Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
PREPARATION OF COORDINATION COMPOUNDS Coordination compounds cannot really be purchased from BDH, Fisher scientific company, Alfa productions etc. they are rather synthesized from starting materials like CrCl3, CoCl2, NiCl2, K2PtCl4 either through substitution of H2O and or halide ions in the primary coordination sphere of the starting materials. Other preparative methods of coordination compounds include: a. SIMPLE ADDITION REACTION example: i. BF3 + :NH3 → [BF3NH3] → This is a gas-phase addition and each reactant is carefully controlled into a large evacuated flask with a resultant deposit of a white powder (product). For heterogeneous reactions ii. BF3 + Et2O →[BF3 . OEt2] Here, the ether and Boron trifluoride are condensed separately into an evacuated flask and cooled in liquid nitrogen. When warmed gently, a controlled reaction takes place. Other coordination compounds prepared by simple addition reaction include. iii. SnCl4 + 2NMe3 trans [SnCl4 (NMe3)2] iv. 8NH4F + 2H3BO3 + 3H2SO4 → 3(NH4) SO4 + 2 N4 [BF4] = 6H2O v. 2KCL + TiCl4 → K2[TiCl6] b. SUBSTITUTION REACTIONS Labile complexes can easily be substituted e.g. i. [Cu(H2O)]2+ 4NH3 (aq) → [Cu (NH)4]2+ + 4H2O. Though the reaction shows complete substitution, the reactions occurs in steps and the species [Cu(H2O)4]2+ will be present in the solution even though at low concentration. ii. [Pb (H2O)6]2+ 6SC(NH2) →[Pb(Sc(NH2)2)6]2+ + 6H2O (thiourea) iii. [Fe(H2O)6]3+ + 3acac- → [Fe(acaca)3] (insoluble in water). Some examples of preparations involving substitution reactions of inert complexes are given below: i. [Co(NH3)5CO3]+ + 2HF ii. [Co(NO2)6]3- + 2en [Co(NH3)5F]2+ + F- + CO2 + H2O Cis[Coen2(NO2)2]+ + 4NO2- (Cis-dinitrobis(ethylenediamine)cobalt III) Other preparative methods involving substitution reactions in non-aqueous media are: i. [Cr(H2O)6]3+ + 6NCS- [Cr(NCS)6]3- + 6H2O ii. 2NEt4Cl + NiCl2 c. OXIDATION – REDUCTION REACTION (NEt4)2[Net4[NiCl4] Substitution reactions cannot be totally relied upon as a preparative method for coordination compounds especially inert complexes. A better method is to take a compound containing the metal in a different oxidation state and oxidize or reduce it as appropriate in the present of a coordinating ligand. This method is used extensively in the preparation of complexes of Co 3+ with Co2+ as the precursor and either air or H2O2 being the oxidizing agent. i. CoCl2 + 2NH4Cl + 10NH3 + H2O2 2[Co(NH3)6]Cl3 + 22O ii. K2Cr2O7 +H2C2O4 + 2K2C2O4 → 2K3[Cr(C2O4)3] + 6CO2 + 7H2O iii. 8[Mn(H2O)6]2+ + 2[MnO4]- + 25HF2 → [MnF5(H2O)]2- + 9H+ + 46H2O. d. THERMAL DISSOCIATION REACTIONS Some complexes can be degraded to others through controlled heating, expelling the volatile ones. Examples, i. CuSO4 . 5H2O CuSO4 . 4H2O CuSO4 CuSO4 . 