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©. Coordination Compounds 12th Science 12th- Chemistry Syllabus 1. Introduction 2. Types of ligands 2.1 Monodentate ligands 2.2 Polydentate ligands 2.3 Ambidentate ligand 3. Terms used in coordination chemistry 3. Coordination sphere 3.2 Charge number of complex ion and oxidation state of metal ion 3.3 Coordination number (C.N.) of central metal ion 3.4 Double salt and coordination complex 3.5 Werner theory of coordination complexes 4. Classification of complexes 4.1 Classification on the basis of types of ligands 4.2 Classification on the basis of charge on the complex 5. IUPAC nomenclature of coordination compounds 6. Effective Atomic Number (EAN) Rule 7. lsomerism in coordination compounds 7A Stereoisomers 7.2 Structural isomers (Constitutional isomers) 8. Stability of the coordination compounds 8.1 Factors which govern stability of the complex g. Theories of bonding in complexes g.1 Valence bond theory (VBT) 9.2 Octahedral, complexes g.3 Tetrahedral complex 9.4 Square planar complex g.5 Limitations of VBT 9.6 Crystal Field theory (CFT) 9.7 Factors affecting Crystal Field Splitting parameter (Ao) g.8 Colour of the octahedral complexes 9.9 Tetrahedral complexes 10. Applications of coordination compounds “Theory Notes ©. Define simple salt and double salt Q. Define the terms- a) Central metal atom/ion b) Coordination compound Q. What are coordination compounds? What is coordi- | The compound which conti nation chemistry? Q. Explain, why F Simple Salt Simple salt is a crystalline compound that dissociates in water producing jons and shows properties © f individual constituents. Ex. NaCl, K2SOxete. Double Salt Double salt is a crystalline compound containing more than one salt that dissociates in water producing jons and shows properties of individual constituents. Ex. KeSO;-Al:($Os);.24H:0, Mohr’s salt (FeSO, (NHi),S0.-6H20) Central metal atom, /ion The metal atom orion incoordination comp by many oppositely charged ions or neutral molecules coordinate bonds is called central metal atom or jon. ound which is surrounded (ligands) by Coordination Compounds ainsacentral metalion or atom surrounded by number of oppositely charged ions or neutral molecules is called as coordination compound or complex. The pond between central metal ion (or atom) and surrounding ions (or neutral molecules) is coordinate bond. On dissociation it does not show the properties of individual constituent. It dissociate in water with at least one complex ion. Ex. In complex [cu(NHs),]", copper is the central metal ion and ammonia is surrounding neutral molecules. Ans. solution of Fe(NH,),(SO.)p(H20), gives positive test for e(NH,),(SO.)p(H20), isa | 1) Aqueous. Fe”, NHi and 50%. This shows that given salt dissociates into salt but Ki[Fe(CN)s] és a complex compound. Q. Give postulates of Werner theory of coordination complexes. constituent ions. Hence it is a salt. 2) Aqueous solution of Ky [Fe(CN)s] gives positive test for K* ions but negative tests for Fe” and CN_ ions. ‘This shows that given salt does not dissociate into constituent ions. Hence it is a complex compound. It forms K* and Fe(CN),| ions in water. Postulates of Werner Theory of Coordination Complexes Postulate (i) The metal in a complex has two types of valencies i.e. primary valency and secondary valency. Postulate (ii) Primary valencies are generally satisfied by anions and are enclosed in ionizable sphere. Postulate (iii) Secondary valencies are satisfied by anions or neutral ligands and are enclosed in non ionizable coordination sphere. Number of secondary valencies is equal to the coordination number. Postulate (iv) The secondary va’ the metal ion. Jencies have a fixed spatial arrangement around Q. Define the term ligand. Ligands i See iain the term ligand. 1) The species surrounding the central metal atom or lon are calleq (Mr. 16, Mark 1) ligands. They are neutral molecules or negatively charged (rarely positively charged) ions bonded to central metal atom (or ion) jn coordination compound by coordinate bonds. — 2) They satisfy secondary valency of central metal ion. 3) In Greek, ‘ligane’ mean ‘to bind’. 