to empty π* on CO = BACKBONDING
backbonding
π-donation from filled d-orbital on M to empty π* on CO
effect of backbonding
strengthens M-C bond
[reason]
e- density is put into CO antibonding orbital
weakens C-O bond
decreases in v(CO) from 2143 cm-1 for CO(g)
thermodynamics
[extent of reaction]
relates to ΔG
quantified by equilibrium constant = K
stable vs unstable
kinetics
[speed of reaction]
relates to activation energy
quantified by rate constant = k
inert vs labile
substitution reactions
K1 > K2 > K3 [step-wise formation constants decrease]
less likely to be sub. as number of H2O ligands decreases (being replaced with NH3)
formation constant
β6 = K1K2K3K4K5K6
high = large CFSE (means ligand being subbed onto molecule is a stronger field ligand)
chelate effect
bidentate/polydentate = enhanced stability
ΔS = +ve (increase in disorder) = large K
+ less rearrangement required
hard
high charge density
non-polarisable
soft
low charge density
polarisable
ligand sub. reactions
MLxX + Y ⇌ MLxY + X
X = leaving group
Y = entering group
L = spectator ligand
L + X covalently bonded to metal = inner-sphere ligands
outer sphere of solvent molecules loosely associated
why is Pd(II) used for kinetic work on square planar complexes?
relatively inert to oxidation/reduction
virtually always square planar
rate of ligand sub. = slow (t1/2 > 60s) - easy to study
what type of mechanism do square planar complexes undergo?
associative mechanisms
evidence for associative mechanisms
- k values for displacement of Cl- by H2O in [PtCl4]2-, [PtCl3(NH3)]-, [PtCl2(NH3)2], [PtCl(NH3)3]+
suggests associative pathway since dissociative pathway would be expected to be dependent on charge of complex
- most reactions occur with stereoretention at Pt (3-coordinate intermediate = all ligands same; no cis/trans)
- all reactions accompanied by large, -ve ΔS = loss of molecular freedom approaching transition state
- if pressure increases, sub. accelerated and large -ve vol. of activation (ΔV) observed
k(obs) vs [Y] graph - effect of solvent
changes intercept
polar = increases
non-polar = decreases
substitution reactions - influence of spectator ligands
more steric bulk = slower reaction
effect is more prominent when bulky ligand = cis to leaving group
substitution reactions - trans effect
effect of ligand on sub. rate for ligand trans to it
trans effect - cause of increased RoR
- destabilisation of ground state
- stabilisation of transition state
σ-effects on trans effect
trans ligand and leaving group, X, compete for same metal orbitals
competition = relaxed in trigonal bipyramidal state
if trans ligand = strong σ-donor = less orbital for interaction with X
π-effects on trans effect
if ligand = π-acceptor, charge delocalisation eases formation of 5-coordinate transition state/intermediate
strong π-acceptor ligands accept e- density donated by incoming Nu
helps spread charge over complex = more stable
= lower energy of transition state
trans influence
effect of ligand on ground state properties
i.e. bond angles + NMR
just σ-components that have influence
sub. in octahedral complexes - effect of charge
higher charge = less labile ligands
stronger M-L bond strength (suggests it’s RDS)
sub. in octahedral complexes - CFSE
loss of CFSE going from ground state -> transition state
= increased in activation energy
= decrease in rate
=CFAE (crystal field activation energy)
evidence for dissociative mechanisms
- rates generally unaffected by nature of entering group
- rates depend on nature of leaving group - correlates with M-X bond strength
[easiest to displace] NO3- > I- >Br- > Cl- > MeCo2- NCS- ~ NH3 > OH-
- rate increases by increasing bulk of spectator ligands
- increasing pressure slows reaction (ΔV = +ve)
which type of sub. mechanism does square planar undergo?
associative
which type of sub. mechanism does octahedral undergo?
dissociative
acid catalysis
protonates leaving group
**leaving group must have lone pair that’s not bonded to metal
base catalysis
ion doesn’t attack metal centre
instead, ligand is deprotonated to give base complex
what is tunnelling?
complexes get close and e- “hops” from 1 M to another
tunnelling - requirements
Ea must overcome electrostatic repulsion between ions of like charge
when reactants have different bond lengths, vibrationally excited states with equal bond lengths must be formed (allows e- transfer to occur)
greater change in bond lengths = slower rate of e- transfer
inner sphere e- transfer
covalently-bound bridging ligand that may transfer with e-
- bridge formation
- e- transfer
- bridge cleaving
bioinorganic chemistry - roles
- structural - stabilising protein structures
- functional - metal ion involved in reactivity
[transport, enzymes, metal storage/transport, photoredox]
oxygen transport and storage
Hb = metalloprotein
Mb = monomer of Hb (only contains iron atom)
in both compounds, iron is bound to porphyrin ring = HAEM GROUP
each haem group can absorb 1 molecule of O2 = red colour
haemoglobin - coordination of oxygen
O2 = π-acceptor
enters 6th coordination site
configuration changes to LS
since antibonding orbitals are no longer occupied, ion = smaller -> therefore moves to haem ring
O2 coordinated in bent, end-on manner + supported by H bond to distant a.a.
haemoglobin - binding of O2 vs CO
O2 - reversible [when conc. of O2 decreases in blood, it’s released from haem group]
CO = irreversible
haemoglobin - cooperative binding
as O2 binds to 1 iron, affinity of other iron atoms for O2 increases
due to conformational changes in protein chains [movement into porphyrin ring - HS->LS]
myoglobin
binds O2 better at low conc.
in tissues, O2 is released by Hb and taken up by Mb
haemoglobin - pH
low pH - O2 released more readily
metabolism => CO2 released = lower pH; helps transfer O2 from Hb to Mb
haemocyanin
copper containing
present in molluscs + anthropoids
blue colour
haemoerythrin
non-haem di-iron protein
present in marine worms
purple colour
cis-platin - function
anti-tumour agent
trans-plantin?
trans-platin = inactive
unable to bridge between guanine-N atoms
Pd cis/trans complexes
cis = inactive
ligands = more labile (break bonds more readily
cis-trans isomerism occurs more readily - rapid interconversion to thermodynamically trans complexes
problems with cis-platin
- affects narrow range of tumours
- toxic - lots of side effects
- not v. soluble in water (can’t be taken orally)
- development of resistance in tumour cells
2nd and 3rd generation of Pt drugs
[carboplatin]
-dicarboxylate = less labile than Cl
-lower toxicity (larger doses possible)
-hydrolysis = slower
[satraplatin]
-Pt(IV) = more soluble
[JM335]
-no cis-amines (different mechanism?)
key features of Pt anti-tumour drugs
cis amines
at least 1 amine group
optimised leaving group
good water solubility and stability in 0.1M NaCl
reactions with competing S-donors suppressed
ability to cross cell boundary
cis-platin - mechanism
conc. of Cl- inside cells = lower than outside (why hydrolysis only occurs inside cells)
labile H2O replaced by N atoms from 2 of DNA base pairs - occurs w/o breaking H bonds linking strands together
Pt bridges are between 2 neighbouring guanine bases
chelation tilts guanine rings from normal stacked position - disrupts helix + interferes with replication
why is H bonding important in cis-platin?
stabilises both intermediate + platinated DNA
no N-H = no anti-tumour activity
H bonding occurs both the P-backbone and carbonyl O of guanine = explanation for preference of G over A
