Metal coordination compound complexes

At a very early stage in their course work in chemistry, students are introduced to a class of compounds referred to as coordination compounds, metal complexes, or just complexes. These compounds contain a central atom or ion, usually a metal, surrounded by several ions or molecules. The complex tends to retain its identity even in solution, although partial dissociation may occur. It may be a cation, an anion or be nonionic, depending on the sum of the charges of the central atom and the surroimding ions and molecules. It is the chemistry of this type of compound that is described in this book. 

The hydrated ion [Cu(H20)6]2+ is an example of a complex, a species consisting of a central metal atom or ion to which a number of molecules or ions are attached by coordinate covalent bonds. A coordination compound is an electrically neutral compound in which at least one of the ions present is a complex. However, the terms coordination compound (the overall neutral compound) and complex (one or more of the ions or neutral species present in the compound) are often used interchangeably. Coordination compounds include complexes in which the central metal atom is electrically neutral, such as Ni(CO)4, and ionic compounds, such as K4[Fe(CN)6]. 

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

In summary, many attempts have been made at achieving enantioselective reduction of ketones. Modified lithium aluminum hydride as well as the ox-azaborolidine approach have proved to be very successful. Asymmetric hydrogenation catalyzed by a chiral ligand-coordinated transition metal complex also gives good results. Figure 6-7 lists some of the most useful chiral compounds relevant to the enantioselective reduction of prochiral ketones, and interested readers may find the corresponding applications in a number of review articles.77,96,97... 

In addition to the ability of transition metals to adopt a variety of oxidation states, they have the ability to form coordination compounds. Coordination compounds contain complex ions. The ability to form a complex ion is not restricted to transition metals however, most examples you will see involve a transition metal. 

Forms a number of coordination compounds (ammonia complex) with several metals adds to AgCl forming soluble complex [Ag(NH3)2]Cl forms tetraamine complex [Cu(NH3)4]S04 with CUSO4 and forms many hexaamine complexes with cobalt, chromium, palladium, platinum and other metals. 

Metal ions can act as electron-pair acceptors, reacting with electron donors to form coordination compounds or complexes. The electron donor species, called the ligand, must have at least one pair of unshared electrons with which to form the bond. Chelates are a special class of coordination compound which results from the reaction of the metal ion with a ligand that contains two or more donor groups. 

In this respect, a classification was reported whose basis is the nature of ligands together with the characteristic bonding and structural peculiarities of the metal complexes [112,113], This allowed the identification of four types of coordination compounds molecular complex compounds (MCC) metal-cyclic complex compounds, metal chelates (MC) complexes with multicenter coordination bonds (CMCB) and di- and polynuclear coordination compounds (DPCC). The expediency of such a classification is based on the following considerations. 

A large class of coordination compounds, metal chelates, is represented in relation to microwave treatment by a relatively small number of reported data, mainly p-diketonates. Thus, volatile copper) II) acetylacetonate was used for the preparation of copper thin films in Ar — H2 atmosphere at ambient temperature by microwave plasma-enhanced chemical vapor deposition (CVD) [735a]. The formed pure copper films with a resistance of 2 3 pS2 cm were deposited on Si substrates. It is noted that oxygen atoms were never detected in the deposited material since Cu — O intramolecular bonds are totally broken by microwave plasma-assisted decomposition of the copper complex. Another acetylacetonate, Zr(acac)4, was prepared from its hydrate Zr(acac)4 10H2O by microwave dehydration of the latter [726]. It is shown [704] that microwave treatment is an effective dehydration technique for various compounds and materials. Use of microwave irradiation in the synthesis of some transition metal phthalocyanines is reported in Sec. 5.1.1. Their relatives - porphyrins - were also obtained in this way [735b]. 

The functioning of homogeneous catalysts involves the properties of coordination compounds. Metal porphyrin complexes such as exist in... 

The most striking feature in diastereoisomerism of metal complexes is the very rapidly growing number of isomers, by introducing different elements of chirality in the basic framework of a coordination compound. The complex formation with l,8-diamino-4-methyl-3,7-diazaoctane (5-Metrien) for example — a ligand with a single asymmetric center — leads not only to four geometric isomers. Three of these show a structure containing all four types of chiral elements mentioned, so that the system offers 28 possibilities of isomers. 

In another very insightful application, Bertrand et al. employed ligand lib for the isolation of low-coordinate transition metal complexes. In these compounds 16 and 17 (Fig. 7), the cyclohexyl ring shields one coordination site of the metal and stabilizes it by means of agostic interactions [65]. 

Complex-formation titration — A method based on the -> titration between a metal ion and electron-pair donor species to form coordination compounds named -> complexes. The donor species is usually called complexing agent or ligand and it can have one or several pairs of unshared electrons available to bond a metal species [i]. 

Metal cations in solution are surrounded by ligands (i.e., solvent molecules, anions, or non-solvent neutral molecules). The bonding that develops is normally a result of the sharing of one or more pairs of electrons of the ligand with the metal ion, which makes a covalent coordinate bond thus, the resulting species is called coordination compound or complex. 

A common method for the preparation of carbene complexes without heteroatom substituents is the reaction of a coordinatively unsaturated metal complex with a diazoalkane. This method is effective for the preparation of CpOsCl(P Pr3)(=CHPh) from CpOsCl(PTr3)2. The carbene in this complex is electrophilic and reacts with aUcyl and aryl lithium compounds and with Grignard reagents see Grignard Reagents) (Scheme 14). ... 

Thermodynamic aspects of 1,3-diborolanes, 2,3-dihydro-l//-l,3-diboroles, 1,3-azaborolidines, 2,3-dihydro-l,3-thia-boroles 2,3-dihydro-l//-l,3-stannaboroles, or 2,3-dihydro-l//-l,3-silaboroles are only sparsely mentioned. It has been found that the 127t-electron antiaromatic heterocycle 23 is stabilized by electron delocalization via the boron atom (cf. compound 9) <2002ZN1125>. Noteworthy is the comparison between the 8jt-electron antiaromatic 2,3-dihydro-l,3-benzothiaborole 24 or 4jt-electron antiaromatic 2,3-dihydro-l,3-thiaborole 26 and the corresponding lOtt-electron 25 or 67t-electron 27 aromatic lithium compounds, the latter forming stable Jt-coordinated transition metal complexes. 

Coordination compound or complex A compound containing coordinate covalent bonds between electron pair donors and a metal. 

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