4) Ex. In complex [Co(NHs .|Cls, the ligands are neutral NH, molecules. 5) Nature of some ligands is given below- Negative CN cyanide, NO: nitrate, OH” hydroxide, ions NOs nitrate, F- fluoride, Cl chloride, Br bromide, O” oxide. Positive NO" nitrosylium, NoH;' hydrazinium ions Neutral CO carbon monoxide, NHs ammonia, HO molecules | water, NH»OHhydroxylamine, © CHs.NH. methylamine. 8. Give the classification of | Classification of Ligands ligands. Ligands are classified on the basis of number of donor atoms in it. i) Mono or Unidentate Ligand 1) whe ligand having only one donor atom is known as monodentate ligand. 2) It coordinates with metal ion at only one site in the complex. 3) Ex. NHs,Cl,OH ,H,0 ete. ii) Poly or Multidentate Ligand 1) The ligand having more than one donor atoms is known as polydentate ligand. 2) It coordinates with metal jon at more than one sites in the complex. 3) Polydentate ligands are further classified on the basis of number of points of attachments as below- a) Bidentate ligand- It has two donor atoms. Ex. Ethylene diamine (en) b) Tridentate ligand-It has three donor atoms. Ex, Diethylene triamine (dien) c) Tetradentate ligand- It has four donor atoms. Ex. Triethylene tetramine (trien) d) Hexadentate ligand- It has six donor atoms. Ex. Ethylene diamine tetra acetato (EDTA) _— CH2COO" —~ CH,COO™ _— CH:COO” “~ CH:COO" H.C —— N HCc—— N iii) Ambidentate ligand ; 1) The ligand having two or more donor atoms but it uses only one donor atom to attach with central metal atom (or ion) during complex formation is known as ambidentate ligand. 2) Ex. CN” has two donor atoms C and N. It uses only one donor atom to attach with metal as M-CNorM-NC. I EE ee"... Q. Explain the following terms related with coordina- tion compounds- i) Coordination number ii) Oxidation number iti) Complex ron iv) Coordination entity Q. Define the term coordina- tion entity. Q. Define the term Homoleptic and heteroleptic complexes. i) Coordination Number (CN) - 1) Coordination Number (CN) of the central metal atom (or ion) in a complex is the number of monodentate ligands directly bonded to it. Ex. in K,[Fe(CN)], six cyanide ligands are bonded to Fe”. hence CN of Fe” is 6. 2) Metals have CN form 2 to 10 where 4 and 6 are very common coordination numbers. 3) In case of polydentate ligands, CN is the number of electron pairs involved in bonding between the metal and the ligands. Ex. in [cu(en),[* four electron pairs are involved in bonding. Hence CN of Cu” is 4. 4) Coordination number of metal depends upon- i) Charge on the metal ion ii) Charge on the ligand iii) Relative sizes of metal ion and the ligands iv) Fores of repulsion between the ligands ii) Oxidation Number of Central Metal Ion . The charge present on the central metal ion after removing all the ligands along with shared electron pairs is called oxidation number (ON) of central metal ion. It is shown by Roman number in parenthesis. Ex, in [Fe(CN),|” oxidation number is +3 and it is written as Fe(III). iii) Complex Ion The charged aggregate formed between a metal ion and ligands is known as complex ion. Ex. [Cu(NH,),] iv) coordination Entity The central metal ion (or atom) along with bonded ligands is known as coordination entity. It may be charged or neutral. Ex. (Coch NHa)is neutral coordination entity while [Fe(CN),]" is charged coordination entity. Homoleptic Complex Complex in which central metal is attached to only one type of ligands is known as homoleptic complex. Ex. Co(NH)),|” is homo- leptic complex because central metal ion Co” is attached to only one type of ligands ie. NHs Heteroleptic Complex Complex in which central metal is attached to more than one type of ligands is known heteroleptic complex. Ex. [Co(NH3), Cl] is heter- oleptic complex because central metal ion Co* is attached to two type of ligands ie. NHs and Cl’. Q. Define cationic, anionic and neutral complexes. Q. Explain cationic complexes and anionic complexes of coordination compounds. (Oct. 15, Mark 2) Cationic Complexes The complex in, which the complex ion