3H2O CuSO4 . H2O ii. Cis[Cr(en)2Cl2]2+ can be prepared by heating [Cr(en)3]Cl3. i.e. [Cr(en)3]Cl3 iii. Cis[Cr(en)2Cl2]Cl + en Trans[Cr(en)2(SCN)2]SCN is prepared by heating [Cr (en)3](SCN)3 to 1300C SYNTHESIS OF COORDINATION COMPOUNDS Transition metal chemistry started from the study of coordination complexes by Werner and his students. To date, coordination complexes remain the most widely studied compounds because they were the first to be prepared. A great variety of cobalt compounds have been synthesized, most of which were made possible due to the lability of Co2+ compounds to substitution and inertness of Co3+ .Conversion of Co2+ to Co3+ was therefore easy with most starting materials being converted to the Co2+ intermediate. Most of the preparative work in Co–chemistry usually begins with the halides (Cl-, Br-, & I-) because of their easeness to substitution by stronger liquids. SYNTHESIS OF SOME COORDINATION COMPOUNDS i. Synthesis of Pentaamine Dichlorocobalt (ii) Chloride Co(NO3)2.6H2 → [Co(H2O)6](NO3)2. The starting materials are first converted to an intermediate Co (ii) carbonato complex which is then oxidized to the final product. Co(NO3 ) + N3(aq) + (NH4 )2 Co3 + H2 O2 NH3 O Nh3 + No3- C=0 Co + NH4 NO3 + H2 O O NH3 Unbalanced NH3 HCl(aq) NH3 NH3 H2 O NH3 H2 O NH3 2+ Co Co NH3 NH3 NH3 Cl NH3 NH3 NH3 HCl(aq) NH3 Cl NH3 Cl2 Co NH3 Nh3 NH3 ii. Synthesis of Hexaamine Dichromium III Nitrate NH3 NH3 CrCl3 + 6NH3 NH3 2 NH | NH3 Co Cl3 NH3 NH3 NH3 brown HNO3(aq) NH3 NH3 NH3 Co NH3 NH3 NH3 iii. (NO3 )3 Preparation of Cobalt Compounds Yellow 2+ When a Co2+ salt is placed in aq NH3, the solution gradually turned brown in the presence of air and finally wine red on boiling. A purple colour cpd is obtained if excess chloride is used for the same reaction. Some of the possible compounds include the following: Co Cl3 . 6NH3 Co Cl3 . 5NH3 . 5N2O Co Cl3 . 5NH3 NH3 NH3 Cl NH3 NH3 Co NH3 Cl3 NH3 NH3 Co yellow NH3 Co Cl3 NH3 red Cl2 NH3 NH3 Because of the ease with which Co-cplexes are prepared, a great variety of them have been synthesized and so transition metal chemistry stems from the study of co-cplexes. Much of the preparative work in Co-chemistry usually begins with the halides (Cl-, Br- & I-). As expected, the halides are the lowest in the spectrochemcial series and so they are easily substituted by stronger ligands to form many successive classes of complexes. iv. Synthesis of Platimium Compounds Pt (II) cpds are the most abundant and interesting. The halides are also the route through which Pt compounds are prepared. Pt . sponge 1. dissolve in aqua regia 2. Add. Kcl. (N2H6)(SO4)2 K2PtCl6 (yellow) K2PtCl4 (red) or Pt. Cl4 (red) M M2PtCl4 Pt . + xcl2 PtCl2 (Olive green) Pt. Cl2 Using M2PtCl4 and PtCl2 as starting materials, many Pt (II) cpds can be synthesized e.g. 2[PtCl4]2- + 4L → [PtL4][Pt. Cl4] + 4ClPtL2X2 + 2L + 2AgNO3 → [PtL4](NO3)2 + 2Ag X [Pt(NH3)4]Cl2 + K2PtCl4 → [Pt (NH3)4][PtCl4]. K2 PtCl4 + NH3 Cl NH3- Cl NH3 Pt Pt Cl Cl Cl + 2Kcl NH3 K2PtCl4 + 4NH3 → [Pt(NH3)4]Cl2 + 2KCl When [Pt(NH3)4]Cl2 is heated in the presence of excess, Cl-, a trans isomer is produced. Pt Cl NH3 NH3- Cl Pt Cl2 Cl NH3 -NH3 Cl Cl NH3 -NH3 NH3 Cl Pt Cl - trans NH3