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Coordination complexes-bonding and magnetism.pdf

  1. 1. Coordination Complexes: Bonding & Magnetism Dr. Anjali Devi J S Assistant Professor (Contract Faculty), Mahatma Gandhi University, Kerala
  2. 2. Bonding in coordination compounds • Werner’s theory- primary secondary valence- Alfred Werner • Valence Bond Theory (Linus Pauling in 1930s) • Crystal field theory (Hans Bethe in 1929) • Ligand Field Theory • Molecular orbital theory
  3. 3. 1. Valence Bond Theory Assumptions 1. Formation of a complex involves reaction between Lewis bases (ligands) and Lewis acid (central metal atom or metal ion) with the formation of coordinate covalent (or dative) bonds between them . 2. The model utilizes hybridization of metal s, p, and d, valence orbitals to account for the structure and magnetic properties of complexes. Fe NH3
  4. 4. Coordination Complex geometry • Complex geometry can be linked to orbital hybridization. Coordination number Geometry Hybrid orbitals 2 Linear sp 4 Tetrahedral sp3 4 Square planar dsp2 6 Octahedral d2sp3 or sp3d2
  5. 5. Demonstration: Structure of Ni(II) complexes –[Ni(Cl)4]2-
  6. 6. Structure of Ni(II) complexes –[Ni(CN)4]2-
  7. 7. Valence Bond Theory-Magnetism And Diamagnetic And Paramagnetic
  8. 8. Strong & Weak field Ligands
  9. 9. Describe the bonding in (a) Ni(NH3)6]2+, (b) Pd(NH3)6]2+ and (c) Pt(NH3)6]2+ with valence bond theory. Question
  10. 10. 2. Crystal field theory • Ligand lone pair is modelled as a point negative charge (or as the partial charge of an electric dipole) that repels electrons in the d orbitals of central metal ion. • The resulting splitting of the d orbitals into groups with different energies , and uses that splitting to rationalize and correlate the optical spectra, thermodynamic stability, and magnetic properties of complexes. Purely electrostatic interaction
  11. 11. d orbitals
  12. 12. Crystal field splitting
  13. 13. Crystal Field Splitting
  14. 14. Crystal field theory • In the presence of an octahedral crystal field, d orbitals are split into a lower energy triply degenerate set (t2g) and a higher energy doubly degenerate set (eg) separated by an energy Δo; the ligand field splitting parameter increases along a spectrochemical series of ligands and varies under the identity and charge of the metal atom.
  15. 15. Crystal field theory
  16. 16. Crystal Field Theory (CFT)
  17. 17. • The ligand field strength depends on ligand (spectrochemical series) • The ligand field strength depends on identity of central metal atom. • The values of Δo increases with increase in oxidation state (compare Co spexcies and Fe species). • And Δo increases down the group (see Co, Rh and Ir) Mn2+ < Ni2+<Co2+ <Fe2+<V2+<Fe3+> Co3+ <Mo3+< Rh3+ <Ru3+<Pd4+< Ir3+ <Pt4+ Factors affecting crystal field splitting parameter, Δo
  18. 18. Crystal field stabilization energy (CFSE) In the d1 case: t2g 1 It has an energy of -0.4 Δo relative to the barycenter of the d orbital. For d2 :t2g 2 The electron obey Hund’s rule and occupy different degenerate t2g orbitals, which has an energy of -0.4 Δo relative to the barycenter of the d orbital. System Configuratio n CFSE d1 t2g 1 0.4 Δo d2 t2g 2 0.8 Δo d3 t2g 3 1.2 Δo
  19. 19. In the d4 case: (1) For Δo< pairing energy(P) { weak field or high spin condition} t2g 3eg 1 CFSE= (3X+0.4 Δo) –(1X+).6 Δo )=0.6 Δo ] relative to the barycenter of the d orbital. (2) For Δo> pairing energy(P) { strong field or low spin condition} t2g 4eg 0 Crystal field stabilization energy (CFSE)
  20. 20. • Determine the CFSE for the following octahedral ion: (a) d3 (b) High spin d5 (c) Low spin d6 (d) d9 Question (a)1.2 Δo (b)0 Answer (c)2.4 Δo-2P (d) 0.6Δo
  21. 21. Crystal field stabilization energy of high spin octahedral complexes dn Example N (high spin complexes) CFSE/ Δo d0 Sc3+ 0 0 d1 Ti3+ 1 0.4 d2 V3+ 2 0.8 d3 Cr3+ 3 1.2 d4 Cr2+ 4 0.6 d5 Mn2+, Fe3+ 5 0 d6 Fe2+ 6 0.4
  22. 22. Crystal field stabilization energy of low spin octahedral complexes dn Example N (high spin complexes) CFSE/ Δo d4 Cr2+ 2 1.6-P d5 Fe3+, Mn2+ 1 2.0 -2P d6 Fe2+ 0 2.4-2P d7 Co2+ 1 1.8-P
  23. 23. Six negative charges arranged octahedrally around a central metal ion- Visual
  24. 24. Tetrahedral arrangement of four negative charges around a cation - Visual ∆𝑡 = 4 9 ∆𝑜
  25. 25. Energy level diagram showing splitting of a set of d orbitals by octahedral and tetrahedral crystal field.
