Transition-metal compounds bonding characteristics

The simplest reaction of zirconacyclopentadienes is their hydrolysis. It is characteristic for organo-early transition metal compounds the metal—carbon bond is easily hydrolyzed with acids to give free organic compounds. Similarly, deuterolysis of zirconacyclopentadienes, rather than protonolysis, affords deuterated compounds as expected (Eq. 2.9). The position of the deuterium is indicative of the position of the metal—carbon bond in the organozirconium compound. 

The low values of dissociation energy of a bond metal-carbon are characteristic particularly for alkyl transition-metal compounds. Some of them are the key intermediates of Ziegler-Natta low-pressure polymerization of alkenes. Outside the laboratory or chemical plant [43, 44], Nature gives us also important representatives of such compounds namely vitamin B12 or coenzyme B12. 

The term hydro has been used throughout in this article in preference to the more commonly used terms hydride or hydrido. These latter terms imply that the hydrogen atom bonded to a transition metal has a high electron density comparable to the saline hydrides of Groups lA and IIA. A number of hydro-transition metal compounds do, indeed, show chemical behavior characteristic of a hydridic hydrogen. However, this is in no way general, and, since it is a particularly poor assumption for the compounds of platinum, the use of the term hydro should avoid any misconception on the part of the reader not familiar with this area of chemistry. 

In this chapter, we look primarily at the main transition metals, also referred to as the d-block elements. (Note that the other transitional metals are covered in Chapter 14.) The elements we present in this chapter are important for industry and chemistry research for use as magnetic materials and catalysts. We start you out with a description of the key characteristics of the d-block elements and then explain how the filling of d-orbitals drives their chemical interactions amd bonding with other elements. At the end of this chapter, you find important information about the simple steps for naming transition metal compounds. 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

Coordination of olefins at the transition metal compounds with formation of n complexes is well documented experimentally and theoretically [199]. Whether a particular cycloolefin will coordinate first and then insert into the metal-carbon bond, or will insert directly, depends on the steric environment imposed on the cycloolefin by complexation in addition, the reactivity of the cycloolefin plays an important part. This latter characteristic also influences double-bond opening during the insertion process. Since both the structure and the reactivity of the monomer extend over a wide range (from small to... 

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. 

Low oxidation states - An important characteristic of transition metal chemistry is the formation of compounds with low (often zero or negative) oxidation states. This has little parallel outside the transition elements. Such complexes are frequently associated with ligands like carbon monoxide or alkenes. Compounds analogous to Fe(CO)s, [Ni(cod)2] (cod = 1,4-cyclooctadiene) or [Pt(PPh3]3] are very rarely encountered outside the transition-metal block. The study of the low oxidation compounds is included within organometallic chemistry. We comment about the nature of the bonding in such compounds in Chapter 6. 

Most of the above reactions occur via a mechanism involving intermediates with a metal-silicon bond (i.e. silicometallics) and a metal-hydrogen bond, accompanied (or sided) only occasionally by compounds containing metal-carbon bonds (i.e. organometallics) that are characteristic of the key intermediates of transition-metal-catalyzed transformations of organic compounds (for recent reviews, see Refs [11, 13]). 

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