Chemistry of Transition Metals Part 2. Theories/ Concepts Bonding - - PowerPoint PPT Presentation

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Chemistry of Transition Metals Part 2. Theories/ Concepts Bonding - - PowerPoint PPT Presentation

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Chemistry of Transition Metals Part 2. Theories/ Concepts Bonding in transition metal compounds Theories : (i) Werner Coordination Theory (ii) 18 electron rule/ EAN (iii) Valence Bond Theory (iv) Crystal field theory (v) Molecular

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Chemistry of Transition Metals

Part 2. Theories/ Concepts

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Theories: (i) Werner Coordination Theory (ii) 18 electron rule/ EAN (iii) Valence Bond Theory (iv) Crystal field theory (v) Molecular orbital approach

Bonding in transition metal compounds

Consequences: (i) High spin - low spin complexes (ii) Spectrochemical series (iii) Crystal Field Stabilization Energy (CFSE) (iv) Jahn-Teller distortions (v) Spinels

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CFT- Octahedral Field

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dz2 dx2-y2 dxy dxz dyz

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dz2 dx2-y2 dxy dxz dyz

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dz2 dx2-y2 dxy dxz dyz

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dz2 dx2-y2 dxy dxz dyz

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dz2 dx2-y2 dxy dxz dyz

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Octahedral Field

  • Interaction of d-orbitals with six point charges at

+x, -x, +y, -y, +z and -z axes are not same.

  • The orbitals lying along the axes (i.e. x2-y2, z2) will

be destabilized more than the orbitals lying in- between the axes (i.e. xy, xz, yz).

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CFT-Octahedral Complexes

For Oh point group x2-y2, z2 orbitals: eg xy, xz, yz orbitals: t2g

  • Difference between t2g and eg = Δ0 or 10 Dq.
  • Conservation of barycenter from a spherical field to
  • ctahedral field indicates t2g set must be stabilized as much

as the eg set is destabilized.

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CFSE: Octahedral complex

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  • Nature of the ligands
  • The charge on the metal ion
  • Whether the metal is a 3d, 4d, or 5d element

Dependence: Δo

Ligands: Weak field ligands ; small splitting (Δο ~7000 – 30000 cm-1)

Strong field ligands; large splitting (Δο > 30000 cm-1)

3d < 4d < 5d M2+ < M3+ < M4+

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Tetrahedral field

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Δt = 4/9 Δo

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Octahedral vs Tetrahedral Field

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Spinels- Use of CFSE

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Spinels- Use of CFSE

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Spinels- Use of CFSE

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Spinels- Use of CFSE

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Special case of d8 Octahedral

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Jahn-Teller Distortion

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Jahn-Teller Distortion

Non-linear unsymmetrical molecule: Higher energy

  • J. T. distortion

Lower degeneracy/ lower energy.

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Tetrahedral, Octahedral and Square Planer

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Magnetism

CH CH-105

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Magnetism

Why do we need it?

How does it work??? Familiar world

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Magnetism

Why do we need it?

Outer world

Weather and Sky? Life

  • n

Earth?

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Magnetism

Magnetism is everywhere!!!!

Origin: Lets move inside……

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Transition & Lanthanide ions and their complexes

Magnetism Chemistry

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Magnetism

Origin: Paired & unpaired electron spins How ?

Spinning of electron Paired electron – mutual neutralization Elements with unpaired electron – no cancellation They are magnets (Fe, Co, Ni)

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N2 diamagnetic

Dia.M. ex. H2O, KCl organic ligands, etc. Bismuth metal (most diamagnetic of all metals)

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when dioxygen is in its ground state it is a triplet (spin S=1) and its reactivity is weak.

O2 paramagnetic

Liquid O2

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Magnetic Levitation (Suspension) : Property of diamagnetic molecules

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Magnetochemistry

Electron spin: An electron has two intrinsic spin states, which are referred as up and down or alpha and beta. Electron orbital motion: A magnetic field is generated due to the electron moving around the nucleus. Nuclear spin: Some nuclei, such as hydrogen, have a net spin, which generates a magnetic field. Mutual interaction also and with external magnetic field Shows effect strong/weak and negligible.

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Magnetism

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Molar Susceptibility

Volume  mass  molar SUSCEPTIBILITY Χg = κ/ρ where ρ is density Xm = Xg x M.Wt. Where, M. Wt. is molecular weight of the sample Measurable quatity (Xm) - related to atomic properties

Magnetism

Type:

Mass (gram) susceptibility, χg Volume susceptibility, κ Molar susceptibility, Xm

Interrelation: Summary:

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Magnetic moment (µ) from susceptibility ()

Magnetism

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Magnetism

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e- µorbital µspin µtotal

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Magnetism

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Conditions of orbital angular momentum (µL)

The orbitals must not contain electrons of identical spin during this transformation and the movement of electron These conditions are fulfilled only when one or two orbitals contain partially filled electrons in t2g and NOT in eg

Magnetism

The orbitals should be degenerate (t2g or eg) Interconvertible by rotation eg: t2g orbitals into each other by 90o rotation. Such transformation is not possible with the orbitals of eg. Similar in shape and size

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e.g. the dxz orbital is transformed into the dyz orbital by a rotation of 90° about the z-axis – during this rotation the electron is orbiting the nucleus The degenerate t2g orbitals (dxy, dxz, dyz) can be interconverted by 90° rotations Octahedral complexes Thus, an electron in a t2g orbital can contribute to orbital angular momentum

x y

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z x y z x y 90o rot. dxz dyz z x y dyz

90o rot.

