Transition metal complexes compounds formed with

Transition metal complexes can form with coordination numbers ranging from 2 to 9. They display such a diverse and interesting chemistry with a large range of possible compounds. We cover more detail on these compounds in Chapter 15 that deals with organometallic compounds. 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Recently the oxidative coupling of olefins and alkynes, not only with late transition metal complexes, but also with low valent early transition metal complexes to give metallacyclopentane or metallacyclopentadiene complexes is attracting increasing attention. For example, titanium(II) and zirconium(II) complexes react with olefins and form metallacycles. Various stoichiometric processes to produce useful compounds that are otherwise unavailable can be produced by exploit-... 

Transition metal complexes can interact with the DNA biomolecule either covalently, as with c/i-platin, or noncovalently, when coordinatively saturated octahedral [Ru (dimine)3] " complexes or related are employed. The latter exists in two enantiomeric forms designated as the A and A optical isomers (Figure 4.15). In solution at room temperature they are configurationally stable and kinetically inert to ligand substitution. Due to their geometry these compounds are ideally suited for DNA binding studies. There are three kinds of noncovalent interactions ... 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Ziegler-Natta catalysts-—there are many different formulations—are organometallic transition-metal complexes prepared by treatment of an alkyl-aluminum with a titanium compound. Triethylaluminum and titanium tetrachloride form a typical preparation. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

This chapter is concerned with the simplest reactions of inert transition metal complexes. Fig. 1 shows a typical compound. This is Co(III) coordinated to six NHj molecules to form a triply positive cation [Co(NH3)6]. It is indicated in Fig. 1 to be in aqueous solution where water molecules occupy positions in what... 

Molecular Metal Complexes Compounds of this type do not form delocalized electronic bands in the sohd state, and their color is due to intramolecular electronic transitions. Many complexes of transition metals with organic ligands belong to this class. complexes with phenanthroline (red/colorless) and Ru + + with 2,2 -... 

Certain transition metal complexes may act like carbenes, and give three-membered metallocycles with ADC compounds.74 For example, complexes 34 and 35 are readily formed. The carbene analogy also extends to the formation of 1,4-addition products (e.g., 36)7 5... 

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

A kinetically stabilized diarylgermylene, Tb(Tip)Ge, is also stable in hexane solution with no tendency to dimerize. The structure has not yet been measured but it was characterized as the base-free mononuclear transition metal complex formed with the reactive M(CO)s. THF adduct (M = Mo,W). The crystal structure of the W complex466 shows Ge=W = 259.3 pm, Ge-Tb = 198.8 pm, Ge-Tip = 199.9 pm, CGeC = 108.4°, TbGeW = 138.9° and TipGeW = 112.2°. Thus the Ge is pyramidal and the structure is obviously the result of a balance of steric repulsions between the carbonyls, Tb, and Tip. Older work on similar Ge=W compounds, RR,Ge=W(CO)5 with RR = Cp (Cl) or (Bsi)2467, show Ge=W lengths of 251.1 and 263.2 pm, respectively, indicating the combined steric and electronic effects. 

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