Class Note for CHM 218 with Professor Berger at IPFW
Class Note for CHM 218 with Professor Berger at IPFW
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191 Transition Metal and Coordination Chemistry Transition metal A metal which has a partially lled d subshell either in the neutral atom or in a common oxidation state These are generally hard and strong metals which conduct heat and electricity quite well They also tend to have high melting points These metals o en form colored and paramagnetic compounds due to their partially lled d subshells Coordination complex A metal ion or atom with one or more molecules or ions referred to as ligands joined formally by coordinate covalent bonds Coordination number The number of donor atoms directly bonded to the metal atom ion Coordination numbers of 4 and 6 are the most common and important Ligand A Lewis base ion or molecule covalently bonded to the metal atom or ion Coordination Sphere The metal center along with all the ligands bonded to it Types of Ligands l Monodentate Ligands literally meaning onetoothed are those ligands which coordinates through only one atom Some examples are F39 Cl39 H20 NH3 PCH33 etc 2 Polydentate Ligands are those ligands which coordinate through two or more donor atoms simultaneously These are also referred to as chelating ligands when the donor atoms are bonded to the same metal center Bidentate ligands coordinate through two donor atoms Some examples are 22 bipyridine 110phenanthroline ethylene diarnine and acetylacetonate O Q mowim HscbcHC E cm Q Q 22Lbjpyrjdjm bpy ethylenediamjm en acetylacetonate acac 110plemnhrolim phen 192 Tridentate ligands coordinate through three donor atoms Some examples are 22 6 2quot terpyridine and diethylenetliamine CHz CHZ 39 2 H CHZ CH I Hsz k 22396392quot terpyridine tlpy diethylenetriamjm dien Tetradentate ligands coordinate through four donor atoms Some examples are macrocycles such as the valious porphyrins and phthalocyanines porphyn39n phthalocyanine Hexadentate ligands coordinate through II II O c CH2 CHz O O s1x donor atoms A very common example is ethylenediaminetetraacetate N CHZ CHZ N 27 EDTA O cH CHz C O ethylenediamjnetetraac etate Ambidentate ligands are those ligands which can coordinate through either of two diiTerent donor atoms Some common examples are the thiocyanate anion which can coordinate through either the S or N atom and the nitrite ion which can coordinate through either the O or N atom g CEN 39 o o SCN39 thiocyanatoi S NOZ39 ninitoiN formerly called nitro NCS39 thiocyanatoiN ONO39 nitritoio Some Common Ligands Common name uoro F39 chloroC139 bromo Br iodo I cyano CN thiocyano SCN isothiocyano NCS hydroxo OH aqua HZO carbonyl CO thiocarbonyl CS nitrosyl NO nitro NOZ nitrito ONO39 phosphine PR3 pyridine CSHSN ammine NH3 cthylenediamine N39HZ CH2 CH2 NHZ diethylinetriamine N39HZ CZH4NHCZH4N39HZ triethylenetetramine N39HZ CZH4NHCZH4NHCZH4N39HZ 3 3 triaminotriethylamine NC2H4NH23 IUPAC name uoro chloro bromo iodo cyano thiocyanatoS Sbonded thiocyanatoN N bondcd hydroxo aqua carbonyl thiocarbonyl nitrosyl nitritoN N bonded nitritoO Obonded phosphane pyridine ammine 12ethanediamine 1 47triazaheptane 22 diaminodiethy1amine 14710tetraazadecane 3 3 tris2aminoethy1amine Abbreviation F Cl39 CN SCN NCS OH HZO co cs No No ONO39 PR Py Cl l dien trien tren 193 Common name acetylacetonato CH3COCHCOCH339 2239 bipyridine csH4NicsH4N 110phenanthIoline CanNz 2 26 2quotterpyridine dialky 1 dithiocarbamate S ZCNRZ 12bisdipheny1phosphinocthane th PCZH4PPhZ 0pheny1enebisdimethy1arsine C6H4ASCH3 Z Z dimethylglyoxime ethylenediaminetetraacetate 139UPAC name 24 pentanediono 22 bipyridy1 1 10diaminophenanthrene dialkylcarbamodithioate 12ethanediy1bisdipheny1pho sphane 12pheny1enebisdimethy1arsane butanediene dioxime 12ethanediy1dinitrilotetraac etate 194 Abbreviation acac bpy bipy phen FY th dtc dppe diars DMG ED TA CH3 CH3 c c OH N N OH DMG 0 oc CH2 N CHZ CHZ N oc CH2 ED TA 0 MefS SMe 391 Me Me Ph diaIs i CHZ CO s R AH I CHZ CO s R dtc ll 0 195 The Chelate Effect It is generally the case that formation constants for complexes involving chelating ligands are substantially greater than those for complexes involving monodentate ligands For example we can compare the formation constant of CuNH342 with that of Cuen22 The latter is 75 orders of magnitude higher CuH20412 aq 4NH3aq e C11IIH34l2aqJr 4H20a 10991259 CuH20412 aq zen aq e Cuen22aq 4H20a IogKf2003 A larger Kf re ects a more negative value of AG which could result from a more negative AHO or a more positive AS RTan AGO AHO TASO There is little dilTerence in the values of AH0 for the two reactions so the difference must lie in the values of AS The reaction of NH3 with CuH2042 involves no change in the number of molecules in solution and is therefore to a rst approximation entropy neutral The reaction of en with CuH2042 however leads to an increase in the number of molecules