carries positive charge is called as cationic complex. Ex. [Co(NH)),|Cls,[Fe(H20),|Cl,[Ni(NH3),|Che ete [Co(NHs),JCls = [Co(NHs),}" + 3CP [Fe(H20),|Cl. = [Fe(H20),)" + 3CP [Ni(NH)|Cl. = [Ni(NH3).” + 21° _ Q. What is effective atomic number? Q. Explain IUPAC nomencla- ture of mono nuclear coordi- nation compounds. Q. Write the rules which are applied while writing the formulas of coordination compounds. Q. Explain the impromptu of principles of additive nomenclature. Effective Atomic Number (EAN) Effective atomic number is the total numbe central metal ion present in a complex. It is calculated by using the following formula- EAN=Z-X+Y r of electrons around the Where 'Z! ig atomic number of the central metal. 'x! js the number of electrons Jost during the formation of the metal ion from its atom 'y! ig the number of electro Ex. In [Fe(CN)]" , Atomic number of hate c Number of electrons lost Total number of electrons =. BAN = Z-X+Y =26-2412=36 ns donated by ligands entral metal (Z = 26), during formation of Fe" (X = 2) donated by six cyanide ligands (¥ = 12) EAN Rule ‘A metal ion continuous to accept electro: its Effective Atomic Number becomes equ the next inert (noble) gas. Ex, In frecnn EAN is 36 which is the atomic number of rare gas krypton. Note i) EAN rule is useful to fin: around a metal ion. ii) Some complexes are excep mn pairs from the ligands till al to the atomic number of d the number of possible ligands tions to EAN rule. IUPAC Nomenclature of Coordination Compounds 1) Name of cation is named first followed by the name of the anion. 2) While naming complex entity, names of ligands are written first followed by name of the central metal. Name of the complex is one single word without any space. 3) Oxidation state of the metal ion is written next to the metal ion in Roman number within the parenthesis. There should be no space between metal name and parentheses. 4) Name of central metal in complex anion ends with suffix-ate. The metal in cationic or neutral complex is specified by its usual name. Metal Name of metal in anionic complex Al Aluminate Cr Chromate Co Cobaltate Cu Curprate Au Aurate Fe Ferrrate Pb Plumbate Mn Mangenate Mo Molybdate Ni Nickelate Ag Argentate Sn Stannate Zn Zincate 5) Names of anionic ligands are obtained by changing the ending -ide to -O and -ate to -ato while names of neutral and cationic ligands are written as it is. (except aqua for H,O, ammine for NHz, carbonyl for CO and nitrosyl for NO) mapas tion compounds I——,-_—_—«| Bromo |——7a- Chloro —~ yO CN Cyano OH Hydroxo — QO Oxo “tor Carbonato Ethylene ; Ethylene diamminetetracetate diamminetetraacetato NO; Nitro C,0} Oxalato NH; Ammine ca Carbonyl H,0 Aqua Ethylammine Ethylammine 6) If more than one different ligands are present, they are written 7 8 ) in alphabetical order. If more than one same etc. is used. If more than one ligand present, prefix bis-, tris-, ligand is written in bracket. ligand are present, prefix di-, tri-,tetra- having numerical prefix in its name are tetrakis- etc. is used. Name of such Complex IUPAC name i, Anionic complexes : a. [Ni(CN),J° b.[Co(C20,),° c. [Fe(CN),f? Tetracyanonickelate(II) ion Trioxalatocobaltate(III) ion Hexacyanoferrate(II) ion ii. Compounds containing complex anions and metal cations: a. Nas[Co(NO;),] b. Ks[Al(C:04),] c. Nas [AIF] Sodium hexanitrocobaltate(III) Potasium trioxalatoaluminate(III) Sodium hexafluoroaluminate(III) iii. Cationic complexe a, Cu(NH,),” b. [Fe(H20),(NCS)f° c. [Pt(en), (SCN),]” Tetraamminecopper(I]) ion Pentaaquaisothiocyanatoiron(III) ion, Bis(ethylenediamine) dithiocyanatoplatinum(IV). iv. Compounds containing complex cations and anion ‘ a.[PtBr:(NHs),|Bre b.[Co(NHs),COs]Cl c.[Co(H.0)(NHs),|Is Tetraamminedibromoplatinum|IV) bromide, Pentaamminecarbonatocobalt(III) chloride, Pentaammineaquacobalt(III) iodide. v. Neutral complexes : a. [Co(NO.), (NHs),] b. Fe(CO), c. [Rh(NH), (SCN),] Triamminetrinitrocobalt(III) Pentacarbonyliron(0) Triamminetrithiocyanatorhodium(I) Q. Draw the geometrical isomers of [Co(en),Cl.] ‘ai _ i: Zi mpounds ii) Six Coordination oe th CN 6 are octahedral in. sha dination Compo cs otanedall complexes of type Mac and MAsB do not show geome, rical isomerism. Cc exes of type MA.B» . | ‘t monodentate ligands B are adjacent to each other, it is catieg cis-isomer and if ligands B are opposite to each other, it is caljeg trans-isomer. " Ex. Cis and trans isomers of complex [CoCl.