properties of transition metals coordination chemistry
TRANSCRIPT
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Outline
• Properties of Transition Metals
• Coordination Chemistry
• Naming Coordination Compounds
• Structures of Complexes
• Theories in Coordination Chemistry
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Properties of Transition Metals
1. Electron Configurations
Transition Metal Atoms
4s orbital filled before 3d orbital
Sc [Ar]4s23d1 Ti [Ar]4s23d2 V [Ar]4s23d3
Cr [Ar]4s13d5 Cu [Ar]4s13d10 ... are exceptions!
Transition Metal Ions
4s e−’s removed before 3d e−’s
Ti3+ [Ar]3d1 V3+ [Ar]3d2 Cr3+ [Ar]3d3
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2. Atomic Radii
except Sc thru V, little variation
of radii in first series (3d)…
d-electrons shield nuclear charge!
metals in the second series (4d) and third series (5d) have similar radii: lanthanide contraction
f-electrons don’t shield the increasing nuclear charge…
electrons are drawn inward by increasing charge!
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3. Catalytic Activity
transition metals are often key elements in catalysts
Ni hydrogenation of oils
Pt, Pd, Rh catalytic converter in automobiles
Fe3O4 synthesis of NH3
V2O5 manufacture of H2SO4
4. Color and Magnetism
partially filled d-sublevels imparts color and magnetic properties to transition metal compounds
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Coordination Chemistry
Coordination Compounds are compounds with a metal atom or ion surrounded by 2 or more atoms or ions
[Pt(NH3)6]Cl4
Pt4+ ion surrounded by 6 NH3 molecules
Pt(NH3)64+
Complex Ions (or Coordination Complexes) are ions with a metal atom or ion surrounded by 2 or more atoms or ions:
Counterions are anions or cations added to produce a neutral compound
Cl−
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Properties of coordination compounds and complex ions vary…
1. Color Co(NH3)63+ yellow
Co(NH3)5NCS2+ orange
Co(NH3)5H2O3+ red
Co(NH3)5Cl2+ purple
The color depends on the chemical groups attached to the transition metal
The attached chemical groups are called ligands
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2. Coordination Number
The number of ligands surrounding a central atom or ion
Ox. No. Typical Co. No. Example
+1 2 Ag(NH3)2+
+2 4, 6 Pt(NH3)42+
+3 6 Co(NH3)63+
3. Magnetism
Some are diamagnetic (no unpaired e−’s), and some are paramagnetic (1 or more unpaired e−’s)
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Frog levitated in very strong magnetic field…
… as a result of the diamagnetic behavior of water!
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4. Influence of d-electrons
d0 very stable, difficult to produce moving to right
Sc3+ (III)
TiF62− (IV)
CrO42− (VI) Stable when
MnO4− (VII) bonded to oxygen!
K2CrO4 KMnO4
Color of CrO42− and MnO4
− due to charge transfer… electrons on O2− are excited to empty d-orbitals by visible light
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d5 and d10 very stable
Mn2+ and Fe3+ (d5)
Ag+ and Zn2+ (d10)
d3, d6, d8 form inert (stable) complexes
d3 Cr(H2O)63+
d6 Co(NH3)63+
d8 PtCl42−
d4 very unstable
Cr2+ (d4) → Cr3+ (d3)
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Naming Coordination Compounds
1. name cation first, anion second
2. ligands precede the central metal atom in the complex ion
3. negatively charged ligands end in –o
4. number of ligands indicated with a prefix (no mono-)
5. ligands are ordered alphabetically (ignore prefixes)
6. the name of the metal ion ends in –ate if the complex is negatively charge
7. a roman numeral (or 0) in parentheses indicates the ox. no. of the metal atom
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[Pt(NH3)6]Cl4 hexaammineplatinum(IV) chloride
[Pt(NH3)5Cl]Cl3 pentaamminechloroplatinum(IV) chloride
Pt(NH3)2Cl4 diamminetetrachloroplatinum(IV)
K[Pt(NH3)Cl5] potassium amminepentachloroplatinate(IV)
Fe(CO)5 pentacarbonyliron(0)
K4[Fe(CN)6] potassium hexacyanoferrate(II)
K3[Fe(CN)6] potassium hexacyanoferrate(III)
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Structures of Complexes
The coordination number determines the structure…
If CN = 2, then linear
CN = 4, then tetrahedral (square planar if d8)
CN = 6, then octahedral
Structural Isomers are compounds with the same chemical formula, but with the atoms bonded differently
H – C = C – C – H
H
_H H H__
_
C
C C___
H
H
H
H
H
H
Both are C3H6...