in solution and therefore has a positive value of AS The chelate effect is therefore a re ection of the more favorable entropy change accompanying the formation of a chelate complex relative to that of a complex with monodentate ligands The effect is even more pronounced when ligands such as EDTA form complexes and these complexes have very large formation constants EDTA in fact is used as an analytical reagent for tittating metal ions Isomerism 196 Coordination complexes or compounds may have the same formula but different structures These di erent forms are referred to as isomers Co ordination Comp ounds Structural Isomers Stere ois omers different bonds identical bonds Linkage C oordination Sphere Coordination Geometric Optical Isomers Isomers Isomers Isomers Isomers Ionization Hydration Isomers Isomers Stereoisomerism Geometric Isomerism Geometric isomers typically exhibit distinct chemical and physical properties Geometric isomerism does not occur in tetrahedral complexes however it does occur in square planar and octahedral complexes oistrans isomerism occurs in square planar complexes of the general formula MXZYZ and octahedral complexes of the formula MX2Y4 As in organic chemistry the cis designation refers to the like ligands being adjacent to each other while the trans designation refers to the like ligands being opposite each other tetraaquadhhlorochromimn Ill diamminedicl oroplatinmn H CI CI NH3 HC OHZ CI CI P1 quot C CI P1 quot NH3 HZO CHHZ HZO CF39 OH2 H3N H3N HZO HZO CI 01 12 trans 0139 s trans 0139 s 197 Facial and meridional fac and mer isomerism occurs in octahedral X X complexes of the formula MX3Y3 The I 39X I IX fac designation refers to the like ligands MW occupy adjacent comers of a triangular I I face of the octahedron all at 900 to y X each other while the mer designation refers to the like ligands occupying fac mel coplanar sites with two of them at 1800 with each other Optical Isomer39ism Molecules which are nonsuperimposable on their mirror images are optical isomers enantiomers These molecules rotate plane polarized light in equal but opposite directions They have identical chemical properties and reactivities except in a chiral environment Optical isomerism occurs when a molecule lacks an improper axis of rotation including a plane of symmetry and a center of inversion Therefore square planar molecules will not exhibit optical isomerism but octahedral and tetrahedral molecules can In the case of octahedral enantiomers the molecules are designated as A or A depending on their absolute con gurations X X N I N quot39 39 quot39 z MI y i y IMZ N M N N M z l I Z N I I I NJ y l y N i N I I mirror plane mirror plane A i A N i N l l M39 I i 3 39C39D D B EN7AN mirror plane mirror plane 198 Structural Isomerism Linlmge Isomerism Linlmge isomerism can occur when a complex contains an ambidentate ligand such as SCN39 or NOZ39 The complex ion CoNH35N022 illustrates this type of isomerism The complex ion with the N bonded nitrite ion is yellow in color while the ion with the O bonded nitrite ion is red in color 0 2 N OTO 2 3 NH3 NH3 H3N7Co NH3 H3N Co NH3 H3N H3N NH3 NH3 yellow red It should be pointed out that although such isomerism is always possible when an ambidentate ligand is present it is not always the case that both isomers are isolable In some instances a particular metal center will prefer to bond to one donor atom or the other For example SCN39 typically bonds to so metal ions using the S atom and to harder metal ions using the N atom Coordination Sphere Isomerism Coordination sphere isomerism includes ionization isomerism and hydration isomerism Ionization Isomerism Ionization isomers give di erent ions when dissolved in solution Two di erent compounds with the formula CoNH34C12N02 exist One of them yields a precipitate of AgCl when silver nitrate is added to a freshly prepared solution while the other does not A Cl ion within the coordination sphere will not be precipitated while one outside the coordination sphere will be Therefore the compound that yields the precipitate must have its Cl ions within the coordination sphere while the other has one outside the coordination sphere coNH34CI21 N02 s coNH34CI21 aq N0239aq corNH34cIN021cI s E corNH34cIN021 aq C39oq 199 Hydration Isomerism Hydration isomerism is similar to ionization isomerism but it is water molecules that may be inside or outside the coordination sphere For example there are three diiTerent compounds with the empirical formula CrCl3 39 6 H20 One of these is Violet in color loses no water over sul lric acid and all of its chloride is precipitated from a freshly prepared solution From this data we can determine that the water OH2 3 molecules are all within the coordination sphere and all of II 0H2 the chloride ions are outside the coordination sphere H20 Cr LOH2 Therefore this coordination compound is H20 I 0H2 CrH2053 3 or or CrH205 013 The second