Chapter 5 - Coordination Compounds

Think of a king central metal ion surrounded by guards ligands that donate electron pairs and hold king tightly. The whole group king+guards called coordination sphere inside square brackets [ ]. Counter ions outside balance charge.

Example [Fe(CN)6]4- central Fe2+ 6 CN- ligands donating lone pairs. [Cu(NH3)4]2+ deep blue solution you see in lab copper sulphate plus ammonia king Cu2+ 4 NH3 guards.

Ligand donates lone pair to metal empty orbital forms coordinate bond. Coordination number is number of donor atoms bonded to metal directly how many guards holding king.

Ligands classified by number of donor sites denticity teeth that bite metal.

Monodentate one donor site one tooth bite NH3 ammonia, H2O water, Cl- chloride, CN- cyanide only one bond. Like one hand holding.

Bidentate two donor sites two teeth forms ring chelate ring en ethylenediamine H2N-CH2-CH2-NH2 two N donors forms 5-member ring with metal, oxalate C2O4 2- two O donors also 5-member ring more stable.

Polydentate many donor sites many teeth EDTA ethylenediaminetetraacetate 6 donor 2 N +4 O hexadentate wraps like octopus very stable chelate used to trap metal ions. Chelating ligands form ring structures more stable than monodentate due to entropy chelate effect.

Coordination number 2 linear [Ag(NH3)2]+, 4 tetrahedral like [ZnCl4]2- or square planar [Ni(CN)4]2-, 6 octahedral most common [Fe(CN)6]4-.

Naming coordination compounds like naming full name with surname alphabetical order.

Rules: 1. Cation first then anion like salt. 2. Inside coordination sphere ligands alphabetical order ignore prefixes di tri penta. Anionic ligands end -ido chlorido for Cl- cyanido for CN-, neutral ligands ammine NH3 aqua H2O carbonyl CO. 3. Metal name after ligands with oxidation state Roman numeral in brackets Co(III) Fe(II). If complex anion metal name ends -ate ferrate for Fe cobaltate for Co.

Example [Co(NH3)5Cl]Cl2 pentaamminechloridocobalt(III) chloride 5 NH3 ammine alphabetical c before p so chlorido before pentaammine? Actually alphabetical ignore prefix c comes before p so chlorido before ammine but IUPAC 2005 says alphabetical by ligand name ammine before chlorido? Simple rule alphabetical ammine a before chlorido c so pentaamminechlorido. Counter 2 Cl- chloride outside.

Daily: just remember alphabetical ligands then metal oxidation then anion.

Same molecular formula but different arrangement isomers.

Structural: ionization isomerism counter ion exchange [Co(NH3)5SO4]Br vs [Co(NH3)5Br]SO4 bromide outside vs sulphate outside gives different precipitate AgNO3 test. Linkage isomerism ambidentate ligand NO2 can bind via N nitro or O nitrito [Co(NH3)5NO2]2+ vs [Co(NH3)5ONO]2+ different colours. Coordination isomerism ligands exchange between cation anion [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6]. Solvate hydrate water inside vs outside [Cr(H2O)6]Cl3 vs [Cr(H2O)5Cl]Cl2.H2O.

Stereoisomerism: geometrical cis trans same bonds different spatial. Square planar [Pt(NH3)2Cl2] cisplatin anticancer cis isomer both Cl same side active trans inactive different. Octahedral [Co(NH3)4Cl2]+ cis Cl adjacent trans opposite.

Optical isomerism non-superimposable mirror images enantiomers like left right hands chiral [Co(en)3]3+ en bidentate three rings propeller left right rotates plane polarized light opposite. Daily hands mirror cannot superimpose.

Valence Bond Theory explains bonding hybridization and magnetic.

Metal provides empty orbitals hybridize with ligand orbitals. For coordination number 6 octahedral: if inner d orbitals used d2sp3 inner orbital complex low spin pairing due to strong field, if outer d used sp3d2 outer orbital complex high spin no pairing weak field.

Examples: [Fe(CN)6]4- Fe2+ d6 CN- strong field low spin inner d2sp3 all paired diamagnetic mu=0. [Fe(H2O)6]2+ H2O weak field outer sp3d2 4 unpaired paramagnetic mu= sqrt(4(6))=4.9 BM. [Ni(CN)4]2- Ni2+ d8 CN- strong square planar dsp2 0 unpaired diamagnetic.

Magnetic moment mu = sqrt(n(n+2)) BM Bohr magneton n unpaired electrons more unpaired stronger magnet. Used to find structure.

Limitations cannot explain colour and CFSE.

Crystal Field Theory ligand negative point charges repel d electrons split d-orbitals.

Octahedral: 6 ligands along axes eg dz2 dx2-y2 higher energy repelled more +0.6 Delta0, t2g dxy dyz dzx lower -0.4 Delta0. Delta0 crystal field splitting energy difference. Strong field large Delta0 pairing energy P small so low spin electrons pair in t2g before eg. Weak field small Delta0 high spin electrons go eg before pairing.

Tetrahedral: 4 ligands between axes opposite splitting t2 higher e lower Delta t = 4/9 Delta0 small always high spin.

Colour due to d-d transition electron jumps t2g to eg absorbs visible light complement colour seen. [Ti(H2O)6]3+ d1 t2g1 -> eg absorbs green shows purple. [Cu(H2O)6]2+ blue.

CFSE Crystal Field Stabilization Energy = (-0.4 x t2g electrons +0.6 x eg electrons) Delta0 + pairing energy considerations stabilizes complex more negative more stable.

Stability constant K large means stable complex does not dissociate easily. Factors: charge higher metal charge more stable, small size more stable, chelate effect.

Chelate effect: bidentate polydentate ligands form ring structures more stable than monodentate due to entropy increase more particles released. Example [Ni(en)3]2+ three en bidentate more stable than [Ni(NH3)6]2+ six NH3 monodentate because 3 en replace 6 NH3 increases particles entropy. Entropy disorder increases stability.

Macrocyclic effect cyclic polydentate even more stable like crown ether heme porphyrin ring very stable.

Daily: EDTA chelates Ca Mg hardness removes, traps heavy metal poisoning lead chelation therapy EDTA removes lead.

Coordination compounds everywhere daily life.

Haemoglobin blood red Fe2+ coordinated to porphyrin ring heme 4 N plus protein carries O2 from lungs to tissues Fe binds O2 reversibly red blood O2 transport. Myoglobin similar stores O2 muscle.

Chlorophyll green plants Mg2+ in porphyrin ring photosynthesis absorbs sunlight CO2 to glucose green colour.

Vitamin B12 cyanocobalamin Co3+ complex helps blood formation.

Cisplatin [Pt(NH3)2Cl2] cis isomer anticancer drug kills cancer cells by binding DNA stops division trans isomer inactive geometrical importance.

EDTA ethylenediaminetetraacetate hexadentate used to measure hardness of water Ca2+ Mg2+ chelate, remove heavy metals lead poisoning, in detergent softens water.

Nickel Ni(CO)4 carbonyl extraction of Ni Mond process, Ag(NH3)2+ Tollen's reagent silver mirror test for aldehydes.