CHE 201; DR GERVAS ASSEY; LECTURE 08

ELECTONIC COUNTING IN ORGANOMETALLIC CHEMISTRY

This lecture comprises of the following parts:

1.     The eighteen electron rule

2.     Electron counting scheme for common ligand

3.     Tutorial questions.

1.     ELECTRONIC COUNTING IN ORGANOMETALLIC CHEMISTRY.

Ø Just as in the main group chemistry requires valence shell of 8 electrons (octet rule), the organometallic chemistry is based on a total valence electron count of 18 on the central metal atom.....

CHE 201,DR,GERVAS ASSEY LECTURE 03

A.    THE CHEMISTRY OF d-block ELEMENTS COMPARISON BY ELECTRON    

                                        CONFIGURATION

The d⁰ Configuration

This configuration occur for the ions such Sc³⁺ and Mn(VII). All metals d⁰ configuration are hard acids and prefer to interact with hard bases such as Oxide. Complexation chemistry is less extensive than in other configurations. Complexes such as [ScF₆]^3- and [Sc(OH)₆]^3- are known and result when excess F^- or OH^- is added to insoluble ScF₃ or Sc(OH)₃.

The higher oxidation states [Cr(VII)] and [Mn(VII)] tend to form oxyanions which are good oxidizing agents especially in acidic solution: The oxides of intermediate species are insoluble (TiO₂) or amphoteric V₂O₅ [VO4]^3-

d⁰  is the most stable configuration for Titanium. E.g. TiO₂, TiCl₄ and [TiCl₆]^2-

The d¹ Configuration

This configuration does not tend to be a stable configuration. It is completely unknown for Scandium and strongly reducing in Ti(III). The latter members of the series tend to disproportionate to more stable configuration.

3[CrO₄]^3- + 10H⁺                                    2[HCrO₄]^- + Cr³⁺ + 4H₂O

3[MnO₄]^2- + 4H⁺                                     2[MnO₄]^- + MnO₂ + 2H₂O

The only d¹ species of importance is Vanadyl VO²⁺, which is the most stable form of vanadium in aqueous solution.

The d² Configuration

This configuration ranges from Ti(II),  very strongly reducing to Fe(VI) Very strongly oxidizing. Both Ti(II) and V(III) are reducing agents.

The Fe(VI) ion [FeO₄]^2- is formed by oxidation of iron or iron compounds. It is reasonably stable in basic solution and becomes a more powerful oxidizing agent as the pH is lowered.

The d³ Configuration

This is not a stable configuration. V(II) is a strongly reducing while Mn(IV) Is strongly. However Cr³⁺ is the most stable chromium in aqueous solution.

The d⁴ configuration

The Cr²⁺ is strongly reducing agent but may be prepared readily

Cr + H⁺        Zn(Hg)           Cr²⁺ + H₂

But require addition of Cr²⁺ solutions and sodium acetate precipitates chromium (II) acetate.

2Cr²⁺ + 4CH₃COO^- + H₂O                             Cr₂(OOCCH₂)₄(H₂O)₂

Complexes of Mn(III) are relatively unstable except [Mn(CN)₆]^3- which forms readily upon exposure of Mn(II) solution and cyanide to air. The d⁴ configuration contains both high spin and low spins octahedral complexes. The cyanide complexes of Mn³⁺ and Cr²⁺ are low spin.

The d⁵ configuration

Important species in this configuration are Mn³⁺ and Fe³⁺.

Almost all the known complexes of this configuration are high spin. Exceptions to this are the complexes of [Mn(CN)₆]^4- and [Fe(CN)₆]^3-

The d⁶ configuration

The complexes that are in this category are the octahedral complexes with strong field ligands which provide the highest possible ligand field stabilization energy (LFSE). Cobalt (III) and nickel (IV) are oxidizing agent. Most Fe(II) complexes are high spin exception being ferrocyanides, [Fe(CN)₆]^4-.