is green in color loses 16 of its water over sul lric acid and b of its chloride is precipitated from a freshly prepared solution From this data we can determine that one of the water Cl 2 molecules is outside the coordination sphere and only two IIIIOH2 of the three chloride ions are outside the coordination H20 Crquotquot OH2 sphere Therefore this coordination compound is H20 0H2 CrC1H2052 2 cr H20 or CrC1H205C12 H20 The third is also green in color loses a of its water over sul lric acid and a of its chloride is precipitated from a freshly prepared solution From this data we can determine that two of the water CF I 0H2 molecules are outside the coordination sphere and only H20Cr quot39 0H2 one of the three chloride ions are outside the coordination H sphere Therefore this coordination compound is 2 CI CrC12H204 cr 2 H20 or CrC12H204C1 2 H20 Incidentally the Cl ions in the coordination sphere are trans to each other 1910 Coordination Isomerism Coordination isomerism not to be confused with coordination sphere isomerism occurs when both the cation and the anion of a coordination compound are complex ions In this case the distribution of ligands in the cation and anion may vary For example each of the following is a pair of coordination isomers C0mNH36lC1mCN6l and CIHINH3s C0mCN6l Crul NH3s C111 ISCNkl and C1111 NH34SCN2 CIJJI NH32SCN4 PtHNH34PtIVC6 and PtWNH34012lPtll Ch Nomenclature of Coordination Compounds 1 The names of neutral coordination complexes are written without any spaces Ionic coordination compounds are written as two words with the cation first and the anion second In naming a coordination complex or a complex ion the names of the ligands listed alphabetically are given first followed by the name of the metal and nally the oxidation state of the metal or the charge on the complex is given last A The number of ligands of one kind is given by the following pre xes Ifthe ligand name includes one or more of the these pre xes or is otherwise complicated it is set OH in parentheses and the second set of pre xes is used 2 di bis 3 tri His 4 tetra tetrakis 5 penta pentakis 6 hexa hexakis 7 hepta heptakis 8 octa octakis 9 nona nonakis l 0 deca decakis B Ligands are named in alphabetical order according to the name of the ligand not the pre x although exceptions to this rule are common An earlier rule gave anionic ligands first then neutral ligands each listed alphabetically 1911 C Anionic ligands are given an o suf x Neutral ligands retain their usual name with some exceptions Neutral ligands are usually given the same name as the uncoordinated molecule but with spaces omitted Speci c examples are CH3ZS O dimethylsulfoxide DMSO NH22CO urea C 5H5N pyridine terpy terpyridine bpy 22 bipyridine 02 sulfurdioxide N2 dinitrogen O2 dioxygen PC13 trichlorophosphine PPh3 triphenylphosphine OPCH33 trimethylphosphineoxide POCH33 trimethylphosphite There are however some neutral molecules which when serving as ligands are given special names These are NH3 ammine H20 aqua NO nitrosyl CO carbonyl CS thiocarbonyl Anionic ligands are given names that end in the letter quot0quot When the name of the free uncoordinated anion ends in latequot the ligand name is changed to end in quotatoquot Some examples are CH3C0239 acetate acetato 804239 sulfate sulfato CO3239 carbonate carbonato acac acetylacetonate acetylacetonato When the name of the free uncoordinated anion ends in quotidequot the ligand name is changed to end in quotidoquot Some examples aie N339 nitride nitrido N339 azide azido NHZ39 amide amido H39 hydride hydrido When the name of the free uncoordinated anion ends in quotitequot the ligand name is changed to end in quotitoquot Some examples are NO nitrite nitrito 803239 sul te sulfrto 1912 Certain anionic ligands ale given special names all ending in quot0quot CN39 cyanide cyano F39 uoride uoro Cl chloride chloro Br bromide bromo I39 iodide iodo 0239 oxide oxo 02239 peroxide peroxo 0239 superoxide superoxo OH39 hydroxide hydroxo CH3O39 methoxide methoxo Organic groups although implicitly considered to be anions are given their regular names without an quotoquot ending Some examples are CH3 Me methyl C2H5 Et ethyl C3H7 Pr propyl C5H5 Ph phenyl 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 a er the name of the coordination sphere This is the more common convention although there are cases where it is di icult to assign oxidation numbers B The EwensBassett system puts the charge on the coordination sphere in parentheses a er the name of the coordination sphere This convention is used by Chemical Abstracts and olTeIs an unambiguous identi cation of the species In either case if the charge is negative the suf x ate is added to the name of the coordination sphere In some cases the suf x is merely added to the name of the metal cobalt Co cobaltate bismuth Bi bismuthate zinc Zn zincate nickel Ni nickelate In some cases the ending of the