Coordination Compounds. Suggested reading: Shriver & Atkins, Chapter 7

Transcription

1 Lecture 15 Coordination Compounds Suggested reading: Shriver & Atkins, Chapter 7

2 From last lectures: 3 classes of nanomaterials Alex Zettl with a carbon nanotube Metallic Nanoparticles Semiconducting Nanocrystals

3 Ligand Ligand Metal cluster Metal atom Nanoparticle Complex

4 1706: German paint maker Diesbach Prussian blue Cochineal Potash Fe 4 [Fe(CN) 6 ] 3 + = The Great Wave off Kanagawa (1830)

5 Pigments are Coordination Complexes Colors of various coordination complexes

6 Photosynthesis

7 Photosynthesis Electron/oxygen transport in biology

8 Photosynthesis Photovoltaics Electron/oxygen transport in biology

9 Coordination Compounds Metal-ligand li compounds play crucial roles in photosynthesis, h Gratzel solar cells, chemotherapy, electron & oxygen transfer in biological processes, pigments & dyes, and catalysis Complex a 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 one of the ions is a complex

10 Terminology Tris(bipyridine)ruthenium(II) ( ) chloride Donor atom: the atom in the ligand that bonds to the central atom Acceptor atom: the metal atom or ion that accepts electrons from the ligand 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

11 Outer Sphere Complex probed via XRD CoCl 2 6H 2 O (Cobalt(II) chloride hexahydrate) : Contains the neutral complex [CoCl 2 (OH 2 ) 4 ] and two uncoordinated H 2 O molecules occupying well-defined positions in the crystal CoCl 2 6H 2 O Invisible ink, developed by potassium ferricyanide 3K +

12 Typical Ligands monodentate polydentate ambidentate

13 Ru-bpy, revisited bpy ligands are polydentate (attachment to the central metal can occur at each N) Polydentate ligands can produce a chelate (Greek for claw ): a complex in which a ligand forms a ring that includes the metal atom Ru-bpy dye is an effective stabilizer for semiconducting nanoparticles such as IrO 2, TiO 2 [Ru(bpy) 3 ] 2+

14 Dye Sensitized (Gratzel) Cell TiO 2 -bound Rubpy dye molecules act as the light harvester Electrons are injected into the TiO 2, flow to the collector electrode, and through the circuit to the counter electrode. the dye is regenerated by electron donation to the I3-/3I- redox couple (0.536V)

15 Dye Sensitized Photovoltaic Cell Energy Ti 4+/3+ Ligand π LUMO Goal of next 2-3 lectures: understand the bonding, electronic structure, and spectra of complexes Ru (II/III) (6 spin-paired electrons in d xy,d xz,d yz ) Absorption of UV-Visible radiation causes π π* and metal-toligand charge transfer electronic transitions

16 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 Bulky ligands result in lower coordination numbers, especially if the ligands are charged 3) Electronic interactions between the central atom or ion and the ligands

17 Consititution High coordination numbers: metal ion has a small number of valence electrons can accept more electrons from Lewis base ligands Low coordination numbers: right of d-block (metals are rich in electrons) Very high coordination numbers (10-12): large ions can accommodate many ligands

18 Low coordination number compounds: CN=2 Common C coordination i number 2 compounds are linear species of the group 11 ions (i.e., Cu+, Ag+) examples: [AgCl 2 2] -, dimethyl mercury, Au(I) complexes of the form L-Au-X (X is a halogen, L is a neutral Lewis base, such as a thioether or phosphine) HgMe 2 complexes with cysteine (an amino acid) to cross blood-brain barrier: Two-coordinate complexes often gain additional ligands to form 3 or 4 coordinate complexes CuCN CN appears to have CN=1, but in fact exists as linear Cu- CN-Cu-CN chains CN of Cu is 2

19 Low coordination number compounds: CN=3 Three-coordination is rare, but is found with bulky ligands, such as tricyclohexylphosphine MX 3 compounds, where X is a halogen, are usually chains or networks with ih a higher hih CN and shared ligands [Pt(PCy 3 ) 3 ], Cy=cyclo-C 6 H 11

