• Co Dmg 2bf2 Pyridine Chloride 1
• Co Dmg 2bf2 Pyridine Chloride 2
• Co Dmg 2bf2 Pyridine Chloride 1

A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group and its identity. In crystal field theory, ligands modify the difference in energy between the d orbitals called the ligand-field splitting parameter for ligands or the crystal-field splitting parameter, which is mainly reflected in differences in color of similar metal-ligand complexes.

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A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group and its identity. In crystal field theory, ligands modify the difference in energy between the d orbitals (Δ) called the ligand-field splitting parameter for ligands or the crystal-field splitting parameter, which is mainly reflected in differences in color of similar metal-ligand complexes.

Spectrochemical series of ligands[edit]

The spectrochemical series was first proposed in 1938 based on the results of absorption spectra of cobalt complexes.^[1]

A partial spectrochemical series listing of ligands from small Δ to large Δ is given below. (For a table, see the ligand page.)

I⁻ < Br⁻ < S²⁻ < SCN⁻ (S–bonded) < Cl⁻ < N₃⁻ < F⁻< NCO⁻ < OH⁻ < C₂O₄²⁻ < O²⁻< H₂O < acac⁻ (acetylacetonate) < NCS⁻ (N–bonded) < CH₃CN < gly (glycine) < py (pyridine) < NH₃ < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO₂⁻ < PPh₃ < CN⁻ < CO

Weak field ligand: H₂O,F-,Cl-,OH-Strong field ligand: CO,CN-,NH₃,PPh₃

Ligands arranged on the left end of this spectrochemical series are generally regarded as weaker ligands and cannot cause forcible pairing of electrons within the 3d level, and thus form outer orbital octahedral complexes that are high spin. On the other hand, ligands lying at the right end are stronger ligands and form inner orbital octahedral complexes after forcible pairing of electrons within 3d level and hence are called low spin ligands.

However, keep in mind that 'the spectrochemical series is essentially backwards from what it should be for a reasonable prediction based on the assumptions of crystal field theory.'^[2] This deviation from crystal field theory highlights the weakness of crystal field theory's assumption of purely ionic bonds between metal and ligand.

The order of the spectrochemical series can be derived from the understanding that ligands are frequently classified by their donor or acceptor abilities. Some, like NH₃, are σ bond donors only, with no orbitals of appropriate symmetry for π bonding interactions. Bonding by these ligands to metals is relatively simple, using only the σ bonds to create relatively weak interactions. Another example of a σ bonding ligand would be ethylenediamine, however ethylenediamine has a stronger effect than ammonia, generating a larger ligand field split, Δ.

Ligands that have occupied p orbitals are potentially π donors. These types of ligands tend to donate these electrons to the metal along with the σ bonding electrons, exhibiting stronger metal-ligand interactions and an effective decrease of Δ. Most halide ligands as well as OH⁻ are primary examples of π donor ligands.

When ligands have vacant π* and d orbitals of suitable energy, there is the possibility of pi backbonding, and the ligands may be π acceptors. This addition to the bonding scheme increases Δ. Ligands that do this very effectively include CN⁻, CO, and many others.^[3]

Spectrochemical series of metals[edit]

The metal ions can also be arranged in order of increasing Δ, and this order is largely independent of the identity of the ligand.^[4]

Co Dmg 2bf2 Pyridine Chloride 1

Mn²⁺ < Ni²⁺ < Co²⁺ < Fe²⁺ < V²⁺ < Fe³⁺ < Cr³⁺ < V³⁺ < Co³⁺

In general, it is not possible to say whether a given ligand will exert a strong field or a weak field on a given metal ion. However, when we consider the metal ion, the following two useful trends are observed:

• Δ increases with increasing oxidation number, and
• Δ increases down a group.^[4]

See also[edit]

References[edit]

• Zumdahl, Steven S. Chemical Principles Fifth Edition. Boston: Houghton Mifflin Company, 2005. Pages 550-551 and 957-964.
• D. F. Shriver and P. W. Atkins Inorganic Chemistry 3rd edition, Oxford University Press, 2001. Pages: 227-236.
• James E. Huheey, Ellen A. Keiter, and Richard L. Keiter Inorganic Chemistry: Principles of Structure and Reactivity 4th edition, HarperCollins College Publishers, 1993. Pages 405-408.