(NH»)] are shown below- - —* al ‘ [aye fa | "A ___, Ni, NY | \y | LON LZ LN HN Lh NH, Cis AN ‘NH, Fig. Geometrical isomers of [CoCl, | | | | ral Trans (NH.), b) Complexes of type MA.BC Ex. [Pt(NHs),CIBr]” 2 NH. cl Cle} NH NH, | _NH Pt in Br H NH,~—1— NH NH, * ° Br Cis ‘Trans c) Complexes of type M(AA),B: Complex in which central metal is attached to two symmetrical bidentate ligands (AA) and two monodentate ligands B shows cis and trans isomers. Ex. Cis and trans isomers of complex [CoCl.(en)," are shown below- + cl cl AIA LLAIA VY Co. a) oy, AS Loe a Cis Trans Fig: Geometrical isomers of [CoC1,(en),] 2) Optical Isomerism The compound which has clement of symmetry (plane of symmetry or line of symmetry or point of symmetry) is achiral and is optically inactive. The compound having no element of symmetry possesses chirality. It has non superimposable mirror images. Such compounds can rotate the plane of plane polarized light to clockwise or anticlock- wise direction. They are called optical isomers and this phenom enon is known as optical isomerism. The non superimposable mirror images of the same optically active compound are called enantiomers. The optical isomer that rotates the plane of plane polarized light to clockwise direction is calle d-isomer and the optical isomer that rotates the plane of plane polarized light to anticlockwise direction is called 1-isomer. 10 Q. Draw optical isomers of {Co(en),Cl] . Q. Explain following isom- erism in complexes - i) Jonisation isomerism ii) Linkage isomerism iii) Coordination isomerism iv) Hydrate isomerism | i) Four Coordination Compounds a) Tetrahedral complexes Tetrahedral complexes do not show optical isomerism. b) Square planar complexes In square planar complexes all the four ligands bonded the central metal lie in the same plane. Hence they possess a plane of symmetry or axis of symmetry and they are not chiral in nature. Therefore square planar complexes do not show optical isomerism. ii) Six Coordination Compounds Octahedral complexes containing one or more symmetrical bidentate ligands. i) Complexes of type M(AA), Ex. [Co(en),]" we en Co, “OO d 34 c + CA te Miror ii) Complexes of type M(AA),B. Octahedral complexes in which two symmetrical bidentate chelating ligands (AA)and two monodentate ligands B are coordinated to the central metal show optical isomerism. They can be resolved into d- and |-forms. Ex, Complex [CoCl.(en),| shows cis and trans isomers. Trans isomer is symmetrical and is optically inactive. Cis isomer is unsymmetrical and shows optical isomerism. It can be resolved into d- and 1-forms as shown below- + + cl + al am | [ “Lia | \ 7 | \ \ INL] | Co | \ Co \ g | Co | 8 cr AY No | L® % ‘| | cl d- form 1- form Mirror Fig. Optical isomers of Ic oCl,(en),}' i) lonisation Isomerism 1) Complexes having same molecular formula but different ions inside the outside the coordination sphere are called ionization isomers and this phenomenon is known as ionization isomerism. It is also called ion-ion exchange isomerism. 2) Ex. Complex |Co(NHs),SO Br and complex [Co(NH:),Br]SO, are ionization isomers of each other. In first complex SO* ion is present inside the coordination sphere and Br_ ion outside the coordination sphere while in second complex Br ion is present inside the coordination sphere and SO{ ion is outside the coor- dination sphere. Q. Explain the factor saffecting the stability of coordination compounds, Q. Give the salient features of valence bond theory. Q. Explain the magnetic properties of coordination compounds. Q. How is the geometry of complex determined? Factors affecting stability of coordination compounds Thermodynamic stability of coordination compound depends on following factors- i) Charge density of the central metal ion 1) Thermodynamic stability of coordination