Mctal complexes

A currently popular alternative <> the ah initio method is density ftmitwnal theory, m which the energy is expressed in terms of rhe electron density rather than the w-ivi-funcron itself. The idvautogc of this approach is that it is less demanding computationally, requires less computer nine-, and m some cases—particularly for d-mctal complexes—gives bet-ter agreement with experimental values than other procedures. 

Cyclopentene-l-dithiocarboxylic acid, 2-amino-mctal complexes, 800 Cyclophosphazenes metal complexes, 81 Cyclotetraphosphazene, octamethyl-platinum complexes, 82 Cyclotetraphosphazenes metal complexes, 81 Cyclotriphosphorus complexes, 1058 Cysteine... 

Porphyrin, dihydro-metal complexes geochemistry, 862 Porphyrin, hydrometal complexes biology, 546 Porphyrin, tetrahydro-mctal complexes geochemistry, 862 Porphyrin, tetraphenyl-indium complexes radiopharmacology, 971 zinc... 

A d-mctal ion may also be responsible for color, as is apparent from the varied colors of many d-metal complexes (see Chapter 16). Two types of transitions may be involved. In one, which is called a d-to-d transition, an electron is excited from a d-orbital of one energy to a d-orbital of higher... 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

Methods 1 and 2 are intended to control the boiler water pH and to precipitate the calcium and magnesium compounds as a flocculenl sludge, so that they can be removed in the boiler blowdown rather than being deposited on heat-transfer surfaces. Method 1 maintains an excess of hydroxide alkalinity. The effects of alkalinity are discussed later under Steam Purity. Method 3 involves the addition of a complex mctal-chelant compound such as ethylenediainine-tetraacetic acid i a.(FDTA) or niirilolriacelic acid (NTA). In Method 4, as ihe name implies, no solid chemicals are added to the boiler or pre-boiler cycle. The pH of the boiler water and condensate cycle is controlled by adding a volatile amine. 

Such J-mctals as Cu(I) [but not Cu(II)], form a variety of compounds with ethenes, for example [Cu(C2H4)(H20)2]C104 (from Cu, Cu2+, and C2H4) or Cu(C2H4)(bipy)+. It is necessary to mention that, of all the metals involved in biological systems, only copper reacts with ethylene [74b]. Such homoleptic alkene complexes can be useful intermediates for the synthesis of other complexes. The olefin complexes of the metals in high formal oxidation states are electron deficient and therefore inert toward electrophilic reagents. By contrast, the olefin complexes of the metals in low formal oxidation states are attacked by electrophiles such as protons at the electron-rich metal-carbon a-bonds [74c]. 

All these reactions arc very significant in explaining the mechanisms of several important industrial processes see below. It is noteworthy that the mctal-carbon bond of acyl metal complexes presents some double bond (and, therefore, carbenoid) characters the C=0 stretclung frequencies arc lower than the normal ketonic frequency [23]. 

Isonitrile is isoelectronic with CO, and iminoacyl complexes can be compared with acyl ones. The most remarkable difference is that isonitrile undergoes multiple insertion, not observed in CO, into transition-mctal-carbon bonds to produce bis-, tris- or poly(imino) complexes . ... 

The story of mctai-salen catalyzed olefin epoxidation began in 80s. when Kochi cl al. found that (salen)chromium(111) [2-3] and (salcn)mangancsc(lll) 4] complexes catalyze the epoxidation of unfunctionalized olefins using iodosylbcnzenc (PhlO) as terminal oxidant. Kochi was also the first one to describe the effect of the ligand substituent and donor additives (c.g. pyridinc-N-oxido) on the epoxidation, and to study the reaction kinetics and mechanism of the proposed systems (by UV-Vis). The epoxidizing species in both systems were postulated to be oxo(salen)mctal(V) species. 

The need in new inexpensive, safe and effective processes for asymmetric sulfide oxidations is determined by pharmaceutical industry requirements [38], Recently, inexpensive and active. systems based on hydrogen peroxide as oxidant and non-toxic chiral iron(III) complexes as catalysts have been reported [39-41 ]. Different mctal-salen complexes have also been previously employed as catalysts for oxidation of sulfides with PhIO Mn "(salen) [42-44], salen) 45], salen) [46], The mechanism proposed in [46] involves intermediate formation of 0x0 iron(lV)-salcn cation radical, that seems doubtful based on the experimental results obtained. In this Chapter we present asymmcinc version of the latter system [(salen )Fc ClJ/PhlO (where salcn stands for the corresponding chiral Schiff base ligands. Scheme 5) and an NMR investigation of the active intermediates. 

---