Werner Cobalt Complexes

[Laila Berri]

Coordination compounds, such as the FeCl4-ion and CrCl3 6 NH3, are called suchbecause they contain ions or molecules linked, or coordinated, toa transition metal. They are also known as complex ions orcoordination complexes because they are Lewis acid-basecomplexes. The ions or molecules that bind to transition-metalions to form these complexes are called ligands (fromLatin, "to tie or bind"). The number of ligands boundto the transition metal ion is called the coordination number.

Although coordination complexes are particularly important inthe chemistry of the transition metals, some main group elementsalso form complexes. Aluminum, tin, and lead, for example, formcomplexes such as the AlF63-, SnCl42-and PbI42- ions.

When excess Ag+ ion is added to solutions of theCoCl3 6 NH3 and CoCl3 5 NH3 H2O complexes, three moles of AgCl are formed foreach mole of complex in solution, as might be expected. However,only two of the Cl- ions in the CoCl3 5NH3 complex and only one of the Cl- ions inCoCl3 4 NH3 can be precipitated with Ag+ions.

Werner explained these observations by suggesting thattransition-metal ions such as the Co3+ ion have aprimary valence and a secondary valence. The primary valenceis the number of negative ions needed to satisfy the charge onthe metal ion. In each of the cobalt(III) complexes previouslydescribed, three Cl- ions are needed to satisfy theprimary valence of the Co3+ ion.

The secondary valence is the number of ions ofmolecules that are coordinated to the metal ion. Werner assumedthat the secondary valence of the transition metal in thesecobalt(III) complexes is six. The formulas of these compounds cantherefore be written as follows.

The cobalt ion is coordinated to a total of six ligands ineach complex, which satisfies the secondary valence of this ion.Each complex also has a total of three chloride ions that satisfythe primary valence. Some of the Cl- ions are free todissociate when the complex dissolves in water. Others are boundto the Co3+ ion and neither dissociate nor react withAg+.

One of the chloride ions is bound to the cobalt in the [Co(NH3)5Cl]Cl2complex. Only three ions are formed when this compound dissolvesin water, and only two Cl- ions are free toprecipitate with Ag+ ions.

Werner assumed that transition-metal complexes had definiteshapes. According to his theory, the ligands in six-coordinatecobalt(III) complexes are oriented toward the corners of anoctahedron, as shown in the figure below.

Any ion or molecule with a pair of nonbonding electrons can bea ligand. Many ligands are described as monodentate(literally, "one-toothed") because they"bite" the metal in only one place. Typical monodentateligands are given in the figure below.

Each end of this molecule contains a pair of nonbondingelectrons that can form a covalent bond to a metal ion.Ethylenediamine is also an example of a chelating ligand.The term chelate comes from a Greek stem meaning"claw." It is used to describe ligands that can grabthe metal in two or more places, the way a claw would.

Transition-metal complexes have been characterized withcoordination numbers that range from 1 to 12, but the most commoncoordination numbers are 2, 4, and 6. Examples of complexes withthese coordination numbers are given in the table below.

G. N. Lewis was the first to recognize that the reactionbetween a transition-metal ion and ligands to form a coordinationcomplex was analogous to the reaction between the H+and OH- ions to form water. The reaction between H+and OH- ions involves the donation of a pair ofelectrons from the OH- ion to the H+ ion toform a covalent bond.

The H+ ion can be described as an electron-pairacceptor. The OH- ion, on the other hand, is an electron-pairdonor. Lewis argued that any ion or molecule that behaveslike the H+ ion should be an acid. Conversely, any ionor molecule that behaves like the OH- ion should be abase. A Lewis acid is therefore any ion or molecule thatcan accept a pair of electrons. A Lewis base is an ion ormolecule that can donate a pair of electrons.

The Co3+ ion is an electron-pair acceptor, or Lewisacid, because it has empty valence-shell orbitals that can beused to hold pairs of electrons. To emphasize these empty valenceorbitals we can write the configuration of the Co3+ion as follows.

According to this model, transition-metal ions formcoordination complexes because they have empty valence-shellorbitals that can accept pairs of electrons from a Lewis base.Ligands must therefore be Lewis bases: They must contain at leastone pair of nonbonding electrons that can be donated to a metalion.

