Bonding in complexes of d-block metal ions

1
Bonding in complexes of d-block metal ions
Crystal Field Theory.
eg
energy
d-shell split by presence of ligand donor-atoms
?
3d sub-shell
Co3 ion in gas-phase (d6)
t2g
Co(III) in complex
2
The d-orbitals
the t2g set
z
z
z
y
y
y
x
x
x
dyz
dxy
dxz
z
z
y
y
the eg set
x
x
dz2
dx2-y2
3
Splitting of the d sub-shell in octahedral
coordination
blue ligand donor atom orbitals
the egset
the t2g set
z
z
z
y
y
y
x
x
x
dyz
dz2
dx2-y2
the two orbitals of the eg set lie along
the Cartesian coordinates, and so are adjacent to
the donor atoms of the ligands, which raises the
eg set in energy
the three orbitals of the t2g set lie between the
ligand donor-atoms (only dyz shown)
4
Splitting of the d sub-shell in an octahedral
complex
energy
eg
d-shell split by presence of ligand donor-atoms
?
3d sub-shell
Co3 ion in gas-phase (d6)
t2g
Co(III) in octahedral complex
5
The crystal field splitting parameter (?)

• Different ligands produce different extents of
  splitting between the eg and the t2g levels. This
  energy difference is the crystal field splitting
  parameter ?, also known as 10Dq, and has units of
  cm-1. Typically, CN- produces very large values
  of ?, while F- produces very small values.

energy
eg
eg
? 26,600 cm-1
? 15,000 cm-1
t2g
t2g
Cr(CN)63-
CrF63-
6
High and low-spin complexes
The d-electrons in d4 to d8 configurations can be
high-spin, where they spread out and occupy the
whole d sub-shell, or low-spin, where the
t2g level is filled first. This is controlled by
whether ? is larger than the spin- pairing
energy, P, which is the energy required to take
pairs of electrons with the same spin
orientation, and pair them up with the opposite
spin.
eg
Paramagnetic 4 unpaired es
eg
energy
diamagnetic no unpaired es
? gt P
? lt P
t2g
t2g
low-spin d6 electrons fill the t2g level first.
In this case the complex is diamagnetic
high-spin d6 electrons fill the whole d sub-shell
according to Hunds rule
7
High and low-spin complexes of d5 ions
For d5 ions P is usually very large, so these are
mostly high-spin. Thus, Fe(III) complexes are
usually high-spin, although with CN- ? is large
enough that Fe(CN)63- is low spin (CN- always
produces the largest ? values)
Fe(CN)63- ? 35,000 cm-1 P 19,000
cm-1
Fe(H2O)63 ? 13,700 cm-1 P 22,000
cm-1
eg
Paramagnetic 5 unpaired es
eg
energy
? gt P
? lt P
paramagnetic one unpaired e
t2g
t2g
low-spin d5 (Fe(CN)63-) electrons fill the t2g
level first. In this case the complex is
paramagnetic
high-spin d5 (Fe(H2O)63) electrons fill the
whole d sub-shell according to Hunds rule
8
High and low-spin complexes of d7 ions
The d7 metal ion that one commonly encounters is
the Co(II) ion. For metal ions of the same
electronic configuration, ? tends to increase
M(II) lt M(III) lt M(IV), so that Co(II) complexes
have a small ? and are usually high spin. The
(III) ion Ni(III) has higher values of ?, and is
usually low-spin.
Ni(bipy)33
Co(H2O)62 ? 9,300 cm-1
eg
Paramagnetic 3 unpaired es
eg
energy
? gt P
? lt P
paramagnetic one unpaired e
t2g
t2g
low-spin d7 (Ni(bipy)33) The d-electrons fill
the t2g level first, and only then does an
electron occupy the eg level.
high-spin d7 (Co(H2O)63) electrons fill the
whole d sub-shell according to Hunds rule
9
High and low-spin complexes of some d6 ions
For d6 ions ? is very large for an M(III) ion
such as Co(III), so all Co(III) complexes are
low-spin except for CoF63-.high-spin. Thus,
Fe(III) complexes are usually high-spin,
although with CN- ? is large enough that
Fe(CN)63- is low spin (CN- always produces the
largest ? values)
CoF63- ? 13,100 cm-1 P 22,000 cm-1
Co(CN)63- ? 34,800 cm-1 P 19,000
cm-1
eg
eg
Paramagnetic 4 unpaired es
energy
? gtgt P
? lt P
diamagnetic no unpaired es
t2g
t2g
low-spin d6 (Co(CN)64-) electrons fill the t2g
level first. In this case the complex is
diamagnetic
high-spin d5 (CoF63-) electrons fill the whole
d sub-shell according to Hunds rule
10
The spectrochemical series

• One notices that with different metal ions the
  order of increasing ? with different ligands is
  always the same. Thus, all metal ions produce the
  highest value of ? in their hexacyano complex,
  while the hexafluoro complex always produces a
  low value of ?. One has seen how in this course
  the theme is always a search for patterns. Thus,
  the increase in ? with changing ligand can be
  placed in an order known as the spectrochemical
  series, which in abbreviated form is
• I- lt Br- lt Cl- lt F- lt OH- H2O lt NH3 lt CN-

11
The spectrochemical series

• The place of a ligand in the spectrochemical
  series is determined largely by its donor atoms.
  Thus, all N-donor ligands are close to ammonia in
  the spectrochemical series, while all O-donor
  ligands are close to water. The spectrochemical
  series follows the positions of the donor atoms
  in the periodic table as
• C N O F
• P S Cl
• Br
• I

?
very little data on P-donors may be higher than
N-donors
spectrochemical series follows arrows
around starting at I and ending at C
S-donors between Br and Cl
12
The spectrochemical series

• Thus, we can predict that O-donor ligands such
  as oxalate or acetylacetonate will be close to
  water in the spectrochemical series. It should be
  noted that while en and dien are close to ammonia
  in the spectrochemical series, 2,2bipyridyl and
  1,10-phenanthroline are considerably higher than
  ammonia because their sp2 hybridized N-donors are
  more covalent in their bonding than the sp3
  hybridized donors of ammonia.