Structural isomers!
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Geometric Isomers are compounds with the same chemical formula, but with different arrangements of the atoms
Co(NH3)4Cl2+ is octahedral shape and 2 geometric isomers…
adjacent (cis) opposite (trans)
H3N
NH3
NH3
H3N Cl
Cl
____
Co
____
H3N NH3
NH3H3N
Cl
Cl____
Co
____
++
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Pt(NH3)2Cl2 is square planar (Pt2+ is d8) and has 2 geometric isomers…
Cu(NH3)2Cl2 is tetrahedral (Cu2+ is d9)
and has only 1 structure…
adjacent (cis) opposite (trans)
NH3
Pt
H3N
Cl Cl
NH3
Pt
H3N
Cl
Cl
H3N
___
Cu
Cl
Cl
H3N
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Co(NH3)3Cl3 is octahedral and has 2 geometric isomers…
nonplanar(facial)
or(fac-)
planar(meridional)
or(mer-)
H3N
NH3
NH3
Cl
Cl
____
Co
____
Cl
H3N
NH3
H3N Cl
Cl____
Co
____
Cl
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Draw the trans-square planar coordination compounds...
[Pt(NH3)2Cl2]Br2 [Pt(NH3)2Br2]Cl2
atoms are bonded differently, so structural (coordination) isomers
Coordination isomers have different names...
trans-diamminedichloroplatinum(IV) bromide
trans-diamminedibromoplatinum(IV) chloride
Br – Pt – Br
NH3
——
NH3
2+
Cl−Cl −Cl – Pt – Cl
NH3
——
NH3
2+
Br −Br −
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Draw the tetrahedral coordination complexes...
Cu(NO2)2Cl22− Cu(ONO)2Cl2
2−
atoms are bonded differently, so structural (linkage) isomers
Linkage isomers have different names...
dichlorodinitrocuprate(II)
dichlorodinitritocuprate(II)
ONO
___
Cu
Cl
ClONO
O2N
___
Cu
Cl
Cl
O2N
2− 2−
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Draw the square planar isomers of...
[Pt(H2O)2Cl2]
atoms are bonded the same but arranged differently, so geometric isomers
geometric isomers have similar names...
trans-diaquadichloroplatinum(II)
cis-diaquadichloroplatinum(II)
H2O
Cl – Pt – Cl
——
H2O
H2O – Pt – Cl
H2O
——
Cl
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Ligands that bind to the metal at only 1 point are monodentate ligands
Ligands that bind to the metal at more than 1 point are polydentate ligands
Bidentate:
ethylenediamine (en) oxalate (ox) – “ethanedioate”
NH2
H2N
CH2
CH2
O
O
C
C
O
O═
–
–
____
Co
____
____
Co
____
…bis(ethylenediamine)… …tris(oxalato)…
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Hexadentate: ethylenediaminetetraacetate (EDTA), −4 charge
A polydentate ligand is called a chelating agent; complex that is formed is called chelate
O
O
–
____
Co
____
O
O
–N
N
O
═O
–
O
O
═
–
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Consider the tris(ethylenediamine)cobalt(III) ion: Co(en)33+
Not the same... They’re mirror images of each other that are not superimposable
2 nonsuperimposable mirror images are called enantiomers, and are optical isomers (a type of stereoisomer)
____
Co
____
NN
N
N
NN____
Co
____
NN
N
N
NN
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Draw the isomers of dichlorobis(ethylenediamine)cobalt(III)...
CoCl2(en)2+
Three isomers… 1 trans-chloro and 2 cis-chloro optical isomers
____
Co
____
ClN
N
N
ClN
____
Co
____
NCl
N
N
NCl
____
Co
____
NN
NN
Cl
Cl + + +
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Theories in Coordination Chemistry
They attempt to explain:
geometries (shapes)
magnetism (paired or unpaired e-’s)
color (electronic energy level differences)
1. Valence Bond Theory
assumes ligands are covalently bonded to the metal using hybrid metal orbitals
these orbitals overlap with a ligand orbital containing a lone pair of e-’s, forming a coordinate covalent bond
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CN Shape Hybridization
2 linear s + p → 2 sp
4 tetrahedral s + 3 p → 4 sp3
square planar s + 2 p + d → 4 dsp2
6 octahedral s + 3 p + 2 d → 6 d2sp3
Cu(CN)2− CN = 2, linear sp hybridization
ZnCl42- CN = 4, tetrahedral sp3 hybridization
Pt(en)22+ CN = 4, square planar dsp2 hybridization
Co(H2O)63+ CN = 6, octahedral d2sp3 hybridization
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2. Crystal Field Theory
assumes ionic bonding between the ligands (negative) and the metal cation
lone pair e−’s on ligands affect energies of the d orbitals
dz2dx2 – y2
xy
z
xy
z
xy
z
dxy
xy
z
dxz
xy
z
dyz
dx2 – y2 and dz2 affectedmost by approaching ligands
in octahedral complex!
