Coordinating compounds, naturally

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Metal complexes of ligands containing a sulfur donor in conjunction with nitrogen, oxygen or a second sulfur have been reviewed in the past [11-13]. For example, reviews of the coordination compounds of dithiophosphates [14], dithiocarbamates [15, 16], dithiolates [17], dithiodiketonates [18], and xanthates [16] have appeared. The analytical aspects [19] and the spectral and structural information of transition metal complexes of thiosemicarbazones [20, 21] have been reviewed previously. Recent developments in the structural nature of metal complexes of 2-heterocyclic thiosemicarbazones and S-alkyldithiocarbazates, depicted below, are correlated to their biological activities. 

The most common use of iridium coordination compounds remains in the catalysis field, although interest is developing in the luminescent properties of iridium compounds. The wide range of accessible oxidation states available to iridium (—1) to (VI) is reflected in the diverse nature of its coordination compounds. 

The nature of the target to be attacked by any drug obviously depends on the specific application. Many cytotoxic metal complexes target DNA because of its importance in replication and cell viability. Coordination compounds offer many binding modes to polynucleotides, including outer-sphere noncovalent binding, metal coordination to nucleobase and phosphate backbone... 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

Chemists are involved in both synthesis, that is creating compounds, and analysis, determing the nature of compounds. In this lab, a coordination compound containing Ni2+, NH3 and CL will be synthesized. Once it has been synthesized, the next step will be to analyze the coordination compound to determine its exact formula. 

Coordinative interactions in natural waters change as a result of a variation in coordinative species or coordination number, which in turn leads to a transformation of contaminant properties. Any combination of cations with molecules or anions containing free pairs of electrons (bases) is called coordination (or complex formation). The coordination can be electrostatic, covalent, or a mixture of both. The metal cation is called the central atom, and the anion or molecule with which it forms a coordinative compound is referred to as a ligand. 

It is known that in various coordination compounds cobalt readily undergoes changes in oxidation number, these changes being distinctively influenced by the nature of the Hgands. 

Methyl groups of both the dioxasilane and NMe2 moieties in the coordinative compounds (62) are shown by H NMR spectroscopy to be diastereotopic at 20°C and to coalesce on heating to 55-90°C. The free energies of activation of this process were calculated to be 17.2-18.3 kcal mol and were very little dependent on the nature of X <9iOM1236>. 

The classic Hieber-base reaction 16 is that of a hydroxide with metal carbonyls, which proceeds by nucleophilic attack of the hydroxide at a carbon atom of a carbonyl ligand to give a carboxy group or consequently carbon dioxide and a metal hydride.17 Metal carbonyls are catalysts for the water-gas shift reaction.18 Pentacarbonyl(tetrafluoroborato)rhenium reacts with alkali hydroxide in a similar way however, due to the coordinatively unsaturated nature of the [Re(CO)5]+ group polynuclear compounds are formed.15... 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

No discussion of the Jahn-Teller effect in coordination compounds would be complete without including the special features of chelated compounds (see Chapter 12 for a more thorough discussion of chelated complexes). The very nature of the chelated ring tends to restrict the distortion of a complex from a perfect octahedron because the ligand will have a preferred bite or distance between the coordinating atoms ... 

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