  26. 26. Tetragonally distorted Octahedral complex Octahedral array of ligands becomes progressively distorted by the withdrawal of two trans ligands, especially those lying on the z axis. For a square pyramidal (spy) set of ligands, the splitting diagram has to be qualitatively similar to tat of square set.
  27. 27. Trigonal bi pyramidal complex • The tbp has D3h symmetry. • Taking 3-fold axis as z axis, • dz2, • dxy, dx2-y2 • dxz, dyz
  28. 28. Magnetic Properties
  29. 29. Bohr magnetons • The magnetic moments of atoms, ions, and molecules are expressed in units called Bohr magnetons (B.M.) • 1 𝐵. 𝑀. = 𝑒ℎ 4𝜋𝑚𝑐
  30. 30. Magnetic moment of electron • The magnetic moment 𝜇𝑠 of a singe electron is given by the equation, 𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= g 𝑠(𝑠 + 1)
  31. 31. Question • For a free electron, g has the value 2.00023 which may be taken as 2.00 for most purpose. Find spin magnetic moment of one electron. • 𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= g 𝑠(𝑠 + 1) • Answer: 𝜇𝑠 (𝑖𝑛 𝐵. 𝑀. )= 2 1 2 ( 1 2 + 1) = • 3 = 1.73
  32. 32. Magnetic moment of Metal ions-Special Case • MnII , FeIIIand GdIII (the ions whose ground states are S states) : There is no orbital angular momentum even in the free ion. There cannot be any orbital contribution to the magnetic moment. The observed magnetic moments agrees well with spin only values.
  33. 33. The transition metal ion with in their ground state D, or F being most common, do possess orbital angular momentum. 𝜇𝑆+𝐿 = g 4𝑆 𝑆 + 1 + 𝐿(𝐿 + 1) Magnetic moment of First Series Transition Metal ions
  34. 34. The observed values of 𝜇 frequently exceeds 𝜇S but seldom are as high as 𝜇S+L • Because, the metal ions on its compounds restricts orbital motion of the electrons so tha the orbital angular momentum are wholly or partially quenched.
  35. 35. Temperature independent paramagnetism (TIP) • In many systems that contain unpaired electrons, as well as in a few, eg, CrO4 2-, that do not , weak paramagnetism that is independent of temperature can arise by a coupling of the ground state of the system with excited state of high energy under the influence of the magnetic field. • This TIP resembles diamagnetism in that it is not due to any magnetic dipole existing in the molecule but is induced when the substance is placed in the magnetic field • It also resembles diamagnetism in its order of magnitude 0-500 x 10-6 cgs units per mole
  36. 36. High spin Low spin crossovers • Spin crossover , sometimes referred to as spin transition or spin equilibrium behavior, is a phenomenon that occurs in some metal complexes wherein spin state of the complex changes due to external stimuli such as variation of temperature, pressure, light irradiation or influence of magnetic field.
  37. 37. Spin Crossover High Spin Low Spin
  38. 38. Spin crossovers • This phenomenon is commonly observed with some first row transition metal complexes with a d4-d7 electron configuration in octahedral ligand geometry.
  39. 39. ∆= 𝑃 High spin and low spin states have same energy High spin and low spin states can coexist in equilibrium Spin state equilibrium Spin crossovers
  40. 40. Thank You