P1254413765iDXcm

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Characteristics of Coordination Complexes The
transition metal atom can often exist in more
than one oxidation state. Many coordination
complexes are colored. and paramagnetic.
paramagnetic substance a substance that
contains unpaired electrons paramagnetic
substances tend to move into a magnetic field
diamagnetic substance a substance that contains
no unpaired electrons and tend to move out of a
magnetic field
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The Crystal Field Model The crystal field model
concentrates on the splitting of the d orbitals
of the transition metal atom into groups as a
result of electrostatic interactions between the
ligands and the electrons in the unhybridized
orbitals of the transition metal atom. The model
can be used to understand, interpret and predict
the magnetic behavior, colors and some structures
of coordination complexes.Note that the crystal
field model does not explain the bonding in
coordination complexes it describes only
behavior that can be attributed to electrons on
the transition metal atom in the coordination
complex.
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The orientation of the five d orbitals with
respect to the ligands of an octa- hedral complex
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(a) An octahedral array of negative charges
approaching a metal ion. (b-f) The orientations
of the dorbitals relative to the negatively
charged ligands. Notice that the lobes of the dz2
and dx2-y2 orbitals (b and c) point toward the
charges. The lobes of the dxy, dyz, and dxz
orbitals (d-f) point between the charges.
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Crystal-field theory assigns the first absobtion
maximum at 20300 cm-1 to the transition eg?t2g.
The optical obsorption spectrum of Ti(OH2)63
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Ligand could be arranged in a spectrochemical
series, in which the members are arranged in
order of incresing energy of transition that
oocur when they are present in a complex I-
ltBr- lt S2- lt SCN- lt Cl- lt NO3- lt N3- lt F- lt OH- lt
C2O42- lt H2O lt NCS- lt CH3CN lt py lt NH3 lt en lt
bipy lt phen lt NO2- lt PPh3 lt CN- lt CO The
values of ?o also depend in a systematic way on
the metal ion 1 ?o increases with increasing
oxidation number 2 ?o increses down a group The
spectrochemical series for metal ions is
approximately Mn2 lt Ni2 lt Co2 lt Fe2 lt V2 lt
Fe3 lt Co3 lt Mn4 lt Mo3 lt Rh3 lt Ru3 lt Pd4 lt
Ir3 lt Pt4
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Crystal-field splitting in a series of octahedral
chromium(III) complexes.
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Population of d orbitals in the high-spin
CoF63- ion (small ) and low-spin Co(CN)63-
ion (large )
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Ligand-Field Stabilization Energies The net
energy of tx2geyg configuration relative to The
average energy of the orbitals, which is
called The ligand-field Stabilization
Energy(LFSE), LFSE (-0.4x 0.6y)?o Sample
Exercise calculate ligand field stabilization
energies in 6-coordinate high-spin and low-spin
complexes of Fe3. Solution LFSE(high spin)
(3x(-0.4) 2x(0.6)) 0 ?o LFSE(low-spin)
(5x(-0.4)) -2.0 ?o
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dn example Octahedral Tetrehedral ------------------------------- --------------- N LFSE N LFSE
d0 Ca2,Sc 0 0 0 0 d1 Ti3 1 0.4 1 0.6 d2 V3 2 0.8 2 1.2 d3 Cr3,V2 3 1.2 3 0.8 Strong Field Weak Field d4 Cr2,Mn3 2 1.6 4 0.6 4 0.4 d5 Mn2.Fe3 1 2.0 5 0 5 0 d6 Fe2, Co3 0 2.4 4 0.4 4 0.6 d7 Co2 1 1.8 3 0.8 3 1.2 d8 Ni2 2 1.2 2 0.8 d9 Cu2 1 0.6 1 0.4 d10 Cu, Zn2 0 0 0 0
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Magnetic Measurement Experimental distinctiom
between high-spin and low-spin complexes is based
on the dertermination of their megnetic
properties.Complexes are classified as
diamagnetic if they tend to move out magnetic
field and paramagnetic if they tend to move into
magnetic field.The extent od paramagnetism of a
complexes is commonly reported in terms of the
magnetic dipol moment.the higher magnetic moment
the greater the paramagnetism of the sample. The
spin-only paramagnetism which is characteristic
of many d-metal complexes. µ 2S(S1)1/2 µB S
Total spin quantum number each unpair electron
has a spin quantum number of ½, it allows that S
1/2N, where N is the number of unpaired
electronstherefore µ N(N1)1/2 µB µB is
known as Bohr magneton its value is 9.274x10-24
JT-1
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A measurement of the magnetic moment of a d-block
complex can be usually be interpreted in terms
of the number of unpaired electrons it contains,
and hence the measurement can be used to
distinguish bet- ween high-spin and low-spin
complex. For example, Magnetic measu Rements on
d6 complex permit us to distinguish between a
high-spin t42ge2g ( N 4, µ 4.90) configuration
and a low-spin t62g (N0, µ0) configuration.
