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by: Carmela Kilback
Carmela Kilback
GPA 3.92


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This 11 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 165 at University of Washington taught by Staff in Fall. Since its upload, it has received 18 views. For similar materials see /class/192603/chem-165-university-of-washington in Chemistry at University of Washington.


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Date Created: 09/09/15
Chemistry 165 1871 Chapter 18 Coordination Complexes Oxtoby pages 6597689 The three series of elements arising from the filling of the 3d 4d and 5d shells and situated in the periodic table following the alkaline earth metals are commonly described as quottransition elementsquot though this term is sometimes also extended to include the lanthanide and actinide or inner transition elements They exhibit a number of characteristic properties which together distinguish them from other groups of elements i They are all metals and as such are lustrous and deformable and have high electrical and thermal conductivities In addition their melting and boiling points tend to be high and they are generally hard and strong ii Most of them display numerous oxidation states which vary by steps of 1 rather than 2 as is usually the case with those mainegroup elements which exhibit more than one oxidation state iii They have a strong propensity for forming coordination compounds with Lewis ases Coordination complexes have been known for ca 100 years They were initially called quotcomplex compoundsquot because of the consternation they caused for early chemists These materials are stable more or less and stoichiometrically reliable but they are often made up of combinations of other compounds that are independently stable For example Cu2 ions combine with H20 to give CuH204Z The Cu2 ion coordinates four water molecules The water molecules are called ligands It is very important to realize that the interaction between the cation Lewis acid and the ligand Lewis base is actually a chemical bond The bond has associated with it a highly variable but substantial bond enthalpy Remember that water is just another ligand Much of coordination chemistry is carried out in water and the competition for metal binding between water and various ligands is a central theme Review your notes from Chapter 11 on complex ion equilibria Types of Ligand Ligands are commonly classified according to the number of donor atoms which they contain Unidentate or monodentate ligands include H20 NH3 HS RO etc Bidentate ligands are very common Examples include ethylenediamine 1107 phenanthroline and oxalate These are often quotchelatingquot ligands from Greek x117 crab39s claw Chemistry 165 1872 UH2 NH2 C e OC HN CH239CH2 r CH2 0 H2N NHz OC CHz NHz diethylenetriamine dien Oxalate ethylenediamine en R M82 C Q ASX HC39 9 4 A cquot 39 5 M82 R ophenylenebisdimethylarsine diars diket0nates e R Me 1 10 phenamhmhne phen 12bisdimethylarsin0benzene g acetylacetonate acac O 9 O Tropolonate terpyridine terpy N 9 e gt Oo Nl N 0 00 N N CH21 CH 2 e X N N CH CH O 2 2 C CO N 0 Ge phthalocyanine HOZCCH22NCH22NCH2C02H2 EDTA Chemistry 165 1873 Rules for Naming Complexes 1 The positive ion cation comes first followed by the negative ion anion This is the common order for simple salts as well Examples diamminesilverl chloride AgNH32Cl potassium hexacyanoferratelll K3FeCN6 2 Within the coordination sphere the ligands are named before the metal but in formulas the metal ion is written first The inner coordination sphere is enclosed in square brackets in the formula Examples tetraamminecopperll sulfate CuNH34SO4 hexaamminecobaltlll chloride CoNH36C13 3 Two systems exist for designating charge or oxidation number a The Stock system puts the calculated oxidation number of the metal ion as a Roman numeral in parentheses after the name of the coordination sphere This is the more common convention although there are cases where it is difficult to assign oxidation numbers b The EwingeBassett system