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However the eg orbitals (dz2 and dx2-y2) cannot be interconverted by rotation as they are different shapes Octahedral complexes Thus an electron in an eg orbital can not contribute to orbital angular momentum

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dxy / dx2-y2

  • rbital motion about z axis

dxz / dyz

  • rbital motion about z axis

dxz / dxy

  • rbital motion about x axis

dyz / dxy

  • rbital motion about y axis

But an eg ------> t2g transformation is possible

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d1 think of possible t2g electron arrangements Orbital contribution to the magnetic moment

dxz dyz dxy dxz dyz dxy dxz dyz dxy

Possible t2g arrangements = 3 Orbital contribution = d1 e.g. Ti(III) d2 Possible t2g arrangements = 3 Orbital contribution = yes

dxz dyz dxy dxz dyz dxy dxz dyz dxy

d2 e.g. V(III) high spin octahedral dn ions

YES

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Orbital Contributions in Octahedral Complexes

Magnetism

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  • Q01. Crystalline AgO is diamagnetic.

Explain.

  • AgO

As per formula, oxidation state is +2

Electronic configuration should be Ag: [Kr] 4d10 5s1 Ag(II): [Kr] 4d9

  • Mixture of Ag2O and Ag2O3
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  • Has Ag(I) and Ag(III) configurations
  • Both are diamagnetic

Linear Ag(I) (green) and square planar Ag(III) (grey)

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  • Q02. Work out the hybridization and

geometry for the following complexes using the valence bond approach. (a) Ni(CO)4; (b) [Ni(CN)4]2-; (c) [CoCl4]2-; (d) OsO4; (e) VOCl3; (f) [Pt(NH3)4]2+; (g) [Ag(NH3)2]+; (h) [Pt(PPh3)4]; (i) (Cr2O7)2-

[Ni(CN)4]2-

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(c) OsO4 Electronic configuration [Xe] 4f14 5d6 6s2

d3S hybridization Tetrahedral Ionic approach: Oxidation state of Os(VIII) Electronic configuration: [Xe] 4f14 5d0 6s0 d3S hybridization Tetrahedral Covalent approach: Covalent bond Electronic configuration: [Xe] 4f14 5d6 6s2 The remaining four electrons form π with oxygen

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  • Q03. While the most stable chloride of Zr is ZrCl4,

that of Pd is PdCl2. Why?

  • The third and high I.E. of the d-block metals

increases with increasing atomic number. Owing to the large Zeff making it more energetically unfavourable to attain oxidation state above +2.

  • Further the d-orbitals become more core-like

towards the end of the series and so are less effective in stabilising higher oxidation states.

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  • Q04. When high pressure is applied, what type of

electronic configuration is favoured for a d5 transition metal complex (Octahedral, weak field ligand)?

Low spin; because it leads to low electron density between the metal and the ligand (i.e., along the bond axis).

d5: HS d5: LS

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SLIDE 62
  • Q05. Provide reasons for the fact that a number of

tetrahedral Co(II) complexes are stable, where as the corresponding Ni(II) complexes are not.

The CFSE of d7 tetrahedral complex is greater than that of d8 tetrahedral complex. Similarly, the CFSE of d8 octahedral complex is greater than that of the d7 octahedral complex.

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  • Q06. Using the crystal field stabilization energy as criterion,

indicate whether you expect the following spinels to be normal or inverse: Fe3O4; Co3O4.

Co3O4 has a similar structure with d7 and d6 configurations for 2+ and 3+ ions

  • respectively. Co(III) d6 ion is low spin because (a) high charge (even with

weak ligands like oxo) and (b) maximum gain in CFSE. So the Co3O4 structure is normal spinel. Spinel by definition, the 3+ ion has to go to the Oh site leaving the 2+ ion in Td. Fe3O4 is composed of Fe(II) Td and Fe(III) Oh ions with d6 and d5 configurations

  • respectively. Since d5 has no CFSE, it is more advantageous to put it in a Td

environment than in Oh. In other words, by placing d6 ions in Oh environment there is more gain in more CFSE than keeping this in Td environment. Here the Fe3O4 structure is inverse spinel.

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  • Q07. By showing the details, determine

the CFSE for the following complexes: (a) [FeCl4]2-; (b) W(CO)6.

e3t2

3

CFSE = -0.6 t t2g

6eg

CFSE = -2.4 o

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  • Q08. Explain what is meant by the

term “synergic bonding”?

In the synergic bonding the -donation of charge from the ligand to the metal is reinforced by π- back-donation from the metal to the ligand. In a valence bond model it may be represented by M-CO+ and M=C=O. The filled CO to empty M -donation. Filled M d-

  • rbital to empty π* - back-donation.
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  • Q09. The Cr2+ ion in CrF2 is surrounded by six fluoride

ions. Of these, four are at a distance of ~2.00 Å, while the other two are at a distance of 2.43 Å from the metal ion center. Explain this

  • bservation.

CrF2: Cr2+ is surrounded by 6 fluoride ions in an Oh environment. Cr2+ is d4 high spin; t2g

3eg 1.

The unsymmetrical distribution of electrons in eg leads to Jahn-Teller Distortion. This observation suggests that the eg electrons in dz2 orbital and dx2-y2 is empty.