Transcript Chapter 8 (2014 fall)
8
CHAPTER
BONDING IN TRANSITION METAL COMPOUNDS AND COORDINATION COMPLEXES
8.1
8.2
8.3
8.4
8.5
8.6
Chemistry of the Transition Metals Introduction to Coordination Chemistry Structures of Coordination Complexes Crystal Field Theory: Optical and Magnetic Properties Optical Properties and the Spectrochemical Series Bonding in Coordination Complexes
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General Chemistry I
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347
Emerald 3BeO∙Al
2
O
3
∙ 6SiO
2
with some Al
3+
replaced by Cr
3+
8.1 CHEMISTRY OF THE TRANSITION METALS
348
General Chemistry I
3
349 Decreasing radii for small Z transition atoms → Increase in Z eff Increasing radii for large Z transition atoms → Increase in electron-electron repulsion General Chemistry I
4
349
Lanthanide contraction : bad shielding by 4f orbitals → the radii of the 6 th period ~ the 5 th period → decrease in atomic and ionic radii by increasing Z along the 6 th period General Chemistry I
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350
melting point : function of the bond strength in solids - roughly correlated with the number of unpaired e General Chemistry I
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General Chemistry I 351
Enthalpy of hydration of M 2+ ions
M 2+ (g ) → M 2+ (aq):
H
hyd =
H o f (M 2+ (aq)) –
H o f (M 2+ (g)) Lowering of
H
hyd from a line → due to crystal field stabilization
Anomalies of Mn → due to the stable half-filled d shell
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351
Oxidation states
more common oxidation state
Increasing tendency toward higher oxidation states among heavier transition elements in the same group: Fe (2, 3 ) → Ru (2,3,4,6, 8 ), Ni(2, 3 ) → Pd(2, 4 ) General Chemistry I
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Hard and Soft Acids and Bases
Pearson (1963) ~ Extension of Lewis’ definition – electron pair acceptor (acid) and donor (base) – by adding categories ‘hard’ and ‘soft.’ ~ 'Hard' species: small, high charge states, low electronegativities, weakly polarizable ~ 'Soft' species: large, low charge states, high electronegativities, strongly polarizable ~ ‘ Borderline ’ species General Chemistry I Ralph Pearson (US, 1919 - )
9 353
General Chemistry I
10 354
Prediction of chemical reactivities of inorganic reactions ~ Preferred direction: hard acid /hard base or soft acid /soft base HgF 2 (g) + BeI 2 (g) → BeF 2 (g) + HgI 2 (g) s / h h / s h / h s / s AgBr(s) + I – (aq) → AgI(s) + Br – (aq) s /b s s / s b EXAMPLE 8.2
Predict whether the following reactions will occur.
(a) CaF 2 (s) + CdI 2 (s) → CaI 2 (s) + CdF 2 (s) (b)Cr(CN) 2 (s) + Cd(OH) 2 (s) → Cd(CN) 2 (s) + Cr(OH) 2 (s) NO YES 354 General Chemistry I
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8.2 INTRODUCTION TO COORDINATION CHEMISTRY
Formation of Coordination Complexes
Werner’s investigation: Compound 1: CoCl 3
6NH 3 (orange-yellow) Compound 2: CoCl 3
5NH 3 (purple)
Compound 3: CoCl 3
4NH 3 (green) Compound 4: CoCl 3
3NH 3 (green) Alfred Werner (Swiss,1866-1919)
Nobel prize in chemistry(’13)
Treatment with HCl → did not remove NH 3 AgNO 3 + Cl → AgCl(s) in the ratio of 3 : 2 : 1 : 0 355 General Chemistry I
12
Conductivity measurements: Compound 1: [Co(NH 3 ) 6 ] 3+ (Cl – ) 3 → Conductivity of Al(NO 3 ) 3 Compound 2: [Co(NH 3 ) 5 Cl] 2+ (Cl – ) 2 → Conductivity of Mg(NO 3 ) 2 Compound 3: [Co(NH 3 ) 4 Cl 2 ] + (Cl – ) → Conductivity of NaNO 3 Compound 4: [Co(NH 3 ) 3 Cl 3 ] → Nonelectrolyte → Concept of “coordination sphere” around the central metal ion inner and outer coordination sphere → Formation of an octahedral complex In the above complexes, NH 3 and Cl attached to Co are called LIGANDS that are General Chemistry I
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356
CuSO 4 ∙5H 2 O → [Cu(OH 2 ) 4 ]SO 4 ∙H 2 O General Chemistry I anhydrous CuSO 4
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Monodentate ligands mono “one” and dens “tooth”
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General Chemistry I
15
Bidentate ligands (‘ox’)
Chelating ligands: chelate (G. chele, “ claw ” ) (‘en’)
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[Pt(en) 3 ] 4+ ~ 3 bidentates General Chemistry I [Co(EDTA)] – ~ 1 hexadentate
