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Lecture 15Lecture 15
Coordination Compounds
Suggested reading: Shriver & Atkins, Chapter 7
Alex Zettl with a carbon nanotube
From last lectures: 3 classes of nanomaterials
Metallic NanoparticlesMetallic Nanoparticles
d lSemiconducting Nanocrystals
Li dLigand
Ligand
Metal l
Metal atom
cluster
C lN i l
http://pubs.acs.org/cen/news/87/i34/8734news9.html
ComplexNanoparticle
1706: German paint maker DiesbachPrussian blue
Cochineal PotashPrussian blue Fe4[Fe(CN)6]3
+ =
The Great Wave off Kanagawa (1830)
Pigments are Coordination Complexes
Colors of various coordination complexes
http://en.wikipedia.org/wiki/Coordination_complex
Photosynthesis
Photosynthesis
l b lElectron/oxygen transport in biology
PhotovoltaicsPhotosynthesis
l b lElectron/oxygen transport in biology
Coordination Compounds
M l li d d l i l l i h h i G l l ll Metal-ligand compounds play crucial roles in photosynthesis, Gratzel solar cells, chemotherapy, electron & oxygen transfer in biological processes, pigments &
dyes, and catalysis
Complex•a central metal atom or ion surrounded by a set of ligands
y y
a central metal atom or ion surrounded by a set of ligands•a Lewis acid (central metal) & a Lewis base (ligands)
Ligand•an ion or molecule that an have an independent existence
Coordination compound•a neutral complex or an ionic compound in which at least •a neutral complex or an ionic compound in which at least
one of the ions is a complex
TerminologyTris(bipyridine)ruthenium(II) chloride
Acceptor D
( py ) ( )
patom: the
metal atom or ion that
Donor atom: the atom in the ligand ion that
“accepts” electrons from
h li d
gthat bonds to
the central atom the ligandatom
C di ti b b f li d di tl tt h d Coordination number: number of ligands directly attached to the central metal. These ligands form the “primary
coordination sphere” or “inner sphere complex.”
“Outer sphere complex:” electrostatically-associated ligands, not directly bound to the central metal
Outer Sphere Complex probed via XRD
CoCl2 · 6H2O (Cobalt(II) chloride hexahydrate) : chloride hexahydrate) :
Contains the neutral complex [CoCl2(OH2)4] and two uncoordinated
H2O molecules occupying well-defined py gpositions in the crystal
CoCl 6H O
3K+
CoCl2 6H2O Invisible ink, developed by
potassium ferricyanide
Typical Ligands
monodentate
polydentate
ambidentate
Ru-bpy, revisited
• bpy ligands are polydentate(attachment to the central metal (can occur at each N)
• Polydentate ligands can Polydentate ligands can produce a chelate (Greek for “claw”): a complex in which a li d f i h i l d ligand forms a ring that includes the metal atom
• Ru-bpy dye is an effective stabilizer for semiconducting nanoparticles such as IrO2 TiO2
[Ru(bpy)3]2+
nanoparticles such as IrO2, TiO2
Dye Sensitized (Gratzel) Cell
• TiO2-bound Rubpy dye molecules act as the light harvester• Electrons are injected into the TiO2, flow to the collector electrode,
and through the circuit to the counter electrodeand through the circuit to the counter electrode.• the dye is regenerated by electron donation to the I3-/3I- redox
couple (0.536V)
Dye Sensitized Photovoltaic Cell
Ti4 /3Ti4+/3+ Ligand π LUMOEnergy
Goal of next 2-3 Goal of next 2 3 lectures: understand the bonding, electronic structure and spectra structure, and spectra of complexes
Ru (II/III) (6 spin-paired electrons in dxy,dxz,dyz)
• Absorption of UV-Visible radiation causes ππ* and metal-to-ligand charge transfer electronic transitions
Consititution
Three factors govern the coordination number of a complex:
1) The size of the central atom
larger radii of atoms and ions lower and to the left of the periodic table favor high coordination numbers
2) Steric interactions between the ligands
favor high coordination numbers
lk l d l l d b ll f hBulky ligands result in lower coordination numbers, especially if the ligands are charged
3) Electronic interactions between the central atom or ion and the ligands
ConsititutionHigh coordination numbers:
Low coordination numbers: right of d-block (metals are rich
metal ion has a small number of valence electrons –
can accept more electrons
right of d block (metals are rich in electrons)
pfrom Lewis base ligands
Very high coordination numbers (10-12): large ions can accommodate many ligands
Low coordination number compounds: CN=2
C di i b 2 d li i •Common coordination number 2 compounds are linear species of the group 11 ions (i.e., Cu+, Ag+)
• examples: [AgCl2]-, dimethyl mercury, Au(I) complexes of the p [ g 2] y y ( ) pform L-Au-X (X is a halogen, L is a neutral Lewis base, such as a
thioether or phosphine)
HgMe2 complexes with cysteine (an
i id) amino acid) to cross blood-brain barrier:
•Two-coordinate complexes often gain additional ligands to form 3 or 4 coordinate complexes
•C CN pp r to h CN 1 b t in f t i t lin r C•CuCN appears to have CN=1, but in fact exists as linear Cu-CN-Cu-CN chains CN of Cu is 2
Low coordination number compounds: CN=3
•Three-coordination is rare but is found with bulky ligands Three-coordination is rare, but is found with bulky ligands, such as tricyclohexylphosphine
• MX3 compounds, where X is a halogen, are usually chains or k i h hi h CN d h d li dnetworks with a higher CN and shared ligands
[Pt(PCy3)3], Cy=cyclo-C6H11
Intermediate coordination number compounds