name of the metal such as um or ium is dropped before the suf x is added chromium Cr chromate ruthenium Ru ruthenate osmium Os osmate Ihenium Re rhenate platinum Pt platinate vanadium V vanadate In some cases more than just the um or ium ending or an ending other than um or ium is dropped before the suf x is added manganese Mn manganate molybdenum Mo molybdate tungsten W tungstate 1913 In some instances the name of the anionic complex is based on the Latin name of the element iron Fe ferrate lead Pb plumbate silver Ag argentate tin Sn stannate antimony Sb stibate gold Au aurate copper Cu cuprate 4 The pre xes cis or trans and fac and mer designate adjacent and opposite geometric locations The pre xes fac and mer also distinguish geometric isomers 5 Bridging ligands between two metal ions have the pre x pi Examples potassium hexacyanoferrateIII K3FeCN5 potassium hexacyanoferrate3 hexaamminecobaltIII chloride CoN H 35C13 hexaamminecobalt3 chloride dichlorobisethylenediaminecobaltIH Coen2C12 dichlorobisethylenediaminecobalt1 tn39Sbipy diIIeiI0nH FebPY32 trisbipyridineiron2 hexacarbonylmolybdenum0 MoCO5 sodium amminetrichloroplatinateII NaPtC l 3NH3 sodium amminetrichloroplatinate2 cz39s tetraamminedichlorocobaltIII cis CoC 12N H3 4 cz39s tetraamminedichlorocobalt1 mer trichlorotristriphenylphosphinerutheniumIII mer RuPPh33Cl3 mer trichlorotristriphenylphosphineruthenium0 1914 Crystal Field Theory Crystal Field Theory is a simple yet surprisingly effective model for understanding the bonding and associated electronic and magnetic properties of transition metal complexes The presence of partially lled d orbitals leads to properties of transition metal complexes not normally observed in main group compounds Among these are pararnagnetism visible absorption spectra and irregular structural and thermodynamic properties A word of caution Crystal Field Theory is simply a model and not a realistic description of bonding in transition metal complexes Consider a free gaseous metal ion M39 with a single d electron In the absence of any perturbation the 5 d orbitals are degenerate that is they are of equal energy Therefore there is an equal probability of the electron being in any of the ve orbitals Now suppose a spherically symmetric shell of negative charge approaches the metal ion The orbitals will be destabilized that is raised in energy However because there is spherically symmetric distribution of charge they will all be destabilized equally However in an octahedral complex the charge is not distributed symmetrically but rather we can assume that they constitute a set of point charges approaching from the vertices of an octahedron Now the effect on the ve d orbitals is not the same Those orbitals which have lobes pointing directly toward the vertices of the octahedron will be destabilized Since the distribution of the charge does not alter the total energy the remaining orbitals those not pointing directly toward the comers of the octahedron will be stabilized 1915 XY The metal ion now has two kinds of d orbitals In an octahedral ligand eld the dxy dXZ and dyz orbitals are known collectively as the t2g orbitals while the C122 and dxzyz orbitals are referred to as the eg orbitals The splitting between the tzg and eg orbitals is referred to as A0 where the 0 stands for octahedral or lODq The magnitude of this splitting depends on several factors that we will address shortly 1916 The center of gravity of the ligands known as the baricenter is maintained In other words the sum of the energies of the tzg and eg orbitals relative to the baricenter is 0 2x6Dq3x4DqO e T 9 35 A0 6 Dq l baricenter 1 quot4 A0 10 Dq l 25A 4Dq l I v I 29 II I Iquot d Free metal ion Metal ion in spherically Metal ion in symmetric field octahedral field 1917 The splitting of the d orbitals will depend on the arrangement of the ligands around the metal center In general the splitting observed for square planar and tetrahedral ligand arrangements will dilTer from that of an octahedral arrangement A square planar ligand arrangement J 2 2 can be obtained from an octahedral x 39 X 39 Y ligand arrangement by withdrawing the ligands along the zaxis This should x give us a clue as to the appropriate 6 T A splitting pattern in a square planar 9 xx 1 ligand eld Orbitals which have lobal T xx density along the z axis will be quot dxy stabilized while those having lobal X density along the x and y axes will be Ao xxquot destabilized The result is the splitting l diagram shown l29 dzz dYZ dXZ octahedral Tefragonally square dis ror red planar octahedral Now consider what happens when the ligands approach the metal from the vertices of a tetrahedron Now the dxy dXZ and dyz orbitals are destabilized while the dzz and dxzyz orbitals are stabilized Therefore the tetrahedral splitting pattem is the exact opposite of the octahedral splitting pattern The orbitals which were labeled t2g in the