compound depends on charge density of the central metal ion. Greater the charge density more stable is the complex. Charge density is charge radius ratio. Hence higher magnitude of the charge and smaller size of ion makes the complex more stable. For the same magnitude of positive charge, the ion with smaller radius forms more stable complex, 2) The order of the stability of complexes formed with Cu".Ni*,Co", Fe”, Mn’ and Cd" is as below - Cu"! > Ni > Co” > Fe” > Mn’ > Cd" This is known as Irving William order of stability of M’ ions. ii) Nature of ligands Thermodynamic stability of coordination compound depends on the nature of ligands i.e. basic strength of the ligands. Greater the basic strength of the ligand, greater is the stability of the complex. Valance Bond Theory (VBT) Valence bond theory was developed by Linus Pauling to explain the formation of coordinate bonds between central metal and ligands in complex. Salient Features of Valence Bond Theory 1) Central metal atom or ion in complex has vacant s, P and d orbitals to form coordinate bonds with ligands. 2) The number of vacant orbitals in central metal is equal to its coordination number. 3) Vacant orbitals in central metal undergo hybridization to form equal number of hybrid orbitals. 4) Ligand has ion pair of electrons to form coordinate bonds with central metal. 5) Vacant orbitals of central metal overlaps with filled orbitals of ligand and form coordinate bonds. 6) Geometrical shape of complex depends upon the hybridization. 7) Greater the overlap between orbitals, stronger is the coordinate bond formed. 8) If (n-1) d orbitals are used for hybridization, the complexes are called inner complexes and if nd orbitals are used for hybridiza- tion, the complexes are called outer complexes. 9) The complexes in which central metal contains unpaired electrons are paramagnetic and the complexes in which central metal contains no unpaired electrons are diamagnetic. 10) Strong field ligands like NH,,CN etc. cause spin pairing i.e. pairing of electrons present in central metal. Geometry of Complex According to Valence Bond Theory, geometrical shape of complex depends upon the hybridization of vacant orbitals in central metal atom or ion. Complexes have different geometrical shapes according to the hybridization as follows- ee Q. Give the steps to under- Stand the metal - ligand bonding in complex. Q. Explain the structure of [Co(NH,),}* on the basis of valence bond theory. at => Hybridization Geometry sp hybridization Lane = a sp’ hybridization Tetrahe' - nar sp’ hybridization Square pla dsp’ hybridization Trigonal bypyramidal d’sp’ hybridization Octahedral | d’sp" hybridization Pentagonal bypyramidal a Steps to Understand the Metal - Ligand Bonding 1) Find oxidation state of central metal ion. 2) Write valence shell electronic configuration of metal ion. 3) Find whether the complex is low spin or high spin (For octahe dral complexes with d' to d° configuration) 4) Find the number of metal ion orbitals required for bonding. 5) Identify the orbitals of metal ion available for hybridisation ang the type of hybridisation involved. 6) Write the electronic configuration after hybridisation. 7) Show filling of orbitals after complex formation. : 8) Determine the number of unpaired electrons and predict magnetic behaviour. Structure of. |Co(NH),)" on the basis of Valence Bond Theory 1) [Co(NHs),)” is a cationic complex. Oxidation number of Co” is +3 and coordination number is 6. 2) In [Co(NH;),|” complex, central metal ion is Co” with atomic number 24. Its electronic configuration in ground state (GS) is [Ar]3d°4s°4p°. 3) The strong field ligands NH, promote two unpaired electrons from 3d orbital to other 3d orbital resulting in all the paired electrons. This process is known as spin pairing. Its electronic configuration in excited state (ES) is [Ar]3d°4s°4p’. 4) Coordination number is 6. Therefore two 3d orbitals, one 4s and three 4pvacant orbitals undergo d’sp’ hybridization (HS) and form six d’sp* hybrid orbitals directed towards the corners of octahedron. 