puts the charge on the coordination sphere in parentheses after the name of the coordination sphere This convention is used by Chemical Abstracts and offers an unambiguous identification of the species In either case if the charge is negative the suffix rate is added to the name of the coordination sphere Examples tetraammineplatinumll or tetraammineplatinum2 PtNH34Z tetrachloroplatinatell or tetrachloroplatinate27 PtCl42 hexachloroplatinatelV or hexachloroplatinateZe PtC162 4 Ligands are named in alphabetical order according to the name of the ligand not the prefix although exceptions to this rule are common An earlier rule gave anionic ligands first then neutral ligands each listed alphabetically Examples tetraamminedichlorocobaltIll CoNH34C12 amminebromochloromethylamineplatinumll PtNHgBrClCH3NH2 5 Anionic ligands are given an o suffix Neutral ligands retain their usual name Coordinated water is called aqua Examples chloro Cl Chemistry 165 1874 bromo Br sulfato 8042 methylamine CH3NH2 ammine NH3 the double m distinguishes NH3 from alkyl amines aqua H20 6 The number of ligands of one kind is given by the following prefixes If the ligand name includes these prefixes or is complicated it is set off in parentheses and the second set of prefixes is used 2 di bis 3 tri tris 4 tetra tetrakis 5 penta pentakis 6 hexa hexakis 7 hepta heptakis 8 octa octakis 9 nona nonakis 10 deca decakis Examples Simple ligands are given above dichlorobisethylenediaminecobaltlll CoNH2CH2CH2NH22Clz trisbipyridineironll FeC5H4N7C5H4N3Z 7 The prefixes cis and trans designate adjacent and opposite geometric locations Examples are given in the figure below Other prefixes are used as well and will be introduced as needed in the text Examples cis and transrdiamminedichloroplatinumll PtClzNH32 cis and transrtetraamminedichlorocobaltIll CoClgNH34 H3N C H3N C NH3 CI H N CI H N NH P PK 3 clo 3 clo 3 H3N CI 0 NH3 H3N CI H3N NH3 NH3 CI cisi and transedichlorodiammineplatinumII cisi and transedichlorotetraamminecobaltlll PtC12NH32l C0C12NH34l 8 Bridging ligands between two metal ions as in the figure below have the prefix pi Examples tristetraammineeuedihydroxocobaltcobalt6 CoCoNH34OH236 ueamidoeuehydroxobis tetraamminecobalt 4 NH34C0OHNH2C0NH34l4 Chemistry 165 1875 NH3 H3 NII 0C6 N H3 NH NH NH3 H HO l NH3 H3N 3 H I 3 NH3 OH H3NIII O IIl Co CO H3N H2 NH3 H3N 0 OOH l I NH H HO NH3 NH3 3 100 NH3 0 H3N I NH3 NH3 tristetraammineipidihydroxocobaltcobalt6 p amid p J quot u u 39 4 C0C0NH34OH23l6 NH34C0OHNH2C0NH34l4 Demonstration A solution of C02 in water can be prepared by dissolving various Co salts in water The solution is pink due to the presence of the octahedral ion CoHZO62 Addition of chloride Cl in the form of concentrated HCl forms the blue tetrachloride dianion CoH2062 4 c1 2 C0C142 GHZO The equilibrium can also be shifted to the left by addition of solid ZnSO4 since Zn2 binds Cl more strongly than Coz Alfred Werner pioneered the study of coordination complexes in the 189039s Among other things he studied four different compounds of Colll chloride with ammonia Compound 1 CoCl 3 6NH3 Orangeeyellow Compound 2 CoCl 3 5NH3 Purple Compound 3 CoCl 3 4NH3 Green Compound 4 CoCl 3 3NH3 Green Treating these compounds with aqueous hydrochloric acid did not remove the ammonia suggesting that it was somehow closely bound with the cobalt ions Treatment with aqueous silver nitrate at 0 C on the other hand gave interesting results With compound 1 all of the chloride present precipitated as solid AgCl With compound 2 only two thirds of the chloride precipitated and with compound 3 only one third Compound 4 did not react at all with the silver nitrate Werner accounted for these facts by positing the existence of coordination complexes with six ligands either chloride ions or ammonia molecules attached to each C03 ion Specifically he wrote the formulas for compounds 1 to 4 as Compound 1 CoNHs613 Cos Compound 2 CoNH35ClZ Cl 2 Compound 3 C0NH34C12 Cl Chemistry 165 1876 Compound 4 CoNH33C13 Only those chloride ions that were not ligands attached directly to cobalt were