octahedral case are labeled t2 in the tetrahedral case and the eg orbitals in an octahedral ligand eld are e orbitals in a tetrahedral ligand eld because a tetrahedron lacls a center of T symmetry The splitting between the e and t2 orbital sets is also referred to as A but is designated as At where the t represents tetrahedral For the same metal ligands and metalligand distance A 1 it can be shown that At 49 A l 1918 Crystal Field Theory allows us to explain many properties which ale unique to transition metal complexes Spectral and magnetic properties are use ll in characterizing coordination complexes Magnetic Properties of Transition Metal Complexes For any paramagnetic species the most fundamental question is how many unpaired electrons are there Our discussion of orbital splitting patterns is the fust step in addressing this question Hund s First Rule If a set of n or fewer electrons n occupy a set of n degenerate orbitals the electrons will spread themselves out among the orbitals to give n unpaired spins Electron pain39ng is energetically unfavorable Furthermore if two electrons are forced into the same orbital there is an additional unfavorable energy contribution because of e39 e e39 repulsions Consider a hypothetical molecule with two electrons and two orbitals separated in energy by AE We can consider two likely orbital occupations a and b Whether the system adopts con guration a or b depends only on the relative magnitudes of AE and P If AE lt P a will have the lower total energy and will correspond to the ground state but if AE gt P b will have the lower total energy and will correspond to the ground state EEOEOAE E2EOP 2EO AE We can use the same type of argument for 1919 octahedral complexes using the appropriate dorbital splitting pattem For 1 d1 d2 d3 d8 d9 and d10 ions there is no question as to the orbital occupation However for the d4 d5 d6 and d7 con gurations two possibilities eXistand the question of which represents the lower energy state can be answered by comparing A0 with the average pairing energy F We can calculate the energy of each con guration in terms of A0 and P relative to the baricenter Keep in mind that the t2g orbitals are 04Ao relative to the baricenter and the eg orbitals are 06Ao relative to the baricenter Therefore each electron in a tzg orbital contributes are 04A0 to the total energy while each electron in a eg orbital contributes 06A0 to the total energy Each pair of electrons in the same orbital contributes P to the total energy Ehigh spin Elow spin d4 706A0 716A0 P d5 0 72A0 2 P d6 704A0 P 724A0 3 P d7 708A02P 718A03P From any of the cases we may nd the conditions under which the two con gurations have the same energy The simplest case is d5 however the result is identical for any d electron count E11 2Ao 0 Elms 2A0 2P A0 P 1LT 1920 Therefore the spin state of any ion in an octahedral ligand eld depends only on whether the magnitude of the eld as measured by A0 is greater than or less than the pairing energy P for that ion Ion gt P we have a low spin complex Ion lt P we have a high spin complex spin state Con guration Ion P Ligands A0 predicted observed d4 Cr 23500 6 H20 13900 high high Mn 28000 6 H20 21000 high high d5 Mn 25500 6 H20 7800 high high Fe 30000 6 H20 13700 high high 6 H20 10400 high high Fe 2 17 600 d6 6 CN 33000 low low 6 F 13000 high high Co3 21 000 6 NH3 23000 low low d7 Colt 22500 6 H20 9300 high high EI giesincnr1 8359 curl 1 m1 high spin low spin Tetrahedral Complexes 1 The same reasoning applies to tetrahedral complexes For d1 1 J39LdsL d2 d7 d8 d9 and d10 ions only one con guration is possible For d3 d4 d5 and d6 ions both high and low spin states are possible in principle and a low spin state would be expected if t 1 1 T At gt P However keep in mind that At 49 A0 all else being the same In reality At is almost never greater than P so low d4 d4 spin tetrahedral complexes are exceedingly rare 1 f f t LL lLll d5 d5 l 1 l 1 l LLL LLLL d6 d6 Square Planar Complexes Low spin d8 states are not possible in octahedral complexes but tetragonal distortion of an octahedral complex may cause suf cient splitting of the eg orbitals so that the splitting exceeds the pairing energy Now the two highest energy d orbitals are no longer degenerate and separated by some energy Q In this case we can have either high spin or low spin complexes depending on the relative magnitudes of Q and P If Q gt P we have a low spin complex If Q lt P we have a high spin complex In the case of extreme tetragonal distortion we have 4 0 5 0 lt N if dyz d high spin PgtQ XZ 1921 the splitting diagram for a square planar complex Due to the large separation of the two highest orbitals and the relatively small pairing energies of real d8 ions e g RhI IrI NiH Ptu Pd1 and Aum the high spin con guration is impossible to obtain Therefore all square planar 18 complexes are diamagnetic Factors In uencing Crystal Field Splittings