•CN=4 5 or 6: most important class of complexesCN=4, 5, or 6: most important class of complexes• include vast majority of complexes that exist in solution
• include almost all of the biologically important complexes
Intermediate coordination number compounds
•CN=4 5 or 6: most important class of complexesCN=4, 5, or 6: most important class of complexes• include vast majority of complexes that exist in solution
• include almost all of the biologically important complexes
Four coordination: tetrahedral complexes (Td symmetry)
• Favored over higher CN when the • Favored over higher CN when the central atom is small and ligands are large L-L repulsions override
d f f M L b dadvantage of forming more M-L bonds• found with s and p-block complexes
with no lone pair on the central atom, pi.e.: [BeCl4]2-, [SnCl4]
• oxoanions of metal atoms on the left of the d-block in high oxidation states the d-block in high oxidation states, i.e.: [MnO4]-, [CoCl4]2-, [NiBr4]2-
Intermediate coordination number compounds
•CN=4 5 or 6: most important class of complexesCN=4, 5, or 6: most important class of complexes• include vast majority of complexes that exist in solution
• include almost all of the biologically important complexes
Four coordination: square planar complexes (D4h symmetry)• Rarely found for s & p block
lcomplexes• abundant for d8 complexes of the
elements belonging to the 4d and 5s series metals: Rh+, Ir+, Pt2+, Pd2+, Au3+
• for 3d metals with d8for 3d metals with dconfigurations (Ni2+), square planar is favored by ligands that f b d form π bonds
• Found with ring ligands(porphyrins)
Intermediate coordination number compounds
•CN=4 5 or 6: most important class of complexesCN=4, 5, or 6: most important class of complexes• include vast majority of complexes that exist in solution
• include almost all of the biologically important complexes
Four coordination: square planar complexes (D4h symmetry)
cis-[PtCl2(NH3)2] trans-[PtCl2(NH3)2]cis [PtCl2(NH3)2] trans [PtCl2(NH3)2]
Isomerism: different spatial arrangements of the same ligands
Applications to Chemotherapy
1964 f d l di h f b i i l i • 1964: fundamental studies on growth of bacteria in solution subjected to an electric field between two Pt electrodes
• discovered that cells continued to grow in size, but stopped replicating – traced to formation of Pt(II)(NH3)2Cl2
•1969: Rosenberg and colleagues find that cis-Pt(II)(NH3)2Cl2
injected into mice completely inhibits cancerous cell division
http://www.cancer‐therapy.org/CT/v5/B/HTML/40._Boulikas,_351‐376.html
Applications to Chemotherapy
http://www.cancer‐therapy.org/CT/v5/B/HTML/40._Boulikas,_351‐376.html
Applications to Chemotherapy
• The kink caused by chelationrenders the DNA incapable of
lreplication or repair. • It also makes the DNA recognizable
by ‘high mobility group’ proteins y g y g p pthat bind to bent DNA and target the molecule for death
Applications to Chemotherapy
• The trans platin molecule does not chelate with DNA pnot bound for very long, and no geometric kink formed
Five-coordination
•Less common than 4 or 6 coordinationLess common than 4 or 6 coordination•Usually square pyrimidal or trigonal bipyrimidal
• energies of 5-coordinate complexes differ very little from each h f fl i l ( i i diff h )other often very fluxional (can twist into different shapes)
Active center of Myoglobin
Six-coordination
•Most common arrangement for metal complexesMost common arrangement for metal complexes•Found in s, p, d, and f-metal coordination compounds
• almost all are octahedral, but some can be trigonal prismatic
Octahedral complex Trigonal prismatic
Higher coordination (CN=7-12)
[Mo(CN)8]3-
ML8 dodecahedron
ML8 square antiprism
ML8 Cube
Polymetallic complexes
•Contain more than one metal atomContain more than one metal atom•Metal cluster: polymetallic complexes with direct M-M bonds•Cage complexes: no M-M bond, only metals held together by
b id i li d i [F S SCH Ph ]2bridging ligands, i.e.: [Fe4S4SCH2Ph4]2-
Cubic structure formed from 4 Fe atoms bridged by RS-
ligands FeS clusters generally ligands. FeS clusters generally serve as electron relays or
long-range electron transfer th i l l pathways in molecules can easily delocalize added
electrons
Formation Constants
•Expresses the interaction strength of the incoming ligandExpresses the interaction strength of the incoming ligandrelative to the strength of the solvent molecules as a ligand
C i f h l ( ll H O) d • Concentration of the solvent (normally H2O) does not appear in the expression of Kf, because it is taken to be constant in
dilute solution and is ascribed unit activityy
[Fe(OH2)6]3+(aq)+SNC-(aq) [Fe(SNC)(OH2)5]2+(aq)+H2O (l)
])H[Fe(SCN)(O 252
K]][SCN)[Fe(OH -3
62fK
•If K is large the incoming ligand binds more strongly than the •If Kf is large, the incoming ligand binds more strongly than the solvent
Stepwise Formation Constants
•If more than one ligand can be replaced stefwise formation If more than one ligand can be replaced, stefwise formation constants are used
U ll K K•Usually, Kfn>Kfn+1
[Hg(OH2)6]2+(aq)+Cl-(aq) [HgCl(OH2)5]+(aq)+H2O (l) log Kf1=6.74
[HgCl(OH ) ]+(aq)+Cl-(aq) [HgCl (OH ) ](aq)+H O (l) log Kf =6 48[HgCl(OH2)5] (aq)+Cl (aq) [HgCl2(OH2)4](aq)+H2O (l) log Kf2=6.48
[HgCl2(OH2)4](aq)+Cl-(aq) [HgCl3(OH2)]-(aq)+3H2O (l) log Kf2=0.95
[HgCl2(OH2)4][HgCl3(OH2)]-