--------------------------------------------------
---------------- Ion N
S µ / µB
-------------------
------
Calc. Exp. --------------------
----------------------------------------------- Ti
3 1 ½
1.73 1.7-1.8 V3 2
1 2.83 2.7-2.9 Cr3
3 3/2 3.87
3.8 Mn3 4 2
4.90 4.8-4.9 Fe3 5
5/2 5.92 5.9
 
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The measurerement of magnetic susceptibility
A Gouy balance, which is used to measure the
magnetic susceptibi lities of substances
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Sample problem The magnetic moment of an
octahedral Co(II) complex is 4.0 µB. What is its
electron configuration? Answer µ N(N2)1/2
µB? 4.0 N(N2)1/2 ?N? 3 A Co(II) complex is
d7. The two possible configu Rations are t52ge2g
(high spin) with three unpaired electron Or
t62ge1g (low-spin) with one unpaired electron.The
spin Only magnetic moment are 3.87 µB and 1.73
µB,repectively (see Tab.).Therefore the only
consistent assigment is the High-spin
configuration t52ge2g.
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In a tetrahedral complex, the eg orbitals lie
below the t2g orbi- tals the only weak-field
case need be considered
Ligand-field splitting parame- ter, ?T in a
tetrahedral comp. is less than ?o.Hence, only
weak-field tetrahedral comp. are common (?T lt1/2
?o
The orbital energy level diagram used in the
application of the buiding-up principle in
crystal field-analysis of a tetrahedral complex
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A tetragonal distortion can be expected when
there are one,two,or three eg electrons in the
comp in a 4d8 or 5d8 complex the distortion may
result in the formation of a square-planar
species.
?SP 1.3 ?O
The orbital splitting parameters for a
square-planar complex
The effect of tetragonal distortion(compression
along x and y and extension along z) on the
energies of d orbitals.The electron occupation
is for a d9 complex
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The Jahn-Teller Effect The tetragonal distortions
just described represent specific examples of
the Jahn-Teller Effect If the ground electronic
configuration of a nonlinear comp. is orbitally
degenerate, the comp. will distort so as to
remove The degeneracy and achieve a lower
energy. An octahedral d9 complex is orbitally
degenerate because the eg(dx2-y2) and eg(dz2)
orbitals have the same energy and the single
electron can ocupy either one of them.A
tetragonal dis- tortion results in the two
orbitals having different energies, and the
energy of the resulting complex is lower than in
the undistorted complex. A tetragonal distortion
can also be seen in case of a low spin
octahedral d8 comp which is orbitally
dege- nerate in its undistorted one. Distortion
to square-planar, with the electrons paired in
dz2, can be regarded as an extreme case of the
Jahn-Teller effect. The axial elongation weakens
only two bonds whereas elongation in the plane
would weaken four, axial elongation is more
common than axial compression.