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Free metal ion... In octahedral complex...
The difference in energy between the d orbitals in a metal ion complex is called the splitting energy (Do)
If Do is large, the complex is called a low-spin complex
If Do is small, the complex is called a high-spin complex
___ ______ ___ ___d
x2 – y2, z2
xy, xz, yz
___ ___
___ ___ ___
DoE
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Co(NH3)63+ is a low-spin complex
Co3+ has 6 d e−’s
With large Do, the 6 e-’s cannot spread through the 5 orbitals
Co(NH3)63+ is diamagnetic
___ ___
___ ___ ___large
___ ___
___ ___ ___↿⇂ ↿⇂ ↿⇂
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CoF63- is a high-spin complex
Co3+ has 6 d e-’s
With small Do, the 6 e-’s can spread through the 5 orbitals
CoF63- is paramagnetic
___ ___
___ ___ ___small
___ ___
___ ___ ___↿⇂ ↿ ↿
↿↿
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The stronger the electrostatic field the metal is in, the greater Do
Co(NH3)63+ large Do strong-field
CoF63- small Do weak-field
Spectrochemical series gives relative field strengths for ligands:
CO > CN− > en > NH3 > H2O > OH − > F − > Cl − > Br − > I −
strong-field ligands weak-field ligands
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Predict the number of unpaired electrons and magnetism in Cr(CN)6
4−.
Cr2+ has 4 d e−’s (ignore ligand e−’s)
CN− is a strong-field ligand... so large Do
2 unpaired e−’s (paramagnetic)
Predict the number of unpaired electrons and magnetism in FeCl6
4−.
Fe2+ has 6 d e−’s
Cl− is a weak-field ligand... so small Do
4 unpaired e−’s (paramagnetic)
___ ___
___ ___ ___↿⇂ ↿ ↿
___ ___
___ ___ ___↿⇂ ↿ ↿
↿↿
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Crystal field theory can be used to explain colors of coordination compounds (complexes)
e−’s are promoted from low d-orbitals to high d-orbitals with absorption of photons of visible light
color we observe is the complement (or leftovers) of absorbed light
A solution of... absorbs... and appears...
Co(NH3)63+ blue (450 nm) orange
Co(NH3)5H2O3+ green (500 nm) red
trans-Co(NH3)4Cl2+ red (680 nm) green
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Red
Green
Blue Orange
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Wavelength of light absorbed determined by Do
h = 6.626 x 10-34 J s
c = 2.998 x 108 m s-1
l = wavelength absorbed (m)
What is the splitting energy of Co(CN)63- which absorbs 290 nm
light?
l
c h o =D
J 10 x 6.8 m 10 x 290
)s m 10 x s)(2.998 J 10 x (6.626
c h 19-
-9
1-834-
o ===Dl
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What’s Do (in kJ/mol) of Co(NH3)63+ which absorbs 440 nm light?
What wavelength of light does CoF63- absorb if Do = 155 kJ/mol?
J 10 x 4.5 m 10 x 440
)s m 10 x s)(2.998 J 10 x (6.626
c h 19-
-9
1-834-
o ===Dl
kJ/mol 270 mol 1
10 x 6.022 x
J 1000
kJ 1 x J 10 x 4.5
2319- =
J 10 x 2.57 10 x 6.022
mol 1 x
kJ 1
J 1000 x
mol
kJ 155 19-
23=
m 10 x 773 J 10 x 2.57
)s m 10 x s)(2.998 J 10 x (6.626
c h -9
19-
1-834-
o
==D
=l
nm) 773(or
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d-Orbital energies are split for other geometries...
tetrahedral
ligands point at dxy, dxz, and dyz
so they have higher energies
square planar
ligands point right at dx2 – y2,
near dxy, and donut of dz2
___ ___
___ ___ ___ xy, xz, yz
z2, x2 – y2
___
___
___ ___
___
x2 – y2
xy
z2
xz, yz