17 original crystals of [Λ-dinitrobis(ethylenediamine)cobalt]X complexes from the Alfred Werner collection of original samples of the University of Zurich were studied by single crystal X-ray diffraction revealing that the complexes with X = Cl, Br can undergo spontaneous chiral resolution upon crystallization. The main focus of this article was the chiral [Λ- and Δ-dinitrobis(ethylenediamine)cobalt]Cl complexes, which crystallize from racemic solution in the space group P 21 mainly as synthetical twins enriched in one enantiomer, but to a small extent also as pure enantiomorphs. The twinning effect was recognized and correctly described by PhD student Richard Hessen of the Alfred Werner group (PhD thesis 1914). Richard Hessen eventually resolved the [Δ- and Λ-dinitrobis(ethylenediamine)cobalt]Cl complex by the conglomerate salt resolution method. Based on the availability of the pure [Δ- or Λ-dinitrobis(ethylenediamine)cobalt]Cl complex, he carried out seeding experiments, which proved that the [Δ- and Λ-dinitrobis(ethylenediamine)cobalt]Cl complexes can be enriched to a great extent in one enantiomer by spontaneous chiral resolution. Already in the period of time from 1900 to 1904, various PhD students of Alfred Werner's group (Adolf Grn, Edith Humphrey, Ernst Zinggeler, Heinrich Schwarz, and Paul Larisch) prepared the [Δ- or Λ-dinitrobis(ethylenediamine)cobalt]Cl complex. Adolf Grn and Edith Humphrey have prepared enantiomerically enriched and rarely also enantiomorphic crystals of the [Λ- or Δ-dinitrobis(ethylenediamine)cobalt]Cl complex and could have separated crystals by manual crystal picking. Admittedly due to the crystal habits this would have been a difficult endeavour, but this 'Louis Pasteur method' was apparently not taken into consideration. Still in a cautiously sounding note one could state that Alfred Werner and his group had missed by this omission the opportunity for spontaneous chiral resolution of the [Δ- and Λ-dinitrobis(ethylenediamine)cobalt]Cl complexes in the period of time from 1900 to 1904. In addition, making this early chiral resolution story even more incredible, we found that Heinrich Schwarz and Paul Larisch applied in these early days of coordination chemistry the S-(D-,d-)camphorsulfonate anion to achieve the separation of the cis- and trans-isomers of the [dinitrobis(ethylenediamine)cobalt] complexes. They did not approach the potentially possible chiral resolution of the [cis-dinitrobis(ethylenediamine)cobalt]+ cation. But based on their synthetic procedure they did indeed accomplish chiral resolution of the cis-isomer and prepared eventually a series of the chiral [cis-,Λ-dinitrobis(ethylenediamine)cobalt]X salts; however all this was in an unintentional manner.

The history of modern coordination chemistry has been the subject of several books, ofwhich perhaps the best known are those by George B. Kauffman. By necessity they allhighlight Alfred Werner, "the Father of Coordination Chemistry" who in 1893,proposed the octahedral configuration of transition metal complexes and in 1913 receivedthe first Nobel prize inInorganic Chemistry.

In this series of experiments, some simple cobalt(III) complexes are to be prepared, whichshow some of the properties that Werner was able to interpret using the octahedral model.These include; optical, geometric and linkage isomerism.

Reaction Schemes as ISIS Draw .tgf file Hexol.html

The oxidation of cobalt sulfate to the tetraamminecarbonate complex can be done by passingair through the solution for 2 hours, or more conveniently using hydrogen peroxide(although this is more costly, it allows the experiment to be easily completed in onesession). The conversion of the bis-aqua complex to "hexol" given here is themethod originally used by Jorgensen. By contrast, Werner added pyridine to a hot, diluteacetic acid solution. "Hexol" exists in several hydrated forms, all of them darkpurple-black crystals. Air-drying the crystals has been found to result in theennea-hydrate, whereas drying at 98C or over sulfuric acid results in the tetrahydrate.
The nitrite ion (NO2-) is an example of an ambidentateligand. That is, it can form a bond to a metal via either of two non-equivalent sites,from the N or from an O.
In the second scheme, two linkage isomers of the type [Co(NH3)ONO]2+ and [Co(NH3)5NO2]2+are prepared starting from [Co(NH3)5Cl]2+.
The actual isomer obtained depends on the pH of the solution. it should be noted that thetwo isomers are in equilibrium and that the nitro compound is the more stable. Whereasisomerisation takes several months at room temperature it is greatly accelerated byheating. Therefore the products should NOT be heated.