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359
Naming coordination compounds
1) Single word for a coordination complex ~ [prefix-ligand-metal] 2) Cation first followed by anion ~ K[…] or […]Cl 3) Ending with the suffix “-o” for anionic ligand, chlorido (Cl) , no change for neutral ligands except aqua (H 2 O), ammine (NH 3 ), carbonyl (CO). Note: “chloro” for Cl in a compound ligand 4) Prefixes for the number of ligands ~ di-, tri-, tetra-, penta-, hexa , … (bis-, tris-, tetrakis , … for ligands with di- (etc) in their names) 5) Alphabetical ordering for many ligands 6) Roman numeral (oxidation state) in (..) after the name of metal ~ […cobalt(III)]Cl or K[…ferrate(III)] anionic complex ions: the ending “-ate” General Chemistry I
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General Chemistry I
18 359
Ligand substitution reactions
360
[Ni(OH 2 ) 6 ] 2+ (aq) + 6 NH 3 (aq) → [Ni(NH 3 ) 6 ] 2+ (aq) + 6 H 2 O Another example _ CuCl 4 (aq) Green HCl(aq) Cu(H 2 O) 6 2+ (aq) NH 3 (aq) Pale blue Cu(NH 3 ) 6 2+ (aq) Deep blue General Chemistry I
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Difference between ‘inert’ and ‘labile’
‘Inert’ coordination complex: thermodynamically unstable, kinetically stable (inert)
3 [Co(NH ) ] (
aq
)
aq
) 6 3
takes a week
aq
)
‘Labile’ coordination complex: thermodynamically unstable, kinetically unstable (labile)
3 6 2 [Co(NH ) ] (
aq
) 6H O ( 3
aq
) [Co(H O) ] 2 6 2 6NH ( 4
aq
)
takes a matter of seconds
Reaction Reaction 361
General Chemistry I
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361
8.3 STRUCTURES OF COORDINATION COMPLEXES
Octahedral complexes with geometrical isomers (complexes of type MA 2 B 4 (or MA 2 B 2 ; B is bidentate) cis-[Co(NH 3 ) 4 Cl 2 ] + cis-[CoCl 2 (en) 2 ] + trans-[Co(NH 3 ) 4 Cl 2 ] + General Chemistry I trans-[CoCl 2 (en) 2 ] +
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Octahedral complexes with mer / fac isomers (Complexes of type MA 3 B 3 )
mer
-isomer: Similar ligands define a mer idian of the octahedron
fac
-isomer: Similar ligands define a fac e of an octohedron -- all three groups are 90 ° apart.
362
mer
-Co(NH 3 ) 3 (Cl) 3 General Chemistry I
fac
-Co(NH 3 ) 3 (Cl) 3
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Tetrahedral complexes ~ Dominant for four-coordinate complexes ~ No geometrical isomers for tetrahedral complexes of MA 2 B 2
Square planar complexes ~ Au 3+ , Ir + , Rh + , Ni 2+ , Pd 2+ , Pt 2+ ~ cis-[Pt(NH 3 ) 2 Cl 2 ] (anticancer drug, ‘cisplatin’) ~ trans-[Pt(NH 3 ) 2 Cl 2 ]
Linear geometry ~ Ions with d 10 configuration: Cu + , Ag + , Au + , Hg 2+ General Chemistry I
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363
366
Chiral Structures
Optical isomers are molecules that rotate plane polarized light Enantiomers (Gk.
e
ά
τιος, “
opposite
” , and μέρος, “
part or portion
” ) are optical isomers whose structures are non superimposable mirror images (they lack reflection-rotation symmetry)
Chiral center (chirality [G.
χειρ (kheir), "hand"] ~ handedness) is a central atom around which enantiomers are formed
A racemic mixture has equal amount of enantiomers (net rotation of plane polarized light = 0) General Chemistry I
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Octahedral complexes of type MA 3 (A is bidentate) E.g. enantiomers of the [Pt(en) 3 ] 4+ ion General Chemistry I Octahedral complexes of type MA 2 B 2 C 2 E.g. enantiomers of all-cis [Co(NH 3 ) 2 (H 2 O) 2 Cl 2 ] +
25 366
EDTA (ethylenediaminetetraacetate) ion Hexadentate ligand, sequestering metal ions Antidote for lead poisoning, preserves freshness of oil
367
General Chemistry I
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8.4 CRYSTAL FIELD THEORY: OPTICAL AND MAGNETIC PROPERTIES
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Crystal Field Theory ~ Ionic description of metal-ligand bonds ~ Ligands are treated as point charges approaching the central metal ion Octahedral coordination complexes
Degeneracy of d-orbitals lifted into two groups :
d z
2 ,
d x
2
y
2 and
d xy
,
d yz
,
d z
General Chemistry I
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367
Crystal Field Theory
•
Ligands such as a halide or oxide are regarded as an electrostatic, point charge, or point dipole type , which set up an electrostatic field.
