Download - Catalyst

5/22/2011
D is influenced by:
D is influenced by:
• The identity of the ligands
• The Mn+ oxidation state
D (M3+) > D (M2+) > D (M+)
Example,
[Fe(II)L6]2+
Example,
D=
Fe(II)(NH3)62+
12,800 cm-1
vs.
Fe(III)(NH3)63+
26,000 cm-1
L=
Δ=
H2 O
8,900
CN–
30,000
Cl–
5,900 cm-1
• The row in which Mn+ lies in periodic table
D (3rd row) > D (2nd row) > D (1st row)
Spectrochemical series
The spectrochemical series
Ligands influence color
Ligands
I- < Br- <S2- < SCN- < Cl- < NO2- < N3- < F- < OH- <
C2O42- < O2- < H2O < NCS- < CH3C=N < py < NH3 < en
< bpy < phen < NO2- < PPh3 < CN- < CO
[Ni(H2O)6]2+
[Ni(en)(H2O)4]2+
[Ni(en)2(H2O)2]2+
[Ni(en)3]2+
Appears:
green
green/blue
blue
purple
Absorbs:
red
red / orange
orange
yellow
Metal ions
Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mo3+
< Rh3+ < Ru3+ < Pd4+ < Ir3+ < Pt4+
DO
increasing d–orbital splitting
1
5/22/2011
Weak vs. strong field ligands
Weak vs. strong field ligands
If we need to fill the d orbitals with four
electrons, where does the fourth electron go?
If we need to fill the d orbitals with four
electrons, where does the fourth electron go?
d
Pairing the electron
requires energy –
“pairing energy” (P)
d
Weak vs. strong field ligands
Weak vs. strong field ligands
If we need to fill the d orbitals with four
electrons, where does the fourth electron go?
If we need to fill the d orbitals with four
electrons, where does the fourth electron go?
Occupying an eg
orbital requires
energy – D
d
d
D < P = Weak field
Examples:
[Cr(OH2)6]2+
D > P = Strong field
[Cr(CN)6]4-
2
5/22/2011
Weak vs. strong field ligands
If we need to fill the d orbitals with four
electrons, where does the fourth electron go?
Demo: Nickel Complexes
Ni(H2O)62+(aq) + 6 NH3(aq) → Ni(NH3)62+(aq) + 6 H2O(l)
(octahedral)
(octahedral)
Ni(NH3)62+(aq) + 3 en(EtOH) → Ni(en)32+ + 6 NH3(aq)
(octahedral)
(octahedral)
d
Examples:
“High-spin”
“Low-spin”
[Cr(OH2)6]2+
[Cr(CN)6]4-
Demo: Ammines
Cu(H2O)42+(aq) + 4 NH3(aq) → Cu(NH3)42+(aq) +
4 H2O(l)
2−
Spectator Ion: SO4
Ni(H2O)6
2+(aq)
2+(aq)
+ 6 NH3(aq) → Ni(NH3)6
+
6 H2O(l)
Spectator Ion: NO3−
Co(H2O)62+(aq) + 6 NH3(aq) → Co(NH3)62+(aq) +
6 H2O(l)
−
Spectator Ion: Cl
Ni(en)32+(aq) + 2 Hdmg(EtOH) + 2 H2O(l) →
Ni(dmg)2(s) + 3 en(EtOH) + 2 H3O+(aq)
(octahedral)
(square planar)
Note: If any green precipitate forms, it is Ni(OH)2(s).
