Earlier Transition Metals

Schwartz to propose a radical-chain mechanism (62) accompanied by initiation processes (59)-(61). 

The oxidative addition of Bu Br, Bu I, PhCH2Cl, and PhCH2Br to the [CpMo(CO)3] ion is first order in each reagent, and reaction rates are somewhat susceptible to counterions, additives, and solvent.  

In general, iron carbonyls react with organic halides by two pathways. However [Fe(CO)3(AsPh3)2] and CCI4 follow only one route, which is believed to involve radicals since there is an induction period, no reaction in the dark, and a decrease in rate in the presence of duroquinone. On the other hand, [Fe(CO)3(PMe3)2] and Mel react only by the alternative pathway, which has no characteristics of a radical process. This reaction is first order in each reactant, A5 being -33 cal deg mol . A concerted nucleophilic displacement (two-center 5n2) is proposed as in (77), in which the first step is rate determining. 

The complexes, [(OC)4Fe(R)SiMe3], R = Me or CHzPh, have been prepared s and are of particular interest to the catalytic olefin hydrosilyla-tion process, since they undergo a reaction postulated but not previously observed directly, namely, alkylsilane elimination as in (78). 

The scope of the - change has been extended by formation of ruthenium(IV) and osmium(IV) species by oxidative addition to metal(II) compounds as in equations (79a) and (79b).  

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The carbides of Cr, Mn, Fe, Co and Ni are much more reactive than the interstitial carbides of the earlier transition metals. They are rapidly hydrolysed by dilute acid and sometimes even by water to give H2 and a mixture of hydrocarbons. For example, M3C give H2 (75%), CH4 (15%)... 

The structural and electronic factors that influence metallaboranes of the earlier transition metals.14... 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

In contrast to these reactions with the earlier transition metals, the reaction with Zn (Fig. 4) is calculated to proceed on a single singlet potential energy surface. 

The theoretical interpretation of the reactions concerning the earlier transition metals are more easily understood on Pauling s picture. Both the Pauling and the Mott and Jones interpretations can be usefully applied in reactions concerning the later transition elements. 

General reviews of transition metal complexes of alkoxides, dialkylamides122 and nitrides123 showed that the main focus of attention was on the earlier transition metal elements, in particular, the Ti, V and Cr triads. 

In contrast to the later transition metal complexes, electron-deficient complexes of the earlier transition metals, e.g., CpV(CO)2 (RC=CR) (94), are mostly inert to acetylene cyclization. Thus, bis- or trisacetylene com-... 

The polarity of the displacement reaction can be reversed where suitable reagents are available. Thus, metal halides can be treated with anionic main group fragments. This has been an important method for extending the E-M chemistry to the earlier transition metals for which fewer readily accessible metal-carbonyl complexes are known. Examples for production... 

It is somewhat surprising that Mn compounds with 02 moieties appear to have such limited existence peroxo compounds of the higher oxidation states are an important feature of the earlier transition metals (and we shall notice a few, poorly characterized, examples for Mn in Section 41.5.3) and the metals which follow manganese—Fe and Co—have an extensive and important chemistry of reversible combination with the oxygen molecule. It is perhaps pertinent to recall that some Mn11 species are very efficient catalysts for the decomposition of hydrogen peroxide (Section 41.3.3.7). 

In general, alloying in the surface alloy cases follows the trends observed in the corresponding bulk systems [31,36]. However, there are exceptions due to several factors. One of these is the crystal structure of the host (or substrate), which may play crucial role in the alloying [37], especially when the substrate is an earlier transition metal. This is, for instance, reflected in the asymmetry of the alloying behavior of A-B and B-A systems (see, for instance, W-Me and Me-W or Ta-Me and Me-Ta). 

The forward steps of H2 oxidative addition, equations (a) and (e), are usually considered to be promoted by coordinatively unsaturated metal centers in initially low oxidation states (i.e., high metal basicity). Loss of electrons via the oxidation is compensated for by a gain of electrons through an increase in the coordination number . This also explains why the earlier transition metal systems (d -d ) tend to form complexes of higher coordination number than those of the later d -d systems (especially Group... 

When earlier transition metals are used in the cofacial DPX and DPD porphyrin motifs, oxidation reactions are promoted by a metal 0x0 intermediate. As for reductive catalysis, the oxidative catalytic reactions of the Pacman porphyrins are derived from PCET. The catalase reaction (H2O2 disproportionation to give 0.5 O2 and H2O) is exemplary of this PCET reactivity as the reaction includes both 0 0 bond making and bond-breaking catalysis coupled to proton transport. 

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