5) Each vacant d’sp* hybrid orbital of central metal ion Co” overlaps with filled orbital of ligand NHsto form a coordinate bond. Thus there are six coordinate bonds. 6) Hexaamminecobalt(III) complex has no unpaired electron (n= 0). Hence it is diamagnetic. Box Diagram Co™ in GS (outer configuration) [iif 11111] J CT 3a i ap Co" in BS oon 4s 4p? Co" in HS ohh (Io Electronic configuration after coplex formation Q. Explain the structure of [NicL] on the basis of Valence Bond Theory. Q. Explain the structure of [Ni(CN), on the basis of Valence Bond Theory. Structure of [NiCl./ on the basis of Valene Bond Theory 1) [NiCl, is a anionic complex. Oxidation number of Ni” is 49 and coordination number is 4. . ce 2) In [NiCl,/ complex, central metal ion is Ni* with atomic number 26. Its electronic configuration in ground state (GS) js [Ar]3d*4s°4p°. . a 3) The weak field ligands Cl do not cause spin pairing. Its elec. tronic configuration in excited state (ES) is same as that in Gs, 4) Coordination number is 4. Therefore one 4s and three 4p vacant orbitals undergo sp’ hybridization (HS) and form four sp’ hybrid orbitals directed towards the corners ofa tetrahedron, | 5) Each vacant sp’ hybrid orbital of central metal ion Ni™ overlaps with filled orbital of ligand Cl to form a coordinate bond. Thus there are four coordinate bonds. . 6) Tetrachloridenickelate(II) complex has two unpaired electrons (n = 2), Hence it is paramagnetic. Box Diagram Ni” in GS (outer configuration) (ween O (10 4 2 ae Ni" in HS aa sp° hybrid orbitals Electronic configuration after complex formation 3d 4s 4p (lit 14] cl NE | Fig: Structure of (NiCL} Structure of [Ni(CN),]’ on the basis of Valence Bond Theory 1) [Ni(CN),} is a anionic complex. Oxidation number of Ni® is +2 and coordination number is 4. 2) In [Ni(CN),| complex, central metal ion is Ni® with atomic number 26. Its electronic configuration in ground state (GS) is [Ar]3d°4s°4p°. 3) The strong field ligands CN’ promote one unpaired electron from 3d orbital to other 3d orbital resulting in all the paired electrons. This process is known as spin pairing, Its electronic configura- tion in excited state (ES) is Tar 3d*4s°4p° 4) Coordination number is 4. Therefore one 3d orbital, one 48 and two 4p vacant orbitals undergo dsp’ hybridization (HS) and form four dsp’ hybrid orbitals directed towards the corners of a plane square. 5) Each vacant dsp’ hybrid orbital of central metal ion Ni” overlaps with filled orbital of ligand CN” to form a coordinate bond. Thus there are four coordinate bonds. Tetracyanonickelate(II) complex has no unpaired electrons (n= 0). Hence it is diamagnetic. Q. What are the limitations of Valence Bond Theory? Q. Write the salient features of crystal field theory (CFT). Box Diagram Ni* in GS (outer configuration) [Lif [1] O C1 Ni* ines ff] OT Ni in us iif] coo Electronic Configuration after complex formation CN CN \ \ \ SN \ \ we ~ | (ZN Fig: Structure of [Ni(CN),]” Limitations of Valence Bond Theory 1) 2) 3) 4) 5) It cannot explain the spectral properties (colours) of complexes. Some complexes show magnetic moment other than those calcu- Jated from number of unpaired electrons on the basic of VBT. It fails to explain inner and outer complexes of the same central metal. It cannot explain why some ligan strong field. Geometry of four coordination compounds (tetrahedral or square planar) cannot be predicted on the basis of VBT. ds are weak field and some are Salient Features of crystal Field Theory (CFT) 1) 2) 3) 4) 5) 6) In a complex, the central metal atom or ion is surrounded by ligands. tively charged ions or neutral molecules. Ligands are nega The central metal and ligands act as point charges and the inter- action between them is purely electrostatic. When the ligands approach the central metal, the degenerate d-orbitals in the central metal split into two groups tag and eg. The energy difference between these two groups is A or 10Dq. The electrons of the metal occupy two groups of d-orbitals according to Hund’s rule. It does not consider the overlapping between orbitals. Hence it rules out any covalent character of the complexes. The stability of the complex depends upon the magnitude of 7 crystal field stabilization energy. nan octahedral crystal ‘eld, draw the figure to splitting of d orbitals, fi show Q. Give the application of crystal field theory to tetra- hedral complexes. (My) ( Energy - Q) dey Average energy Splitting of ¢ orbitals, : of the d orbitals in in octahedral Free metal ion spherical erystal field crystal field Fog. Splitting of d orbitals in an octahedral crystal field. 