Factors affecting Crystal Field Splitting by Dr. Narinderjit Kaur

Factors affecting Crystal Field Splitting by Dr. Narinderjit Kaur

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  FACTORS AFFECTING CRYSTAL FIELD SPLITTING Dr.Narinderjit Kaur Assistant Professor PG Dept. OfChemistry Kanya Maha Vidyalaya, Jalandhar Factors affecting Crystal field Splitting Dr. Narinderjit Kaur Assistant professor PG Dept. of Chemistry Kanya Maha Vidyalaya, Jalandhar
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  Dr. Narinderjit Kaur 2 Whymagnitude of Crystal Field Splitting is different for different complexes?
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  The Magnitde ofΔo depend on following factors: • Nature of the ligand (Spectrochemical Series) • Oxidation state of the transition metal • Type of d-orbitals (transition metals) • Geometry of the Complex Dr. Narinderjit Kaur 3
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  1. Nature ofLigands • Greater the ease with which ligands can approach the metal ion, greater will be the CFS. • Complexes with weak field ligands have smaller Δo than complexes with strong field ligands. How to decide which ligand is weak field & which one is strong field? The ligands which cause small degree of CFS Δo are called weak field ligands while which cause large splitting are called strong field ligands. 4 Dr. Narinderjit Kaur
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  Spectrochemical series The arrangementof ligands in an increasing or decreasing order of CFS Δo is called spectrochemical series. This is an experimentally determined series. The order remains almost same for different metal ions. CO > CN - = C H > PR > NO - = phen > bipy > SO 2- > en = py = NH3 > edta 2 4 3 2 3 - - - > NCS- > H2O > C2O4 2- > OSO3 2- > OH- = ONO- > F- > Cl- > SCN > Br > I Decreasing Crystal Field Strong Weak In fact, it is difficult to explain the order as it incorporates the effects of both σ & π bonding. In general the variation in the splitting depends on number of other factors Such as: 5 Dr. Narinderjit Kaur
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  Other factors:  Smallligands can cause greater CFS.  Ligands containing polarizable electron pair will be drawn more closely to metal ion.  Ligands which can form multiple bonds such as CN- & CO cause greater CFS. CFS by CN- ligand is about double that for weak field ligands like halide ions, this is due to the π bonding in metal. Metal donates e’s filled t2g orbitals into vacant orbitals onligand.  Increasing σ donation occurs in the order: Halide donors < O donors < N donors < C donors 6 Dr. Narinderjit Kaur
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  Weak field ligands,such as Cl- and SCN- , give rise to small values for Δo, while strong field ligands, such as CN- and CO, give rise to large values forΔo. It is also possible to arrange the metals according to a spectrochemical series as well. The approximate order is: 7 Dr. Narinderjit Kaur Mn2+ < Ni2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mn3+ < Mo3+ < Rh3+ < Ru3+ < Pd4+ <Ir3+ < Pt4+
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  Which of thefollowing would have the largest value ofΔo? A. [Cr(H2O)6]3+ B. [CrF6]3- C. [Cr(NH3)6]3+ 8 Dr. Narinderjit Kaur
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  6 2 3For[Co(III)L ], Δ in cm-1: 13,100 (F- ); 20,760 (H O); 22,870 (NH ) For [Cr(III)L ], Δ in cm-1: 15,260 (F- ); 17,830 (H O); 26,280 (CN- ) 6 2 Complex ligand Donor atom Δ in cm-1 [CrCl6]3- Cl- Cl 13640 [CrF6]3- F- F 15260 [Cr(H2O)6]3+ H2O O 17830 [Cr(NH3)6]3+ NH3 N 21680 [Cr(en)3]3+ en N 21900 [Cr(CN- )6]3- CN- C 26280 9 Dr. Narinderjit Kaur
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  Why CFS isdifferent even if complexes have same metal ion and same ligands? Dr. Narinderjit Kaur 10
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  2. Oxidation Stateof Metal ion  Δo =10,200 cm-1 for[Co(NH3)6]2+  Δo =22,870 cm-1 for[Co(NH3)6]3+  Δo =32,200 cm-1 for [Fe(CN)6]4-  Δo =35,000 cm-1for[Fe(CN)6]3- Let’s take an example…… Metal ion with higher oxidation state causes largerΔo As the oxidation state of the transition metal (the charge on the metal) is increased, the surrounding ligands are attracted more closely to the metal centre. The orbitals on the ligands interact more strongly with the d orbitals andΔo increases. 11 Dr. Narinderjit Kaur
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  Which of thefollowing would have the largest value ofΔo? A. [Fe(H2O)6]3+ B. [Fe(H2O)6]2+ 12 Dr. Narinderjit Kaur
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  13 Dr. Narinderjit Kaur
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  3. Type ofd-orbitals (transition metals) -1 Δo = 22,870 cm (Co) Δo = 34,100 cm-1 (Rh) oΔ = 41,200 cm-1 (Ir) In groups, heavier analogues have larger Δ. For hexaammine complexes[MIII(NH3)6]3+ 14 Dr. Narinderjit Kaur
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  On going fromthe first to the second row of a transition metal triad, there is approximately 30 to 50% increase in the size of Δo, and another 30 to 50% increase on going from the second row to the third row. This is due to the fact that as you descend a transition metal triad the size of the d-orbitals increases (3d < 4d < 5d). The larger d-orbitals interact more with the orbitals on the ligands, hence Δo is larger. It should also be noted that the pairing energies for the larger d- orbitals are smaller, which means that low spin configurations are favoured 15 Dr. Narinderjit Kaur
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  4. Geometry ofComplex CFS energy of tetrahedral complexes (Δt) is nearly half the value for octahedral complexes (Δo) 𝟒 ∆ 𝒕 = 𝟗 ∆ 𝒐 Since magnitude of CFS in tetrahedral complex is quite small and is always less than pairing energy. 16 Dr. Narinderjit Kaur
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  Complex Oxidation state of metal GeometryΔ in cm-1 [Co(NH3)4]2+ II Tetrahedral 59,00 [Co(NH3)6]2+ II Octahedral 10,200 VCl4 IV Tetrahedral 7,900 [VCl6]2- IV Octahedral 15,400 Thus, all tetrahedral complexes are high spin complexes. 17 Dr. Narinderjit Kaur
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  Which of thefollowing would have the smallest value ofΔo? A. [Os(OH2)6]3+ B. [Fe(OH2)6]3+ C. [Ru(OH2)6]3+ 18 Dr. Narinderjit Kaur
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  19 Dr. Narinderjit Kaur