Chapter 19: Transition Metals
and Coordination Chemistry
19.1 Survey of transition metals
19.2 1st-row transition metals
19.3 Coordination compounds
19.4 Isomerism
19.5 Bonding in complex ions: The localized electron model
19.6 The crystal field model
19.7 The molecular orbital model
19.8 The biological importance of coordination complexes
3
5/22/2011
Classes of isomers
Fig 19.9
1
Coordination Isomers:
[Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4
SO4
1
2
3
Br
4
Fig 19.10
2
Linkage Isomers:
NO2 can bond to the
metal through one of the
oxygens or through the
nitrogen
yellow
[Co(NH3)5(NO2)]Cl2
red
Pentaamminenitrocobalt(III)
chloride
[Co(NH3)5(ONO)]Cl2
Pentaamminenitritocobalt(III)
chloride
4
5/22/2011
Stereoisomers:
3
3
Cis
Chloride ligands
Cis
Trans
Geometrical isomers
Cis = together
Trans = across, opposite
Trans
green
violet
Fig 19.11
Fig 19.12
a facial isomer (fac) where
the three identical ligands
are mutually cis
a meridional isomer (mer)
where the three ligands
are coplanar
5
5/22/2011
4
Optical Isomers
Figure 19.15
Mirror image of hand
Objects that are not
superimposable
until you make a
mirror image are
called chiral.
Zumdahl: hands are “nonsuperimposable mirror images”
4
Figure 19.16
Isomers I and II
for [Co(en)3]3+
Geometric Isomers not always Optical Isomers
3
[Co(en)2Cl2]+
Trans isomer
Achiral Complex
4
Cis isomer
Chiral Complex
Nonsuperimposable
mirror images!
Fig 19.17
6
5/22/2011
Chiral Amino Acids
C
N
C
C
*
C
O
O
D-Alanine (unnatural)
N
C
*
O
C
O
L-Alanine (natural in proteins)
* denotes “chirality center”, where the C noted has
4 different substituents (-CH3, -H, -COOH, -NH2)
Achiral Complex
Chiral Complex
(I and III are enantiomers)
BIOINORGANIC CHEMISTRY
TMs serve as the active site within many large biological
molecules.
Key is ability of TM metals to
 Coordinate with and release ligands
 Easily undergo oxidation and reduction
Human body contains only 0.01% TM by mass, divided
among 3d Cr, Mn, Fe, Co, Ni, Cu, Zn and 4d Mo. Nature has
used the most abundant TMs:
 3d abundance >> 4d/5d.
 Fe is most abundant 3d element and the most used
biologically.
 Mo is the most abundant 4d/5d element.
7
5/22/2011
BIOINORGANIC CHEMISTRY
Functions of these trace metals:
Electron Carriers. TM have >1 stable oxidation state.
Oxidized form can pick up electrons; reduced form can
release electrons elsewhere as pH or other conditions
change.
Example: Iron-Sulfur Proteins. Ancient; found in
all organisms from bacteria to mammals.
Tetrahedral FeS4 active site. Catalyze metabolic redox
reactions. Cycle between Fe+3 and Fe+2, which are
much closer in stability in proteins (Eo = 0.3 V) than
in H2O (Eo = +0.8 V); hence inter-conversion requires
less energy.
BIOINORGANIC CHEMISTRY
BIOINORGANIC CHEMISTRY
Oxygen Carriers. TM have >1 stable CN. At different O2
partial pressures, can bind or release this metabolically
crucial small molecule.
Example: Hemoglobin, Myoglobin (Hb, Mb). Recently
evolved proteins that carry O2 so efficiently that warmblooded birds and mammals can exist. Blood Hb picks up
O2 in lungs, transfers to Mb in cells. Hb (M = 64,500
g/mol) is 4 Mb units stitched together; binds 4 O2.
Myoglobin…storage of O2
Hemoglobin…transport of O2
Hemoglobin Molecule
Catalysts (Enzymes). Flexibility of both oxidation state
and CN allows TM to bond reactants close together,
allowing reaction under milder conditions than normal.
Critical for organisms, which must carry out all metabolic
reactions near STP.
Example: Nitrogenase. Mo-Fe enzyme. Reduces N2(air)
 RNH2(soil) at STP, within bacteria on roots of legumes
(industrial process requires 400 oC, 250 atm). Converts
dead-organism protein decomposition product (inert N2)
into reactive form suitable for making new proteins.
Hard since N2 is so stable. Fe, Mo together coordinate,
then give electrons to N2, weakening and eventually
severing NN bond.
Figures 19.33,19.36
Heme
• Sickle cell anemia (importance of structure)
• High-altitude sickness (how hemoglobin
works)
• Toxicity of CO and CN- (ligand strength)
8