5) The energy difference between tg and eg in octahedral complexes is Ao or 10Dq. The energy of tg is 0.4A. or 4Dq less than the energy of hypothetical degenerate d-orbitals and the energy of eg is 0.6A, or 6Dq more than the energy of degenerate d-orbitals. 6) Therefore each electron entering into bg orbital stabilizes the complex by 0.4A,or 4Dq and each electron entering into eg orbital destabilizes the complex by 0.6Ao or 6Dq. 7) The total gain of energy obtained by filling electrons in d-orbitals is called crystal field stabilization energy (CFSE) which deter- mines the stability of the complex. Application of Crystal Field Theory to Tetrahedral Complexes 1) In tetrahedral complexes, the metal atom or jon is at the centre of a regular tetrahedron and four ligands are at the corners of tetrahedron. L I. ligands approach the central metal, degeneracy of d-or- 2) Hie ie iost and crystal field splitting takes place. : 3) Out of five d-orbitals, dx°y” and dz’ are axial orbitals. Thy lie Ms between the axes of ligands. Hence they experience less m and acquire lower energy than degenerate d-orbitals eo dxz,dyzare planar orbitals. They are along the axes of ligands. Hence they experience more repulsion and acquire i than degenerate d-orbitals. oe foerystal field splitting, five degenerate d-orbitals split ee ps, higher energy group catdxy,dxz, dyz) and lower into two grou TO) ay? into treroupes (x Y42)- nore Hinn Camnannde Q. What is spectrochemical series? Energy day, de Average energy Splitting of d orbitals pote of the d orbitals in in tetrahedral spherical crystal field crystal field Fig: Splitting of d orbitals in an tetrahedral crystal field 5) The energy difference between t2g and eg in tetrahedral complexes is A, or 10Dq. The energy of tgis0.4A, or 4Dq more than the energy of hypothetical degenerate d-orbitals and the energy of eg is 0.6A, or 6Dq less than the energy of degenerate d-orbitals. 6) Therefore each electron entering into tg orbital destabilizes 7) the complex by 0.4A, or 4Dq and each electron entering into eg orbital stabilizes the complex by 0.6A, or 6Dq. The total gain of energy obtained by filling electrons in d-orbitals in called crystal field stabilization energy (CFSE) which deter- mines the stability of the complex. Spectrochemical Series 1) 2) 3) 4) 5) 6) 7) The series of ligands arranged in decreasing order of their strength is called as spectrochemical series. CO > CN > en > NH; > EDTA > NCS > H,0 > C,0% > OH >F >S? >Cl>SCN >Br>r Field strength is the capacity of the ligand to split d-orbitals of the central metal. Strong field ligands cause more splitting of d-or- bitals while weak field ligands cause less splitting of d-orbitals. Strong field ligands are those in which doner atoms are ¢, N or P. Thus CN ,NC ,CO,NHs, EDTA, en are strong ligands. They favour pairing of electrons and form low spin complexes. Weak field ligands are those in which donor atoms are halogens, oxygen or sulphur. Thus FCI ,Br,],SCN ,C,Oi are weak ligands. They form high spin complexes. For octahedral complexes, in case of d'\d’ andd’ the electrons occupy lower tg orbitals. But in case of d’,the fourth electron may occupy higher energy eg orbital or may pair up with an electron in tog orbital. This depends upon values of P and Ao. Ay is the energy gap between eg and tg orbitals. P is the pairing energy which is required to pair two electrons against elec- tron-electron repulsion in the same orbital. There are two cases of d‘ system i) When Ao> P, the fourth electron occupies t.g level and tends to pair. Low spin (LS), strong field or spin paired octahedral complex ion is obtained. ii) When 4,