The nitrito isomer converts slowly at room temperature to the nitro isomer and theconversion can be conveniently followed by observing the disappearance of the nitrito bandat approximately 1060 cm-1 in the IR spectrum.
Collect an IR spectrum of the penataamminechlorocobalt(III) chloride for comparison.

Diecisiete cristales originales de los complejos [Λ-dinitrobis(etilendiamina)cobalto]X, pertenecientes a la coleccin de muestras originales de Alfred Werner, de la Universidad de Zurich, fueron estudiados mediante difraccin de rayos X de monocristal, encontrndose que los complejos con X = Cl, Br pueden resolver su quiralidad espontneamente a travs de la cristalizacin. El objetivo principal de este artculo fueron los complejos quirales [Λ- y Δ-dinitrobis(etilendiamina)cobalto]Cl los cuales cristalizan desde una solucin racmica en el grupo espacial P 21 primordialmente como twins sintticos enriquecidos en un enantimero, y tambin en una pequea medida como enantiomorfos puros. El efecto twinning fue reconocido y correctamente descrito por el estudiante de doctorado Richard Hessen (tesis doctoral en 1914) perteneciente al grupo de Alfred Wegner. Richard Hessen pudo eventualmente resolver el complejo [Δ- y Λ-dinitrobis(etilendiamina)cobalto]Cl mediante el mtodo de resolucin del conglomerado de sal basndose en la disponibilidad de complejo [Δ- o Λ-dinitrobis(etilendiamina)cobalto]Cl puro; llev a cabo diversos experimentos de sembrado, los cuales concluyeron que los complejos [Δ- o Λ-dinitrobis(etilendiamina)cobalto]Cl pueden enriquecer en gran medida un enantimero mediante resolucin quiral espontnea. Ya en el periodo de 1900 a 1904 varios estudiantes de doctorado del grupo de Alfred Werner (Adolf Grn, Edith Humphrey, Ernst Zinggeler, Heinrich Schwarz, Paul Larisch) prepararon los complejos [Δ- o Λ-dinitrobis(etilendiamina)cobalto]Cl. Adolf Grn y Edith Humphrey prepararon cristales enriquecidos enantiomricamente y, extraamente, a la vez cristales enantiomrficos de los complejos [Δ- o Λ-dinitrobis(etilendiamina)cobalto]Cl, los cristales pudieron ser separados manualmente. Es cierto, que debido al comportamiento de los cristales, esto debera haber sido un esfuerzo muy dificultoso, sin embargo este 'mtodo de Louis Pasteur' aparentemente no fue tomado en consideracin. Aun as, hablando con cautela, se podra decir que Alfred Werner y su grupo, mediante esta omisin, perdieron la oportunidad de lograr la resolucin quiral espontnea de los complejos [Δ- o Λ-dinitrobis(etilendiamina)cobalto]Cl en el periodo de tiempo de 1900 a 1904. Adems, haciendo la historia temprana de la resolucin quiral aun ms increble, encontramos que Heinrich Schwarz y Paul Larisch aplicaron en los inicios de la qumica de coordinacin el anin S-(D-,d-) sulfonato de alcanfor para conseguir la separacin de los ismeros cis y trans de los complejos [dinitrobis(etilendiamina)cobalto]. Ellos no se acercaron a la potencialmente posible resolucin quiral del catin [cis-dinitrobis(etilendiamina)cobalto]+. Pero basados en sus procedimientos sintticos, efectivamente lograron la resolucin quiral del ismero cis y prepararon, eventualmente una serie de sales quirales [cis-,Λ-dinitrobis(etilendiamina)cobalto]X, sin embargo todo esto, de una forma casual.

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