Cobalt (III) complexes in contrast tend to be low spin complexes except in the presence of weak field ligands e.g. [CoF₆]^3- and [Co(H₂O)₃F₃]

The d⁷ configuration

The important species with this configuration are Co(II) and Ni(II). Cobalt (II) occurs in tetrahedral [CoCl₄]^2- square planar [Co(Hdmg)₂],  square pyramidal [Co(ClO₄)(OAsMePh₂)₄], trigonal bipyramidal  [CoBrMe₆trenz]⁺ and octahedral [Co(NH₃)]²⁺ complexes.

Cobalt (II) is stable in aqueous solution but in the presence of strong field ligands it is easily oxidized to form Co(III) complexes.

The d⁸ configuration

This configuration is ideal for the formation of low spin square planar complexes with strong field ligands. Ni(II) complexes are typically red or yellow, although other colors are found. Tetrahedral high spin complexes are formed with bulky ligands such as triphenyl phosphine, triphenylphosphine oxide or halides. Five coordinate complexes of Ni(II) may be either high or low spin complexes depending on the nature of the ligands involved. With soft ligands such as sulfur, phosphorus or arsenic, the complexes tend to be low spin, while they are high spin in similar nitrogen-containing ligands. Both trigonal bipyramidal and square pyramid complexes are known.

Six coordinate Ni(II) complexes may have equivalent ligands as in [Ni(H₂O)]²⁺, [Ni(NH₃)]²⁺ and [Ni(en)₃]²⁺

Only few simple copper (II) salt are known e.g. KCuO₂ and Cs₃CuF₆ but numerous complexes containing organic ligands exist. A few cobalt (I) complexes are known.

The d⁹ configuration

This configuration is found in copper (II) compounds. It has neither the stability of d¹⁰ subshell nor the LFSE possible for d⁸. Copper (II) may be fairly easily reduced to Cu(I).

Six coordinate from pure octahedral by the jahn-teller effect. A number of five coordinate complexes are known both square pyramidal and trigonal bipyramidal.

Four coordination are exemplified by the square planar and the tetrahedral complexes.

The d¹⁰ configuration

For the first transition series, this configuration is limited to Cu(I) and Zn(II), but it is also exhibited by the post transition metals in their highest oxidation states [Ga(III)] and Ge(IV). The copper (I) complexes are good reducing agents being oxidized to Cu(II).

They can be stabilized by precipitation with appropriate counter ions to the extent Cu(I) may form and exclude Cu(II).

 Cu²⁺_(aq) + I^-_(aq)                                             CuI_(s) + 1/2I_2(S)

Cu²⁺_(aq) + 2CN^-_(aq)                                      CuCN_(s) + ½(CN)_2(g)

The preferred coordination for Cu(I) appears to be linear (SP), two coordination. Three coordinate as well as several tetrahedral complexes are known. Zn(II) is typically either tetrahedral [ZnCl₄]^2- or octahedral e.g. [Zn(H₂O)]²⁺ but both trigonal bipyramidal and square pyramidal five coordinate are also known.

B.     COVALENT AND IONIC RADII

The size of atoms and ions are also related to the ionization energies and electron affinities.

As the nuclear charge increases the electron are pulled in towards the center of the atom and the size of any particular orbital decreases. On the other hand as the nuclear charge increases more electrons are added to the atom and their mutual repulsion keeps the outer orbital large.

Factors that influence ionic size include the coordination number of the ion, the covalent character of the bonding, distortion of regular crystal geometric and delocalization of electrons. The radius of an anion is also influenced by the size and charge of the cation.

                   

Lecture 3 translations:

Jahn-teller effect:

Ø  Is a geometric distortion of a non-linear molecular system that reduces its symmetry and energy. Most occur in Cu(II) Complexes.

Ø  The distortion is typically observed among octahedral complexes where the two axial bonds can be shorter or longer than those of the equatorial bonds.