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Thermochemical correlations The concept of
ligand-field stabilization energy helps to
explain the double-humped hydration enthalpies of
3d-metal M2 ions. The nearly linear increse
across a period shown by the filled
circles represents the increasing strenth of the
bonding H2O ligans and the central metal ions as
the ionic radii decrease from left to rigth
across the period.The wave-like deviation of
hydration enthalpies from a straigth line
reflects the variation in the LFSE.
The hydration enthalpy of M2 ions of the first
row the d-block. The general trend to greater
hydration enthalp(more exothermic hydration) on
crossing the period from left to rigth
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• Sample problem
• The oxides formula MO, which all have octahedral
  coordinations
• of the metal ions,have the following lattice
  enthalpies
• CaO TiO VO
  MnO
• 3878 3913
  3810 kJmol-1
• Account for the trends in terms of the LFSE.
• Answer The general trend across the d-block is
  the increase in
• lattice enthalpy from CaO(d0) to MnO(d5) as the
  ionic radii
• decrease. Both Ca2 and Mn2 have an LFSE of
  zero.Because
• O2- is a weak-field ligand, TiO(d2) has an LFSE
  of 0.8 ?o and
• VO(d3) has an LFSE of 1.2 ?o. It follows that the
  graeter lattice
• Enthalpies of TiO and VO arise from the
  ligand-field stabilization
• Energy.

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Sample Exercise Which of the following complexes
of Ti3 exhibits the shortest wavelength
absorption in the visible spectrum Ti(H2O)63
Ti(en)33 TiCl63-? SOLUTION The
wavelength of the absorption is determined by the
magnitude of the splitting between the d-orbital
energies in the field of the surrounding
ligands. The larger the splitting, the shorter
the wavelength of the absorption corresponding to
the transition of the electron from the lower- to
the higher-energy orbital. The splitting will be
largest for ethylenediamine, en, the ligand that
is highest in the spectrochemical series. Thus,
the complex with the shortest wavelength absorptio
n is Ti(en)33.
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Practice Exercise The absorption spectrum of
Ti(NCS)63- shows a band that lies intermediate
in wavelength between those for TiCl63- and
TiF63-. What can we conclude about the place of
NCS- in the spectrochemical series? Answer It
lies between Cl- and F- that is, Cl- lt NCS- lt
F-.
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Sample Exercise Predict the number of unpaired
electrons in 6-coordinate high-spin and low-spin
complexes of Fe3. SOLUTION The Fe3 ion
possesses five 3d electrons. In a high-spin
complex, these are all unpaired. In a
low-spincomplex the electrons are confined to the
lower-energy set of d orbitals, with the result
that there is one unpaired electron
Practice Exercise For which d electron
configurations does the possibility of a
distinction between high-spin and low-spin
arrangements exist in octahedral complexes?
Answer d4, d5, d6, d7
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Sample Exercise Four-coordinate nickel(II)
complexes exhibit both square-planar and
tetrahedral geometries. The tetrahedral ones,
such as NiCl42-, are paramagnetic the
square-planar ones, such as Ni(CN)42-, are
diamagnetic. Show how the d electrons of
nickel(II) populate the d orbitals in the
appropriate crystal-field splitting diagram in
each case. SOLUTION Nickel(II) has an electron
configuration of Ar3d8. The population of the d
electrons in the two geometries is given here
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Sample Exercise The complex ion
trans-Co(NH3)4Cl2 absorbs light primarily in
the red region of the visible spectrum (the most
intense absorption is at 680 nm). What is the
color of the complex? SOLUTION Because the
complex absorbs red light, its color will be
complementary to red. From Figure , we see that
this is green. Practice Exercise The
Cr(H2O)62 ion has an absorption band at about
630 nm. Which of the following colors--sky blue,
yellow, green, or deep red--is most likely to
describe this ion? Answer sky blue.
