crystal field theory Flashcards
does crystal field theory assume that bonding in TM complexes are ionic or covalent?
ionic (no covalent character)
electrostatic interactions (point charge interactions between the metal and the ligand)
which d orbitals lie on the axis and which ones lie inbetween for octahedral
on: dx^2-y^2 and dz^2
between: dxy, dxz, dyz
what is the natural state of the d orbitals in free metal ions?
all 5 d orbitals are degenerate
what happens to degenerate orbitals when placed in a ligand field?
the ligands are negatively charged or have lone pairs of electrons which repel the electrons found in the d orbitals therefore energy of the d orbitals increase BUT this assumes a negative charge field
-> change of POV by assuming ligands as negative point charges
how are the d orbitals distributed in an octahedral ligand?
dx^2-y^2 and dz^2 directly interact with the concentrated negative point charge of the ligands (repulsion) thus become destabilized and go higher in energy
dxy, dxz, dyz are between the axis thus are electronically relived as they do not point directly towards the ligand (stabilized and energy is lowered)
- these are relative to if the ligands created a sphere of negative charge where all orbitals experience the same amount of repulsion
what are the higher energy and lower energy orbitals called in octahedral complexes?
higher: eg (equal)
lower t2g (triplet)
what is the barycenter
energy of a spherical ligand field, energy is raised or lowered relative to this value
what is CFSE? what represents a more stable value
the stabilisation of the particular arrangement of ligands in a specific geometry with respect to the spherical ligand field
the more negative the value the more stable the complex
what is the general CFSE equation for oct and tet?
CFSE(oct) = (-2/5 x no. of electrons in t2g + 3/5 x no. of electrons in eg) x doct + zP
CFSE(tet) = (-3/5 x no. of electrons in t2 + 2/5 x no. of electrons in e) x dtet + zP
zP is the number of extra paired electrons COMPARED TO the barycenter form
what is the CFSE of a barycenter geometry of any metal complex?
0
what is high spin and low spin
high spin: more unpaired electrons (promoted to HIGHER energy level)
-> all electrons pointing in the same direction therefore higher net spin
low spin: less unpaired electrons (electrons are paired up in lower energy orbital)
-> not all electrons spinning in the same direction therefore lower net spin
what determines the size of P
metal + oxidation state
what determines the size of doct
metal, oxidation state, ligand
what is the range of d values that have high/low spin state for octahedral?
d4-d7
which 2 states have no net benefit compared to barycentre?
high spin of d5 and d10
what is the relationship between oxidation state and the size of delta oct?
increase oxidation state means higher effective nuclear charge experienced by the ligands thus are pulled closer to the metal ion. this results in a larger electrostatic repulsion between the ligands and the metal ion causing a greater delta oct
how does delta oct change down the group?
d orbitals get bigger -> better contact with ligands -> more interactions with ligands -> more destabilizing -> increase in delta oct
another effect: P becomes smaller as there is more space for the electrons to pair up
are 2nd and 3rd row TM complexes high spin or low spin and why?
ALL 2nd and 3rd row elements are low spin bc low energy penalty of pairing electrons and greater delta oct
what is the effect of field strength of a ligand on delta oct?
weaker field strength -> less interaction with metal ion -> less d orbital splitting (high spin)
vice versa
how does electronegativity and sigma donating ability effect the field strength of a ligand?
lower electronegativity means that the electrons are held on less tightly therefore more able to donate -> more interactions with metal ion thus more destabilization
halide < O < N < C
which is a stronger field ligand, flourine or chlorine and why?
flourine is stronger because it is smaller thus less distance between the metal and ligand thus more interactions
more electronegative also means that more withdrawing ability thus more interactions d orbital electrons again and more splitting (but at the same time not willing to give up its electrons so less interaction compared to carbon)
what is the relationship between pi donor/acceptor and delta oct?
pi donors are weak field ligands thus smaller delta oct
pi acceptors are strong field ligands thus larger delta oct
why is P a stronger field ligand than N
P has low lying d orbitals that can accept electron density from the metal ion
is water high spin or low spin complex?
high spin complex because it is considered a weaker field ligand
what is the degeneracy of d orbitals in a tetrahedral ligand field?
tetrahedral is occupying alternate corners of a cube
on: dxy, dxz, dyz
between: dx^2-y^2 and dz^2
what are the higher energy and lower energy orbitals called in tetrahedral complexes?
higher: t2
lower: e
no g because tetrahedral is not an inversion centre
compare the size of deltaoct and deltatet
deltatet is much smaller because the ligands are sitting above and below the plane thus do not point directly towards the d orbitals in the metal ion meaning less destabilizing than ligands in octahedral complex which have them pointing directly towards the d orbitals in the metal ion (greater interaction)
are tetrahedral complexes high spin or low spin and why?
high spin because the energy penalty of promoting an electron is small (regardless of metal, oxidation state and ligand)
draw out the d orbital diagram for square planar complex derived from an octahedral complex
L13 check
why would a complex want to be square planar over tetrahedral?
at d8 configuration and large delta, there can be more electrons below the barycenter thus electronic stabilization of the TM complex
what are the conditions for forming square planar complexes
d8, coordinate no. 4
3d8 needs strong field ligand for more splitting
4d8 and 5d8 with any ligand (more splitting naturally with larger orbitals)
what is a conditions where square planar is adopted but not d8?
if the ligand is bulky it can force a certain geometry (eg. for macrocyclic molecules)
are square planar TM complexes high spin or low spin and why?
always low spin due to very large delta
what is the jahn-teller effect?
non linear molecules in a degenerate electronic state will undergo distortion to form a system of lower energy and symmetry which removes degeneracy
(electrons placement ambiguity is removed to create a consistent but distorted structure through movement of the ligands)
what causes distortions in metal complexes?
electrons can be placed in either of the higher spin orbitals, this results in strong and weaker bonding depending on which orbital the electron is placed in, and weaker and stronger bonds lead to different bond lengths and thus distortions in the overall geometry of the TM complex
where does the jahn-teller effect most observed
odd number of electrons in eg orbitals
(high spin d4, low spin d7)
OBSERVATION NOT PREDICTION
