2 Ligands and complex formation

Complex formation complex :is a central metal ion surrounded by ligands. ligand.: An atom, ion or molecule which can donate a lone electron pair Co-ordinate bonding is involved in complex formation. Co-ordinate bonding is when the shared pair of electrons in the covalent bond come from only one of the bonding atoms. Co-ordination number: The number of co-ordinate bonds formed to a central metal ion. Cu OH2 OH2 H2O H2O OH2 OH2 2+. ligands can be unidentate (e.g. H2O, NH3 and Cl- ) which can form one coordinate bond per ligand or bidentate (e.g. NH2CH2CH2NH2 and ethanedioate ion C2O4 2- ) which have two atoms with lone pairs and can form two coordinate bonds per ligand or multidentate (e.g. EDTA4- which can form six coordinate bonds per ligand). A complex with bidentate ligands e.g. [Cr(NH2CH2CH2NH2 )3 ]3+ It has a coordination number of 6 Cu(H2O)6 2+ + 3NH2CH2CH2NH2  [Cu(NH2CH2CH2NH2 )3 ]2+ + 6H2O Cu(H2O)6 2+ + 3C2O4 2- [Cu(C2O4 )3 ]4- + 6H2O Cu(H2O)6 2+ + EDTA4- [Cu(EDTA)]2- + 6H2O Equations to show formation of bidentate and mutidentate complexes N CH2 CH2 N CH2 CH2 CH2 CH2 C C C C O O O -O -O OOO The EDTA4- anion has the formula with six donor sites(4O and 2N) and forms a 1:1 complex with metal(II) ions 3- C C O O C C O O C C O O Cr O O O O O O 3+ H2C NH2 NH2 CH2 H2C NH2 NH2 CH2 CH2 NH2 NH2 Cr CH2 A complex with bidentate ethanedioate ligands e.g. [Cr(C2O4 )3 ]3- Learn the two bidentate ligands mentioned above but it is not necessary to remember the structure of EDTA There are 3 bidentate ligands in this complex each bonding in twice to the metal ion N Goalby chemrevise.org 2 Shapes of complex ions transition metal ions commonly form octahedral complexes with small ligands (e.g. H2O and NH3 ). transition metal ions commonly form tetrahedral complexes with larger ligands (e.g.Cl- ). square planar complexes are also formed, e.g. cisplatin Ag+ commonly forms linear complexes e.g. [Ag(NH3 )2 ]+ , [Ag(S2O3 )2 ]3- and [Ag(CN)2 ] – (all colourless). [Co(NH3 )6 ]2 [Cu(H2O)6 ]2+ [CoCl4 ]2- Isomerism in complex ions Complexes can show two types of stereoisomerism: cis-trans isomerism and optical isomerism Ni NH3 Cl H3N Cl Ni Cl NH3 H3N Cl Cis-Ni(NH3)2Cl2 trans-Ni(NH3)2Cl2. Optical isomerism. Complexes with 3 bidentate ligands can form two optical isomers (non-superimposable mirror images). 2+ CH2 CH2 NH2 NH2 CH2 CH2 H2N H2C NH2 H2C NH2 NH2 NiOptical isomerism The Pt(II) complex cisplatin is used as an anticancer drug. cisplatin transplatin The cisplatin version only works as two chloride ions are displaced and the molecule joins on to the DNA. In doing this it stops the replication of cancerous cells. It can also prevent the replication of healthy cells by bonding on to healthy DNA which may lead to unwanted side effects like hair loss. In the body one Cl ligand is subsituted by a water molecule Pt(NH3 )2Cl2 + H2O  [Pt(NH3 )2Cl(H2O)]+ + Cl– Be able to apply your knowledge of bonding to given information in the question to explain how it bonds to DNA molecule- generally a combination of dative covalent bonding and hydrogen bonding Platin binds to DNA of cancer cells and stops cancer cells dividing. Cisplatin