coordination chemistry

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COORDINAT ION CHEMISTRY II BONDING, INCLUDING CRYSTAL FIELD THEORY AND LIGAND FIELD THEORY BASIS FOR BONDING THEORIES Models for the bonding in transition metal complexes must be consistent with observed behavior Specific data used include stability (or formation) constants, magnetic susceptibility, and the electronic (UV/Vis) spectra of the complexes BONDING APPROACHES Valence Bond theory provides the hybridization for octahedral complexes For the first row transition metals, the hybridization can be: d2sp3 (using the 3d, 4s and 4p orbitals), or sp3d2 (using the 4s, 4p and 4d orbitals) The valence bond approach isn’t used because it fails to explain the electronic spectra and magnetic moments of most complexes CRYSTAL FIELD THEORY In crystal field theory, the electron pairs on the ligands are viewed as point negative charges that interact with the d orbitals on the central metal The nature of the ligand and the tendency toward covalent bonding is ignored D ORBITALS CRYSTAL FIELD THEORY Ligands, viewed as point charges, at the corners of an octahedron affect the various d orbitals differently CRYSTAL FIELD THEORY CRYSTAL FIELD THEORY The repulsion between ligand lone pairs and the d orbitals on the metal results in a splitting of the energy of the d orbitals D ORBITAL SPLITTING e g dz2 dx2-y2 0.6∆o Spherical field 0.4∆o dxy t 2g dxz dyz Octahedral field ∆o TETRAHEDRAL COMPLEXES The size of the splitting, ∆T, is considerable smaller than with comparable octahedral complexes This is because only bonds are formed, and the metal orbitals used in bonding don’t point right at the ligands as they in octahedral complexes TETRAHEDRAL COMPLEXES In general, ∆T ≈ 4/9 ∆o Since the splitting is smaller, all tetrahedral complexes are weak-field, high-spin cases TETRAGONAL COMPLEXES Six coordinate complexes, notably those of Cu2+, distort from octahedral geometry One such distortion is called tetragonal distortion, in which the bonds along one axis elongate, with compression of the bond distances along the other two axes TETRAGONAL COMPLEXES The elongation along the z axis causes the d orbitals with density along the axis to drop in energy As a result, the dxz and dyz orbitals lower in energy TETRAGONAL COMPLEXES The compression along the x and y axis causes orbitals with density along these axes to increase in energy TETRAGONAL COMPLEXES For complexes with 1-3 electrons in the eg set of orbitals, this type of tetragonal distortion may lower the energy of the complex SQUARE PLANAR COMPLEXES For complexes with electrons in the eg set of orbitals, a d8 configuration, a severe distortion may occur, resulting in a 4-coordinate square planar shape, with the ligands along the z axis no longer bonded to the metal SQUARE PLANAR COMPLEXES Square planar complexes are quite common for the d8 metals in the 4th and 5th periods: Rh(I), IR(I), Pt(II), Pd(II) and Au(III) The lower transition metals have large ligand field stabalization energies, favoring four-coordinate complexes SQUARE PLANAR COMPLEXES Square planar complexes are rare for the 3rd period metals Ni(II) generally forms tetrahedral complexes Only with very strong ligands such as CN-, is square planar geometry seen with Ni(II) SQUARE PLANAR COMPLEXES The value of ∆sp for a given metal, ligands and bond length is approximately 1.3(∆o) THE JAHN-TELLER EFFECT If the ground electronic configuration of a non-linear complex is orbitally degenerate, the complex will distort so as to remove the degeneracy and achieve a lower energy THE JAHN-TELLER EFFECT The Jahn-Teller effect predicts which structures will distort It does not predict the nature or extent of the distortion The effect is most often seen when the orbital degneracy is in the orbitals that point directly towards the ligands THE JAHN-TELLER EFFECT In octahedral complexes, the effect is most pronounced in high spin d4, low spin d7 and d9 configurations, as the degeneracy occurs in the eg set of orbitals d4 d7 d9 eg t2g THE JAHN-TELLER EFFECT The strength of the Jahn-Teller effect is tabulated below: (w=weak, s=strong) # e- 10 High * * * s - w w * spin Low w w - w w - s spin * * s - *There is only possible ground state configuration - No Jahn-Teller distortion is expected EXPERIMENTAL EVIDENCE OF LFSE d1 d2 d3 d4 d5 d6 d7 LFSE 4Δo 1.2 d8 d9 d10 1.2
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