13
The bonding interpretation of the spectrochemical
series

• For the first row of donor atoms in the periodic
  table, namely C, N, O, and F, it is clear that
  what we are seeing in the variation of ? is
  covalence. Thus, C-donor ligands such as CN- and
  CO produce the highest values of ? because the
  overlap between the orbitals of the C-atom and
  those of the metal are largest. For the highly
  electronegative F- ion the bonding is very ionic,
  and overlap is much smaller. For the heavier
  donor atoms, one might expect from their low
  electronegativity, more covalent bonding, and
  hence larger values of ?. It appears that ? is
  reduced in size because of poverlap from the
  lone pairs on the donor atom, and the t2g set
  orbitals, which raises the energy of the t2g set,
  and so lowers ?.

14
Crystal Field Stabilization Energy (CFSE)

• When splitting of the d sub-shell occurs, the
  occupation of the lower energy t2g level by
  electrons causes a stabilization of the complex,
  whereas occupation of the eg level causes a rise
  in energy. Calculations show that the t2g level
  drops by 0.4?, whereas the eg level is raised by
  0.6?. This means that the overall change in
  energy, the CFSE, will be given by
• CFSE ?(0.4n(t2g) - 0.6n(eg))
• where n(t2g) and n(eg) are the numbers of
  electrons in
• the t2g and eg levels respectively.

15
Calculation of Crystal Field Stabilization Energy
(CFSE)

• The CFSE for some complexes is calculated to be
• Co(NH3)63 Cr(en)33

energy
eg
eg
t2g
t2g
? 22,900 cm-1 ? 21,900 cm-1 CFSE
22,900(0.4 x 6 0.6 x 0) CFSE 21,900(0.4 x
3 0.6 x 0) 54,960 cm-1 26,280 cm-1
16
Crystal Field Stabilization Energy (CFSE) of d5
and d10 ions

• The CFSE for high-spin d5 and for d10 complexes
  is calculated to be zero
• Mn(NH3)62 Zn(en)33

energy
eg
eg
t2g
t2g
? 22,900 cm-1 ? not known CFSE
10,000(0.4 x 3 0.6 x 2) CFSE ?(0.4 x 6
0.6 x 4) 0 cm-1 0 cm-1
17
Crystal Field Stabilization Energy (CFSE) of d0
to d10 M(II) ions

• For M(II) ions with the same set of ligands, the
  variation of ? is not large. One can therefore
  use the equation for CFSE to calculate CFSE in
  terms of ? for d0 through d10 M(II) ions (all
  metal ions high-spin)
• Ca(II) Sc(II) Ti(II) V(II) Cr(II)
  Mn(II) Fe(II) Co(II) Ni(II) Cu(II)
  Zn(II)
• d0 d1 d2 d3
  d4 d5 d6 d7 d8
  d9 d10
• CFSE 0 0.4? 0.8? 1.2? 0.6? 0
  0.4? 0.8? 1.2? 0.6? 0
• This pattern of variation CFSE leads to greater
  stabilization in the complexes of metal ions with
  high CFSE, such as Ni(II), and lower
  stabilization for the complexes of M(II) ions
  with no CFSE, e.g. Ca(II), Mn(II), and Zn(II).
  The
• variation in CFSE can be compared with the log
  K1 values for EDTA
• complexes on the next slide

18
Crystal Field Stabilization Energy (CFSE) of d0
to d10 M(II) ions
Ni2
double- humped curve
Ca2
Mn2
Zn2
19
Log K1(EDTA) of d0 to d10 M(II) ions
CFSE
double- humped curve
Zn2
Mn2
rising baseline due to ionic contraction
Ca2
20
Log K1(en) of d0 to d10 M(II) ions
CFSE
double- humped curve
Zn2
rising baseline due to ionic contraction
Ca2
Mn2
21
Log K1(tpen) of d0 to d10 M(II) ions

double- humped curve
Zn2
Mn2
tpen
Ca2
22
The Irving-Williams Stability Order

• Irving and Williams noted that because of CFSE,
  the log K1 values for virtually all complexes of
  first row d-block metal ions followed the order
• Mn(II) lt Fe(II) lt Co(II) lt Ni(II) lt Cu(II) gt
  Zn(II)
• We see that this order holds for the ligand
  EDTA, en, and TPEN on the previous slides. One
  notes that Cu(II) does not follow the order
  predicted by CFSE, which would have Ni(II) gt
  Cu(II). This will be discussed under Jahn-Teller
  distortion of Cu(II) complexes, which leads to
  additional stabilization for Cu(II) complexes
  over what would be expected from the variation in
  CFSE.