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)]-