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1. Indicate the coordination number about the
metal and the oxidation number of themetal in the
following complex Na2CdCl4. 6, 2
4, 2 2, 4 6, 4
4, 4
2. Indicate the coordination number about the
metal and the oxidation number of the metal in
the following complex K3V(C2O4)3. 3, 3
6, 6 3, 0 3, 6
6, 3
3. What are the geometries of the following
two complexes? (i) AlCl4-
(ii) Ag(NH3)2 tetrahedral, linear
trigonal pyramidal, bent tetrahedral, bent
trigonal pyramidal, linear octahedral,
tetrahedral
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4. Which of the following is the correct formula
for hexaamminechromium(III) nitrate, using
brackets to indicate the coordination sphere?
Cr(NH3)6 (NO3)
Cr(NH3)6 (NO3)2 Cr(NH3)6
(NO3)3 Cr(NH3)2 (NO3)2
5. Which of the following is the correct formula
for dichlorobis (ethylenediamine)platinum(IV)
bromide? (Include brackets to indicate the
coordination sphere.) PtCl2(en)2Br
PtCl2(en)2Br2
PtCl2(EDTA)2Br2 PtCl2(en)Br2
PtCl2(C2O4)2Br2
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6. Polydentate ligands can vary in the number of
coordination positions they occupy. In the
following compound, identify the polydentate
ligand present and indicate the probable
number of coordination positions it occupies
Cr(C2O4)(H2O)4Br H2O, 1 H2O, 2
C2O4 2-, 1 C2O4 2-, 2 none
7. Indicate which of the following compounds you
would expect to have color (i) ZnO
(ii) Ni(NH3)6Cl2 (iii) NaAlCl4
(iv) Cd(NH3)4(NO3)2 (v)
Fe(SO4)(H2O)4
i ii i, iii, v iii
iv ii, v
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8. For an octahedral complex, how many unpaired
electrons are there in the electronic
configuration of the Ru3 ion, assuming a
strong-field complex? 3 4 1
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9. How many d electrons are in the lower and
upper states for Ru(NH3)5H2O2 (low spin)? 4,
2 6, 0 6, 1 5, 2
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10. The complex Mn(NH3)62 contains five
unpaired electrons. Is the ion a high-spin or a
low-spin complex? How many electrons are in the
lower and higher energy d orbitals?
high--three in the lower and two in the
higher high--two in the lower and
three in the higher low--three in the
lower and two in the higher low--two
in the lower and three in the higher
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What type of structural isomerism is shown by
Cr(NH3)4Cl2Br and Cr(NH3)4ClBrCl? g
eometrical stereoisomerism
optical coordination-sphere
linkage
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11. Why are many cyano complexes of divalent
transition-metal ions yellow, whereas many aqua
complexes of these ions are blue or
green? cyano complexes have a different
geometry cyano complexes are more
labile cyano complexes are more
tightly bound cyano complexes have a
larger mass cyano complexes have
optical activity
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12.What are the geometries of the following two
complexes? (i) CoBr42- (ii)
Mo(CO)6 tetrahedral, tetrahedral
tetrahedral, octahedral octahedral,
tetrahedral octahedral, octahedral
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13. Which of the following compounds would you
expect to have color? (i) Ru(H2O)62
(ii) Mo(CN)7(H2O)4- (iii)
MnO4- (iv) TiF62- i only ii
only iii only iv only
i and ii only iii and iv only
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14. What is a chelating agent?
Answer Ligands with gt1 pair of electrons. They
tend to clamp on to a metal (from the Greek word,
chele, meaning claw).
15. Yet another characteristic of Transition
Metals is that the complexes they form have very
distinct colors, from blue to yellow to red. What
causes this?
16. What determines the magnitude of the
crystal-field splitting?
The nature of the ligands CN(-1) and CO are
called strong-field ligands because they split
the d-orbitals the most. When the common ligands
are placed in order of increasing field splitting
strength, the resultant series is called the
spectrochemical series.
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17a An aqueous solution of cobalt (III) is
diamagnetic (hexaaquacobalt(III) ion). Draw an
orbital energy diagram to illustrate this. 17b
In excess fluoride ion, the above cobalt solution
becomes paramagnetic. Draw an orbital energy
diagram to illustrate this. 17c The stronger
field ligand is ________________________ .
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Answer