Cr 3+ A B
o = crystal field splitting energy metal d orbitals spherical charges General Chemistry I octahedral environment
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General Chemistry I
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Fig. 8.17 An octahedral crystal field increases the energies of all five d orbitals, but the increase is greater for the d orbitals.
z
2
and d x - y
2
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General Chemistry I
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•
Electron configuration of octahedral complexes d 1 -d 3 General Chemistry I by Hund’s rule
31 370
-
From d 4 to d 7 octahedral complexes there are two possibilities, illustrated for d 4 (E.g. Mn(III) complexes) If
o is large (strong-field ligands), t 2g 4 has a lower energy.
: low-spin complex , minimum number of unpaired e If
o is small (weak-field ligands), t 2g 3 e g 1 has a lower energy.
: high-spin complex , maximum number of unpaired e e g e g E t 2g Low spin (t 2g 4 ) configuration t 2g High spin (t 2g 3 e g 1 ) configuration e-e repulsion low-spin configuration General Chemistry I ligand-ligand repulsion high-spin configuration
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369
- Example: d 4 octahedral complexes of Mn(III)
d z
2
e g
d x - y
2
Mn(CN) 6 3 LOW SPIN
3 5 o
Mn(H 2 O) 6 3+ HIGH SPIN
3 5 o
d z
2
e g
d x - y
2 o 2 5 o 2 5 o 5 x degenerate d orbitals (3d 4 )
d xy d yz
t 2g
d xz
5 x degenerate d orbitals (3d 4 )
d xy d yz
t 2g
d xz
Weak field configuration H 2 O weak field ligand Strong field configuration CN – strong field ligand Fig. 8.18. Electron configuration for (a) high spin (large
o ) and (b) low spin (small
o ) octahedral crystal field splitting energies for Mn(III) complexes
o
General Chemistry I
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Crystal Field Stabilization Energy (CFSE) The amount by which the (otherwise equal) energy levels for the d electrons of a metal ion are split by the electrostatic field of the surrounding ligands in a coordination complex.
Energy difference between electrons in an octahedral crystal field and those in the hypothetical spherical crystal field.
370
General Chemistry I
34
Square planar crystal field
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General Chemistry I
sp
> 1.6
0
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Tetrahedral crystal field General Chemistry I
t
= 4/9
o
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Fig. 8.20.
Correlation diagram showing the relationships among d-orbital energy levels in crystal fields of different symmetries.
General Chemistry I
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Magnetic properties
Magnetic susceptibility ~ Strength of a sample’s interaction with a magnetic field
Paramagnetic compounds
~ One or more unpaired electrons ~ Large, positive magnetic susceptibility ~ Attracted by the magnetic field → “weigh” more ~ Prevalent among transition-metal complexes
Diamagnetic compounds
~ All of the electrons are paired ~ Small, negative susceptibility ~ Repelled by the magnetic field General Chemistry I
38 373
8.5 OPTICAL PROPERTIES AND THE SPECTROCHEMICAL SERIES
Transition-metal complexes
~ absorb visible light → colorful E.g. [Co(NH 3 ) 5 Cl] 2+ ion absorbs greenish yellow light (~530 nm) Only red and blue light transmitted → purple (complementary color)
Wavelength of the strongest absorption,
max
E
h
, so o
h
hc
/ max
d
10 complex ~ colorless (no absorption, all d-levels are filled) High-spin d 5 complex ~ weak absorption (spin flip required)
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General Chemistry I
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Cr(CO) 6 [Co(NH 3 ) 5 (OH 2 )]Cl 3 K 3 [Fe(C 2 O 4 ) 3 ] K 3 [Fe(CN) 6 ] [Co(en) 3 ]I 3
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Colors of the hexaaqua complexes of metal ions prepared from their nitrate salts.
E.g. [Co(H 2 O) 6 ] 2+ General Chemistry I
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Spectrochemical series ~ An ordering of ligands according to their ability to cause crystal field splittings.
