d-block - crystal field theory

Question 1

give 2 simplifying assumptions made in crystal field theory

Answer 1

• ligands are treated as point -ve charged
• ignores possibility of metal d-electrons being involved in bonding

Question 2

what are the 2 important interactions happening in metal coordinate complexes?

Answer 2

• interaction of ligands with core (= metal nucleus and inner electrons, +vely charged)
• interaction of ligands with d valence electrons, this would be a repulsion

Question 3

what happens according to crystal field theory when ligands interact with metals?

Answer 3

as ligands (= points of electron density) approach d orbitals from an infinite distance, this creates a symmetrical field around the metal atom
this causes d electrons to increase in energy uniformly as they repel ligand charges
as ligands approach more closely and start to orient themselves around the metal atom the d orbitals lose their degeneracy - the new arrangement created depends on the geometry of the complex

Question 4

what are the 5 types of d-orbital?

Answer 4

dxy, dyz, dxz, dx^2y^2, dz^2

Question 5

explain the movement of d-orbitals in an octahedral complex

Answer 5

in octahedral complexes ligands orient themselves along x/y/z axes
dxy, dyz, dxz = stabilised as they don’t point directly at -ve ligands so less electron repulsion, these form the t2g set
dx^2y^2, dz^2 = destablised as point directly at ligands, form a degenerate set eg

Question 6

what is Δoct?

Answer 6

this is the crystal field stabilisation energy, the gap between the 2 sets of orbitals, and represents the stabilisation relative to spherical field

Question 7

when would Δoct =0?

Answer 7

if the electron configuration is d0, d5 or d10, Δoct would = 0 as stabilisation must offset destabilisation of orbitals, electrons in each t2g set provides -2/5Δoct, and in eg set provides +3/5Δoct
if electrons are evenly distributed in every orbital then Δoct should =0 as stabilisation and destabilisatoin cancel out

Question 8

give 1 piece of observable evidence for crystal field theory

Answer 8

when the energies of hydration for all transition metals is plotted, we expect a linear trend of increasing favourability from L->R
however the measured trend shows only Ca2+ (d0), Mn2+ (d5) and Zn2+ (d10) sit ont he expected line, the rest form a double hump shape with peaks between these enrgies
this is because then electrons are added to more stable t2g set the energy increases on the plot, as when added to less stable eg set energy decreases, this repeats when pairing electrons as well

Question 9

how does electron filling of orbitals become more complicated at d4?

Answer 9

at d4 there are 2 possibilities - pair the electrons in the stable set or keep electrons unpaired and place the fourth in the destabilised set
both cost energy, either unfavourable pairing of electrons or placing electron in a higher energy orbital

Question 10

how can we determine where to place the 4th/etc electron?

Answer 10

if energy to put electron in eg set < electron repulsion then it will be placed in the higher energy set = high spin/weak field arrangement
if energy to put electron in eg > electron repulsion then it will be paired in the lower energy set = low spin/strong field arrangement

Question 11

why is high/low spin only relevant between d4 and d7?

Answer 11

d1-d3 has only 1 possible arrangement, as all electrons are able to go unpaired in the lower energy set
d8-d10 also only has 1 possible arrangement, as the orbitals are so full that electrons can only be paired in high energy orbitals

Question 12

how do we determine if a molecule will have a high or low spin arrangement?

Answer 12

the CFSE needs to be determined fromt he 2 arrangements, using Δoct values and pairing energy

Question 13

state a necessary assumption to enable determination of high/low spin arrangements

Answer 13

it needs to be assumed that all electron repulsion stays constant

Question 14

give 3 factors that affect Δoct

Answer 14

ligands
metal oxidation state
position of metal in d-block

Question 15

how does ligand affect Δoct?

Answer 15

the value of Δoct decreases as you go down the spectrochemical series
therefore ligands low on the spectrochemical series tend to form high spin complexes, whereas ligands low in series form low spin complexes

Question 16

does the impact of ligands on Δoct align with crystal field theory?

Answer 16

no - remember in CFT all ligands are treated as point charged, it is an extreme simplification, this effect actually provides evidence against CFT

Question 17

how does the metal ion affect Δoct?

Answer 17

metals that interact more strongly with ligands are considered to have greater electron repulsion, this depends on the charge of the metal - higher charge = stronger interaction = larger Δoct
going down a group compounds are more likely to be low spin, as metal ligand overlap increases, increasing electron repulsion and therefore increasing Δoct

Question 18

how does high/low spin arrangements impact atomic radii?

Answer 18

in the low spin arrangement, there are no electrons in the higher energy set, therefore smaller radius and less repulsion, so ligands can approach closer (+ vice versa for high spin)

Question 19

give 1 limitation of CFT

Answer 19

jahn-teller distortions

Question 20

what are jahn-teller distortions

Answer 20

these occur when the eg set contains an unpaired electron/unequal arrangement of electrons, its axial bond lengths will be different to its equatorial bond lengths, therefore it will not have a regular octahedral shape
as there is an extra electron among the 2 higher energy orbitals, it can either reside in the dx^2y^2 or dz^2 orbital
if it is in the dz^2 orbital there will be more electron repulsion along z-axis causing elongation along x-axis = jahn-teller distortion

Question 21

how does jahn-teller distortion affect orbitals?

Answer 21

any non linear molecular system in a degenerate electronic state will undergo distortion to remove the degeneracy, leading to a lower energy state

Question 22

explain the movement of d-orbitals in tetrahedral complex

Answer 22

dx^2y^2, dz^2 no longer align directly with bonded ligands, therefore set decreases in energy
dxy, dyz, dxz all point more closely with tetrahedrally arranged ligands so increase in energy
the high energy set is now t2 and the lower energy set is e

Question 23

how does Δtet compare to Δoct?

Answer 23

as stabilisation/destabilisation occurs in a tetrahedral complex to a lesser degree, Δtet is less than Δoct, ~ 4/9ths

Question 24

how does Δtet affect spin of complex?

Answer 24

as Δtet is much smaller than Δoct, tetrahedral complexes tend to adopt high spin state

Question 25

explain the movement of d-orbitals in square planar complexes

Answer 25

as ligands are removed from the z axis, ligand electron repulsion along this axis diminishes so all orbitals with a z component are lower in energy, with the dxz/dyz orbitals becoming the lowest in energy as ligand-metal bonds along z-axis stretch, they also compress along x/y-axes, increasing electron repulsion along these axes dx^2y^2 orbital points directly at these ligands and so significantly increases in energy dxy also increases in energy, though not as much

Question 26

how do orbitals fill in square planar arrangements?

Answer 26

dx^2y^2 is always the highest in energy, it is very unfavourable and will never fill (square planar means only 4 bonds anyway) - square planar can therefore be considered a low spin arrangement for 4-coordinate complexes, only metals with d8 configurations commonly adopt this geometry Δoct is between dx^2y^2 and dxy

Question 27

how does ligand type affect the geometry of 4-coordinate complexes?

Answer 27

weaker ligands towards the bottom of the spectrochemical series tend to favour high spin therefore form tetrahedral structures stronger ligands at the top of the spectrochemical series favour low spin and so form square planar strucutres

Question 28

what is the periodic trend in square planar?

Answer 28

going down the group, low spin becomes more favourable, so square planar is mroe often formed, metals higher up in the periodic table prefer tetrahedral