The d orbital splittings depend on several factors including the identity of the metal its oxidation state the number of ligands and their geometry and nally the identity of the ligands 1 Identity of the metal A values for corresponding complexes of metal ions in the same group and in the same oxidations state increase by about 40 to 50 on going from the rst to the second transition series and by about 20 to 25 from the second to the third transition series This trend is illustrated for the hexaammine complexes of the group 9 metals in the 3 oxidation state There are two important consequences for complexes of the second and third transition series First these complexes o en have A values greater than the pairing energy so these complexes are nearly always low spin Secondly because the splittings are so large absorption bands o en occur in the UV making these complexes colorless dX2y2 I ii ltley l dZ 2 ii d yz d X 2 Complex A0 CoNH363 23000 cm 1 RhN39H363 34000 cm 1 IrNH363 41000 cm 1 1922 2 Oxidation state of the metal A values increase with increasing oxidation state of the metal all else being the same For example A0 values of complexes of the rst transition series are about 7500 to 14000 cm391 for divalent 2 ions while those for trivalent 3 ions are about 14000 to 25000 cm39l 3 The number and geometry of the ligands The number of ligands in uence the magnitude of the crystal eld splitting In general a larger number of ligands leads to a greater splitting As we have already noted a tetrahedral ligand eld results in a splitting of only 49 that of an octahedral ligand eld However we must be care ll because there is also a geometric factor More relevant is the fact that a cubic ligand eld leads to twice the splitting of a tetrahedral ligand eld all else equal 4 Identity of the ligands The dependence of A values on the identity of the ligands follows a regular order known as the Spectrochemical Series This series is based on data for metal ions in common oxidation states and sometimes the order may not be consistent for unusual oxidation states Even in cases of metal ions in common oxidation states inversions of the orders of adjacent or nearly adjacent ligands may occur The Spectrochemical Series The size of the splitting depends on the nature of the ligands surrounding the metal center Ligands with large concentrations of electronic charge lead to large splittings large A values while those with lower charge densities lead to smaller splittings Additionally we can consider the ability of a ligand to be either a 139Idonor or 11 acceptor ligand Ligands which can serve as ndonors tend to cause smaller splittings while those that can serve as nacceptors cause large splittings I39 lt Br lt SZ39 ltSCN39 S bonded lt Cl39 lt NO339 lt F39 lt OH39 lt oxalate C2042 ltHZO ltNCS39 N bonded lt CH3CN ltpy lt NH3 lt en lt bpy lt phen lt N N bonded lt phosphines lt CN39 lt CO Ligands near the beginning of the series cause smaller splittings than those near the end As we shall see in addition to having important consequences for the magnetic properties of complexes ligands also in uence their spectral properties 19 23 clyslal Field Stabililztinn Energy Recall that ln an octahedral llgand eld the a g orbltals are slablllzedby 0 4 Aquot and me eg 69 orbltals are destablllzedby 0 s Aquot A d2 len T would therefore expelenee atotal 35 A0 6 Dq slablllzanen of 0 8 Aquot 8 D lelanye to a hypollneneal d1 len Whats the spllmng didnot occur Consldenng typlcal values of Aquot mime V 7500 to 25000 elnquotllnls addmonal stablllzauon corresponds to about 90 to 300 kJmol Tnls slablllzanon ls r med to as the 25 A 4 Dq clyslal Field Stabililztinn Energy ems Resultsfm lllgn 5pm octahedral complexes are gven ln the table Thegreater V 29 the value of CFSE the more slable acomplEX wlllbe Thlshas nunsmomnamcanplms lmponant consequences for hydration enulalples andlamce chE enthalples aw 04 a ADq my 02 A Man dam l2 a non d d3 I 6 A 6 Dq a elm n n Hydxztinn Enlllalples Hydranon enlnalples of divalmtlons ofthe rsttxansmon senes aeule enthalpy changes assoclated wllll lle followlng geneallzed equanon M g 6 H20 1 MHzOl aq Values may be ealelllaledllslng Lharmodynamlc eyeles as we have already seen Ewe plonlne hydranon Enthalplesfor the divalent lons Ca to Zn we ndthatthe pomts forthe lons Ca 01quotan d5 andznz39 am all lle on a smooth eulye These lonshave 0 CFSE Thermalmnglonshave larga39 neganye hydration enthalples However when the CPSE for eaell lon ls subtracted mm the aemal hydranon enthalpy they all fall on the same eulye 19 23 clyslal Field Stabililztinn Energy Recall that ln an octahedral llgand eld the a g orbltals are slablllzedby 0 4 Aquot and me eg 69 orbltals are destablllzedby 0 s Aquot A d2 len T would therefore expelenee atotal 35 A0 6 Dq slablllzanen of 0 8 Aquot 8 D lelanye to a hypollneneal d1 len Whats the spllmng didnot occur Consldenng typlcal values of