precipitated upon addition of cold aqueous silver nitrate Werner realized that his proposal gave predictions about the electrical conductivity of aqueous solutions of salts of these complex ions Compound 1 for example should have a molar conductivity close to that of AlNOg3 which also forms one 3 ion and three 17 ions per formula unit dissolved in water His experiments confirmed this resemblance and showed that compound 2 resembled MgNOg2 and that compound 3 resembled NaN03 in their conductivities Compound 4 behaved like a nonelectrolyte with a very low electrical conductivity as expected from the absence of ions in the formula for that compound Chemistry 165 1877 Structures of Coordination Complexes We will discuss structures first and then move on to consideration of the bonding theories to attempt to explain the observed structures The coordination number of the central metal in a complex is simply equal to the number of ligands attached Coordination numbers from Z to 12 are known Six is the most common We will focus on coordination numbers 4 5 and 6 in this class Early observations of fourecoordinate platinum ll complexes led to the surprising conclusion that two isomers exist for complexes such as PtNH32Clz This shows that their structure cannot be tetrahedral Werner suggested a square planar structure H3N CI H3N CI Pt Pt H3N CI CI NH3 cis trans Similar observations on sixecoordinate complexes such as Co NH34C12 showed that there are two isomers consistent with an octahedral geometry NH3 CI H N l CI H N I NH 3 CO 3 C 3 H3N C HgN ONH3 NH3 CI cis trans purple green An additional type of isomerism is noted for a complex closely related to cis CoNH 34C12 If the four NH3 ligands are replaced with two ethylenediamine ligands see 1813 we now have the possibility of forming two enantiomers of the complex N NC C Co Co Na l CI my l N N N J Imagine a m1rror plane here Chemistry 165 1878 More on Optical Isomers see p 673 in Oxtoby Complexes such as Pten34 and Feox33 ox oxalate can exist as two optical isomers All complexes of this general type are called tris chelate complexes and their structures can be represented schematically A A These two molecules are enantiomers Official nomenclature calls them lambda A and delta A Chemistry 165 1879 Formal Oxidation States of Metals in Coordination Complexes To figure out the oxidation state or oxidation number of the central metal atom in a complex is very important Proceed as follows identify the charges on the ligands look at the total charge on the molecule 3 charge of ligands formal oxidation state total charge eg FeCl42 has 4 Cl ligands and overall 2 charge so it must contain Fe 2 or Fell Electron Configurations A m Review section 158 pp 564568 For elements up to Argon Zl8 we fill the ls 2s 2p 3s and 3p levels in that order Filling these orbitals causes various changes to the energies of the higher and as yet unfilled orbitals Not all orbitals are affected equally by the screening of the nuclear charge provided by these s and p electrons In particular the 3d orbitals which penetrate the Ar core rather little are relatively unaffected In contrast the 4p level and especially the 4s level penetrate the core quite a bit and drop substantially in energy The next two electrons are added to the 4s level Potassium Zlg and Calcium Z20 result What level do we fill next Recall that we have increased the nuclear charge by Z and that the 4s orbitals are quite diffuse ie ineffective at screening the nuclear charge the 3d orbitals drop in energy and are filled next Thus Scandium Sc has the configuration Ar4s23d1 Titanium is Ar4s23d2 Chromium Cr is peculiar being Ar4s13d5 not Ar4s23d4 This is due to electroneelectron repulsion and results in higher stability for quothalfefilled shellsquot Chemistry 165 18 10 Principal quantum number B Complexes Atomic number In transition metal complexes the nd orbitals end up lower in energy than the n 1 s orbitals eg in Vanadium complexes V0 VI VII VIII VIV 3d5 3d4 3d3 3d2 3d1 not 4s23d3 as in free atoms


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