Spectrochemical series for ligands
I Br Cl F , OH H O 2 : NCS N H 3 en CO , CN Weak-field ligands (high spin) Intermediate-field ligands Strong-field ligands (low spin)
Spectrochemical series for metal ions Mn 2+ < Ni 2+ < Co 2+ < Fe 2+ < Fe 3+ < Co 3+ < Mn 4+ < Pd 4+ < Ir 3+ < Pt 4+
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Crystal field theory cannot explain the spectrochemical series!
General Chemistry I
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8.6 BONDING IN COORDINATION COMPLEXES
Valence bond theory
dsp
3 hybrid orbitals ~ linear combination of one s, three p atomic orbitals and the d z2 atomic orbital ~ five equivalent new hybrid orbitals ~ trigonal bipyramid , PF 5 , CuCl 5 –
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General Chemistry I
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d
2
sp
3 hybrid orbitals ~ linear combination of one s, three p atomic orbitals and d z2 , d x2-y2 orbitals ~ six new hybrid orbitals ~ octahedron , SF 6 General Chemistry I
43 378
Molecular orbital theory
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Ligand field theory ~ Failure of CFT and VB theories to explain the spectrochemical series ~ MO description for ligands
Construction of
s
MOs for octahedral complexes (of 1st row D-block metals) ~ Interaction between the metal 4s orbital with six ligand orbitals →
s
s
and
s
s
* orbitals ~ Interaction between three metal p orbitals with three ligand orbitals → triply degenerate
s
p
and
s
p
* orbitals ~ Interaction of the d z2 and d x2-y2 orbitals with ligand orbitals → a pair of
s
d
and
s
d
* orbitals General Chemistry I
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379
General Chemistry I Fig. 8.27. Formation of
s
bonding MOs from overlap of metal and ligand orbitals.
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General Chemistry I
380
Antibonding MOs
MO correlation diagram for octahedral Cr(III) complex ([CrCl
6
]
3-
):
s
bonding only
Nonbonding MOs Bonding MOs
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381
Formation of
and
* bonds (1) Interaction between an empty metal d orbital with a filled atomic ligand p orbital. E.g. 3p orbitals of Cl – (2) Interaction between a filled metal d orbital with an empty ligand
* antibonding molecular orbital. E.g. CO, CN – → metal-to-ligand (M-L)
donation or
backbonding -
and
* MOs: M d orbital - L p orbital or M d orbital - L
* orbital General Chemistry I
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(3) Overlap of each of the metal nonbonding d
xy
, d
yz
, and d
xz
orbitals with four ligand p orbitals → Formation of three pairs of bonding and antibonding MOs, t 2g and t 2g *.
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General Chemistry I Fig. 8.30. Bonding
MO by constructive overlap of a metal d
xy
orbital with four ligand p orbitals.
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Order of bonding strengths for different ligands
Weak-field ligands (small
o ) → Overlap between occupied p(
) bonding orbitals of
ligands (Br – , Cl – , CO ) with t 2g orbitals of metal → Increase in energy of t 2g and decrease in
o Strong-field ligands (large
o ) → Overlap between unoccupied
* antibonding orbitals of ligands (CO, CN – ) with t 2g orbitals of metal → Lowering of energy of t 2g orbitals by
back-bonding (M →L)
Intermediate-field ligands ~ H 2 O, NH 3
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General Chemistry I
49
E t 2g *
383
Empty ligand p (
*) orbitals Empty ligand p (
*) orbitals e g e g * t 2g * e g e g * Partially filled metal d orbitals (a) t 2g
donor (M ligands L) Filled ligand p (
) orbitals Partially filled metal d orbitals t 2g
acceptor (M ligands L) (b) Filled ligand p (
) orbitals Fig. 8.31. (a) (M
L) [or (b) (M
L)]
(or increase) in Δ o donation showing a reduction compared with that from
s
bonding alone.
(a) Slight increase in energy of t 2g electrons (in t 2g * MOs) (b) Significant lowering in energy of t 2g
electrons
due to
back-bonding → Electrons of t 2g MOs are delocalised into unoccupied
*(L) General Chemistry I
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Summary of the MO picture (Ligand Field Theory) of bonding in octahedral coordination complexes
384
IIlustrated for V 2+ ,Cr 3+ ,Mn 4+ (d 3 ) Cl , Br ligands e.g. [CrCl 6 ] 3 – General Chemistry I H 2 O, NH e.g. [V(H 3 2 ligands O) 6 ] 2+ CO, CN – , NO + Ligands e.g.
Mn(CN) 4 Fig. 8.32. Effect of
bonding on the energy-level structure for octahedral coordination complexes.
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10 Problem Sets
For Chapter 8, 2, 8, 18, 26, 32, 44, 46, 58, 64, 66
General Chemistry I
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