Aquot mime V 7500 to 25000 elnquotllnls addmonal stablllzauon corresponds to about 90 to 300 kJmol Tnls slablllzanon ls r med to as the 25 A 4 Dq clyslal Field Stabililztinn Energy ems Resultsfm lllgn 5pm octahedral complexes are gven ln the table Thegreater V 29 the value of CFSE the more slable acomplEX wlllbe Thlshas nunsmomnamcanplms lmponant consequences for hydration enulalples andlamce chE enthalples aw 04 a ADq my 02 A Man dam l2 a non d d3 I 6 A 6 Dq a elm n n Hydxztinn Enlllalples Hydranon enlnalples of divalmtlons ofthe rsttxansmon senes aeule enthalpy changes assoclated wllll lle followlng geneallzed equanon M g 6 H20 1 MHzOl aq Values may be ealelllaledllslng Lharmodynamlc eyeles as we have already seen Ewe plonlne hydranon Enthalplesfor the divalent lons Ca to Zn we ndthatthe pomts forthe lons Ca 01quotan d5 andznz39 am all lle on a smooth eulye These lonshave 0 CFSE Thermalmnglonshave larga39 neganye hydration enthalples However when the CPSE for eaell lon ls subtracted mm the aemal hydranon enthalpy they all fall on the same eulye 1924 Spine Structures Recall that spinels are mixed oxides of the general formula le M32 02394 In a normal spinel all of 2 ions occupy tetrahedral holes while the 3 ions occupy octahedral holes In an inverse spinel the 2 ions occupy octahedral holes while half of the 3 ions occupy tetrahedral holes and the half octahedral holes We can rationalize some of the spinel structures by considering the CFSE of various ions Magnetite Fe3O4 adopts the inverse spinel Few d6 structure while Mn o4 adopts the normal spinel structure High spin d5 F e3 d5 ions exhibit 0 CFSE so there is no energetic advantage for them to occupy the octahedral holes where splittings and therefore CFSEs will MH2 d5 be larger Therefore in Mn3O4 the Mn ions will preferentially occupy Mn3 d 4 the octahedral holes where there is larger CFSE giving rise to the normal spinel In Fe3O4 the Fe2 ions will preferentially occupy octahedral holes giving the inverse spinel structure More on Magnetic Properties How do we determine the number of unpaired electrons All paramagnetic species are attracted by a magnetic eld while diarnagnetic ones are weakly repelled The pararnagnetism of a substance containing unpaired electrons receives a contribution from the orbital motion of the unpaired electrons as well as from its their spins O en for lighter substances the spin contribution is predominant and we can neglect the orbital contribution The magnetic susceptibility of a substance is a measure of the force exerted by a magnetic eld on a unit mass of the specimen This quantity is related to the number of unpaired electrons per unit mass and therefore per mole of the substance We can measure XM the magnetic susceptibility per mole and correct it for diamagnetic effects which are always present to get XMcoquot the corrected molar magnetic susceptibility XMwquot can be correlated with u the magnetic moment it 284 lxjjrrT From quantum theory it can be shown that the magnetic moment due to the spins of n unpaired electrons is given by u g JS S 1 where S the sum of the spins of the unpaired electrons S n x 12 and g is the gyromagnetic ratio for a free electron 20023 Inn N 5 Male webs V l V l 73 l 771 8 Cu 1 V l 73 V3 2 l 2 83 2 672 8 N1 2 l 2 83 2 874 I 01339 3 32 3 87 3 8 Cuz 3 32 3 87 4175 2 Fez 4 2 4 9D 5 175 S Cuz 4 2 4 9D S 4 Mn 5 52 S 92 S 9 Fe 5 52 S 92 S 9 Electrnnic Ahsnrptinn Spectrnscnpy Consider an octahedral complEX m wnh a single deleemm example TxHzO 3 a if Absorption ofenergy Wm lead to the exmanm of the electron from one of the is was to oneoftheegorbnzls Thxs anagy39xstypxcally T m the form of electromagqu radAauon 3929 iii speci cally visible hm The snag of a a photon Ts gweh by the product ofPlanck39s Constant s 626 x 10W 5 andxts frequmcy E Eh 1 In genemlwhmh 1tpasses through a solunon1 smtansnyxs dammshed The mm of the mtensty of the transmitted by I to the Tummy of the mndenthth a he the hensmmanee T T 110 A o The absorbanceA sthenegame log othe hinsmmance I 710 Trlo 7 A g 31 p 19 25 UL 19 26 The BeerrLambenLaW or sunply Beer39s Lawtells us hat he absorbanee ls dreeuy proporuonal to he eoneentrauon ofthe absorblng peeles and he hght39s pah lengh hrough he absorblng medlum A Ebc where 6 molar absorptwlty M ernquot b ahlengh Em e oneentrauon ofabsorblng specles M 400500 700 10339 t t Ifwe plot he absorbanee of a substanee as a funeuon of wavelenth or frequency ofthe lnndent llght we have an absorpuon spectmm The gure shows he 5 absorpuon speetrurn of TlH20r3 Two features are lrnportant he wavelength or frequency of he mahmum and he absorbanee or rnolar absorpuwty 0 35 30 25 20 IS lo Frequency curl A 104 In ths slrnple d ease we ean look at he Energy where he absoxptwlty has lts mammum and deduee hat he splltung ofhe t1g and eg orbltals m TlaTzohr39 ls about 20000 ernquot When agvmlon has rnore d eleetrons he sltuauonls rnore eornplleated