harlcath yuleena granthem
Camie Fons
Werner's coordination theory from 1893 explained the structure and properties of transition metal complexes. It proposed that metals exhibit primary and secondary valences. Primary valency corresponds to oxidation state and is satisfied by anions, while secondary valency corresponds to coordination number and is satisfied by ligands. Werner's theory could successfully explain the structures of cobalt(III) and platinum(IV) ammine complexes based on these concepts. However, it had limitations and could not explain all properties of coordination compounds.Read less
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.
The concept of octahedral coordination geometry was developed by Alfred Werner to explain the stoichiometries and isomerism in coordination compounds. His insight allowed chemists to rationalize the number of isomers of coordination compounds. Octahedral transition-metal complexes containing amines and simple anions are often referred to as Werner-type complexes.
When two or more types of ligands (La, Lb, ...) are coordinated to an octahedral metal centre (M), the complex can exist as isomers. The naming system for these isomers depends upon the number and arrangement of different ligands.
The number of possible isomers can reach 30 for an octahedral complex with six different ligands (in contrast, only two stereoisomers are possible for a tetrahedral complex with four different ligands). The following table lists all possible combinations for monodentate ligands:
Thus, all 15 diastereomers of MLaLbLcLdLeLf are chiral, whereas for MLa
2LbLcLdLe, six diastereomers are chiral and three are not (the ones where La are trans). One can see that octahedral coordination allows much greater complexity than the tetrahedron that dominates organic chemistry. The tetrahedron MLaLbLcLd exists as a single enantiomeric pair. To generate two diastereomers in an organic compound, at least two carbon centers are required.
The sharing of an edge or a face of an octahedron gives a structure called bioctahedral. Many metal pentahalide and pentaalkoxide compounds exist in solution and the solid with bioctahedral structures. One example is niobium pentachloride. Metal tetrahalides often exist as polymers with edge-sharing octahedra. Zirconium tetrachloride is an example.[7] Compounds with face-sharing octahedral chains include MoBr3, RuBr3, and TlBr3.
Many reactions of octahedral transition metal complexes occur in water. When an anionic ligand replaces a coordinated water molecule the reaction is called an anation. The reverse reaction, water replacing an anionic ligand, is called aquation. For example, the [CoCl(NH3)5]2+ slowly yields [Co(NH3)5(H2O)]3+ in water, especially in the presence of acid or base. Addition of concentrated HCl converts the aquo complex back to the chloride, via an anation process.
Communities of practice are formed by people who engage in a process of collective learning in a shared domain of human endeavor: a tribe learning to survive, a band of artists seeking new forms of expression, a group of engineers working on similar problems, a clique of pupils defining their identity in the school, a network of surgeons exploring novel techniques, a gathering of first-time managers helping each other cope. In a nutshell:
The community. In pursuing their interest in their domain, members engage in joint activities and discussions, help each other, and share information. They build relationships that enable them to learn from each other; they care about their standing with each other. A website in itself is not a community of practice. Having the same job or the same title does not make for a community of practice unless members interact and learn together. The claims processors in a large insurance company or students in American high schools may have much in common, yet unless they interact and learn together, they do not form a community of practice. But members of a community of practice do not necessarily work together on a daily basis. The Impressionists, for instance, used to meet in cafes and studios to discuss the style of painting they were inventing together. These interactions were essential to making them a community of practice even though they often painted alone.
While they all have the three elements of a domain, a community, and a practice, they come in a variety of forms. Some are quite small; some are very large, often with a core group and many peripheral members. Some are local and some cover the globe. Some meet mainly face-to-face, some mostly online. Some are within an organization and some include members from various organizations. Some are formally recognized, often supported with a budget; and some are completely informal and even invisible.
Communities of practice have been around for as long as human beings have learned together. At home, at work, at school, in our hobbies, we all belong to communities of practice, a number of them usually. In some we are core members. In many we are merely peripheral. And we travel through numerous communities over the course of our lives.
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