20 Intermediate coordination number compounds CN=4 5 or 6: most important class of complexes CN=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

21 Intermediate coordination number compounds CN=4 CN=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 (T d symmetry) Favored over higher CN when the central atom is small and ligands are large L-L repulsions override advantage of forming more M-L Lbonds found with s and p-block complexes with no lone pair on the central atom, i.e.: [BeCl 4 ] 2-, [SnCl 4 ] oxoanions of metal atoms on the left of the d-block in high oxidation states, i.e.: [MnO 4 ] -, [CoCl 4 ] 2-, [NiBr 4 ] 2-

22 Intermediate coordination number compounds CN=4 CN=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 (D 4h symmetry) Rarely found for s & p block complexes abundant for d 8 complexes of the elements belonging to the 4d and 5s series metals: Rh +, Ir +, Pt 2+, Pd 2+, Au 3+ for 3d metals with d 8 configurations (Ni 2+ ), square planar is favored by ligands that form π bonds Found with ring ligands (porphyrins)

23 Intermediate coordination number compounds CN=4 CN=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 (D 4h symmetry) cis-[ptcl 2 (NH 3 ) 2 ] trans-[ptcl 2 (NH 3 ) 2 ] Isomerism: different spatial arrangements of the same ligands

24 Applications to Chemotherapy 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)(NH 3 ) 2 Cl : Rosenberg and colleagues find that cis-pt(ii)(nh 3 ) 2 Cl 2 injected into mice completely inhibits cancerous cell division 0._Boulikas,_ html

25 Applications to Chemotherapy therapy.org/ct/v5/b/html/40._boulikas,_ html

26 Applications to Chemotherapy The kink caused by chelation renders the DNA incapable of replication or repair. It also makes the DNA recognizable by high mobility group proteins that bind to bent DNA and target the molecule for death

27 Applications to Chemotherapy The trans platin molecule does not chelate with DNA p not bound for very long, and no geometric kink formed

28 Five-coordination Less common than 4 or 6 coordination Usually square pyrimidal or trigonal bipyrimidal energies of 5-coordinate complexes differ very little from each other often very fluxional l (can twist into different shapes) Active center of Myoglobin

29 Six-coordination Most 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

30 Higher coordination (CN=7-12) [Mo(CN) 8 ] 3- ML 8 dodecahedron ML 8 square antiprism ML 8 Cube

31 Polymetallic complexes Contain 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 bid bridging i ligands, i.e.: [Fe 4 S 4 SCH 2 Ph 4 ] 2- Cubic structure formed from 4 Fe atoms bridged by RS- ligands. FeS clusters generally serve as electron relays or long-range electron transfer pathways in molecules l can easily delocalize added electrons

32 Formation Constants Expresses the interaction strength of the incoming ligand relative to the strength of the solvent molecules as a ligand Concentration of the solvent (normally H 2 O) does not appear in the expression of K f, because it is taken to be constant in dilute solution and is ascribed unit activity [Fe(OH 2 ) 6 ] 3+ (aq)+snc - (aq) [Fe(SNC)(OH 2 ) 5 ] 2+ (aq)+h 2 O (l) K f 2 [Fe(SCN)(OH 2) 5 3 [Fe(OH ) ][SCN 2 6 ] ] - If K is large the incoming ligand binds more strongly than the If K f is large, the incoming ligand binds more strongly than the solvent

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34 Stepwise Formation Constants If more than one ligand can be replaced, stefwise formation constants are used Usually, U K fn >K fn+1 [Hg(OH 2 ) 6 ] 2+ (aq)+cl - (aq) [HgCl(OH 2 ) 5 ] + (aq)+h 2 O (l) log K f1 =6.74 [HgCl(OH ) 5 ] (aq)+cl (aq) [HgCl 2 (OH 2 ) 4 ](aq)+h 2 O (l) log K f2 =6.48 [HgCl 2 (OH 2 ) 4 ](aq)+cl - (aq) [HgCl 3 (OH 2 )] - (aq)+3h 2 O (l) log K f2 =0.95 [HgCl 2 (OH 2 ) 4 ] [HgCl 3 (OH 2 )] -