The eolor hat we pereewe a soluuon to be depends on what eolors wavelengths ofvlslble llght are absorbed 1n he ease of Traizohrz nearly all ofthe vlslble llght 4007700 nrn ls absorbed exeept for sorne vrolet llght between 400 and 420 nm As a result th5 vrolet llght ls transrnltted andhe soluuon appears vlolet The human eye also uuhzes eornplernentary eolors ln eolorvrslon 650 m 580 m andpereewes arnurture of two eornplernentary eolors as whrte hght If a eertarn eolor ofllgnt ls removedfmm whrte llght by absorpuon by a sample 7 0 111 he sarnple wlll have he eornplernentary m 100 nm 560 um 430 um 490 um 1927 The absorption band for TiH2053 is very weak with a molar absorptiVity of about 5 M391 cm39 1 Frequently electronic transitions that are allowed have molar absorptiVities of 104 to 105 M 39 1 cm39 1 This suggests that the transition in TiH2053 is somehow forbidden For an electron transition to be allowed in a system that has a center of symmetry it is necessary but not sufficient that the electron move from a g orbital to a u orbital or Vice versa This is known as the LaPorte Selection Rule The t2g and eg orbitals are all g so such a transition is forbidden All dd transitions in symmetric octahedral complexes are forbidden so the colors of these complexes in solution are fairly pale In tetrahedral systems there is no center of symmetry and so the LaPorte Selection Rule does not apply although dd transitions are still forbidden by the orbital selection rule Solutions of tetrahedral complexes are frequently much more intensely colored than those of octahedral complexes cdeotr M 6 04 2 6H20 palepink chepbue 1928 Molecular Orbital Theory Crystal Field Theory is in many ways incomplete although it does account nicely for the splitting pattems observed in the d orbitals of transition metal complexes However Molecular Orbital theory offers a more complete picture We will consider an octahedral complex MXG where each ligand has only a O orbital to be used in metal ligand bonding The can make a linear combination of the six 0 orbitals to give a set of 6 group orbitals each of these group orbitals has the appropriate symmetry to overlap with a metal s p or d atomic orbital resulting in a total of 6 bonding and 6 antibonding molecular orbitals Three of the metal d orbitals tzg do not have the proper symmetry to mix with any of the group orbitals so these will be nonbonding with respect to metalligand O bonding Three bonding and three antibonding molecular orbitals derived from the metal p orbitals and the appropriate group orbitals are degenerate and these are denoted t1 and tluquot Two bonding and two antibonding molecular orbitals derived from the metal 122 and dxzyz orbitals and the appropriate group orbitals are degenerate and these are denoted eg and egquot Finally the s atomic orbital and the appropriate group orbital give rise to a nondegenerate bonding and antibonding molecular orbital denoted a1g and algquot 25 mum 19 29 19 30 Now we have the sarne qualltauve results as erystal eldtheory gave us regarding the mllmng ofthe d orbltals tn an oetahedral llgandfleld M 0 Theory however shows explmdy how the Orbondmg tn the complex takes plaee that ls by the forrnataon of 6 two eleetron bonds Anothelrnportantresnltls that we ean seethat the e orbltals are not purely metal orbrtals anol that they are aetually annrbondmg wlth respeet to metalrllgand Orbondmg Thls explatns why populanon of sorcalled old or llgandfleld exerted states often leads to llgand dlssoclanon We can also generallze the MO treatment by allowlng the llgands to possess 7E orbltals These 7E orbltals rnay oyerlap wlth the elm ola or dy rnetal orbltals as shown Now lnsteaol of havmg only one set oft1S orbltals whlch are pure rnetal d orbltals we have abmdmg anol annr bonding 53L Hg anol 11g 1 99 569 The posmons ofthe t1S anol t1 orbltals tn the M o diagam are vanable There are two lrnportant eases to conslder Case 1 The ngandls an donor Examplesmclude x H20 OH39 anol other llgands near the begnmng ofthe speetroehernleal sanes The ngand 7E orbltals are lled anol lower tn energy than the metal t1S orbrtals When they lntenaetwlth the rnetal t1S orbltals the etfeet ls to rase the energy ofthe rnetal t1S orbltals anol oleerease the spllmng between thet1S anol e orbltals A nrdonorllgands aretherefore weak eld ngands T A m the absence uf l n lntereetlnns 1931 Case 2 The ligand is a nacceptor Examples include CO CN39 and phosphines In this case the ligand TI orbitals are vacant 139 molecular orbitals The effect of their interaction with the metal tzg orbitals is to decrease the energy of the metal tzg and increase the splitting between the t2g and egquot orbitals A Therefore TI acceptor ligands are strong eld ligands I 13929 I I I I t 139 I e9 1 T29 I A in The absence of I39 A n inferag rions I I I 3d 39 I Eg T2g 13929 Iquot I39
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