1. ^R. Tsuchida (1938). 'Absorption Spectra of Co-ordination Compounds. I.'Bull. Chem. Soc. Jpn. 13 (5). doi:10.1246/bcsj.13.388.
2. ^7th page of http://science.marshall.edu/castella/chm448/chap11.pdf
3. ^Miessler, Gary; Tarr, Donald (2011). Inorganic Chemistry (4th ed.). Prentice Hall. pp. 395–396. ISBN978-0-13-612866-3.
4. ^ ^abhttp://www.everyscience.com/Chemistry/Inorganic/Crystal_and_Ligand_Field_Theories/b.1013.php

Chloro(pyridine)cobaloxime is a coordination compound containing a Co^III center with octahedral coordination. It has been considered as a model compound of vitamin B₁₂ for studying the properties and mechanism of action of the vitamin. It belongs to a class of bis(dimethylglyoximato)cobalt(III) complexes with different axial ligands, called cobaloximes.^[1] Chloro(pyridine)cobaloxime is a yellow-brown powder that is sparingly soluble in most solvents, including water.

Structure[edit]

The complex adopts a distorted octahedral geometry. Cobalt(III) is bound to two dimethylglyoximate ligands, i.e., mono-deprotonated dimethylglyoxime, in the equatorial plane. Completing the coordination sphere are chloride and a pyridine at the axial positions.^[2]

Reactions[edit]

Co Dmg 2bf2 Pyridine Chloride 2

The cobaloxime is slowly decomposed by acids and bases. With acids, the products of decomposition are dimethylglyoxime, cobalt salts, and pyridine; with bases, derivatives of other cobaloximes are formed, usually with the release of chloride ions.

The complex has no reaction with hydrogen gas, and cannot carry oxygen as salcomine does. It would, however, react with hydrogen in the presence of sodium hydroxide, a catalytic amount of platinum metal, or a reduced cobaloxime, therefore once the reduction occurs, the hydrogenation would occur much more rapidly as there is autocatalysis.

The reduction products of cobaloxime depends on the conditions. At pH near 7, a cobaloxime with a Co^II center is formed. With a higher pH, the cobalt center would be further reduced to the Co^I state, which is supernucleophilic.^[3]

Preparation[edit]

The compound is usually prepared by mixing cobalt(II) chloride, dimethylglyoxime and pyridine in an ethanolic solution. This process afford the cobaloxime(II), which is subsequently oxidized by the oxygen in air:^[3]

4 CoCl₂•6H₂O + 8 dmgH₂ + 8 py + O₂ → 4 ClCo(dmgH)₂py + 4 py•HCl + 14 H₂O

Using cobalt(II) acetate in place of cobalt(II) chloride produce aceto(pyridine)cobaloxime. This acetate can be converted to the respective bromide, iodide, cyanate, cyanide, azide and thiocyanate.^[3]

(CH₃COO)Co(DH)₂py + NaX → XCo(DH)₂py + NaCH₃COO (X = Br, I, CNO, CN, N₃ or SCN)

Reactions[edit]

Co Dmg 2bf2 Pyridine Chloride 1

The pyridine base in the axial position can also be replaced by other organic bases containing a sp² hybridized N atom as well. Commonly used bases are morpholine, 4-methylpyridine, imidazole and benzimidazole. The derivatives are again prepared via diacetocobaloxime, followed by the addition of the desired base, such as imidazole.

(CH₃COO)₂Co(DH)₂ + imi → (CH₃COO)Co(DH)₂imi

Alkylation of Co[edit]

One of the methods used for producing the Co-C bond is to make use of the supernucleophilicity of the Co^I center. Chloro(pyridine)cobaloxime(III) is first reduced to Chloro(pyridine)cobaloxime(I) by sodium borohydride in alkaline solution, then an alkyl halide is added into the reaction mixture, and the desired Co-C bond is formed via a S_N2 reaction. This method can be used to produce cobaloximes containing a primary or a secondary alkyl substituent.

For derivatives with phenyl or vinyl substituent, the Grignard reaction is employed. However, since the dimethylglyoxime ligands contains two acidic H atoms in the oxime group, the Grignard reagent must be used in three-fold excess to compensate the loss.^[3]

References[edit]

1. ^Jonathan W. Steed; Jerry L. Atwood (2009). Supramolecular Chemistry, 2nd edition. Wiley. p. 808. ISBN978-0-470-51233-3.
2. ^Geremia, Silvano; Dreos, Renata; Randaccio, Lucio; Tauzher, Giovanni (February 1994). 'Evidence of the interaction between steric and electronic influence in rhodoximes and cobaloximes. Syntehsis of pyRh(DH)2I and X-ray structure of pyRh(DH)2Cl and pyRh(DH)2I'. Inorganica Chimica Acta. Trieste, Italy: Elsevier B.V. 216 (1–2): 125–129. doi:10.1016/0020-1693(93)03708-I.
3. ^ ^abcdG. N. Schrauzer (1968). 'Bis(dimethylglyoximato)cobalt complexes (Cobaloximes) - A. Chloro(pyridine)cobaloxime(III)'. Inorganic Syntheses. XI: 62–64. doi:10.1002/9780470132425. ISBN9780470132425.