Coordinatively unsaturated metal fragments

The catalytic cycle for hydroboration is now widely accepted and direct examples of several intermediate species have been isolated and well characterized (Scheme 3).5-7 These now include (j-borane complexes, which have in some instances been found to be catalytic precursors for hydroboration.8-10 Oxidative addition of an H—B bond to a coordinatively unsaturated metal fragment... 

C(E)R] This method was extended by Wojcicki and co-workers 110,111) to utilize coordinatively unsaturated metal fragments as electrophiles, whereby new synthetic methods for homo- and heterobinuclear and trinuclear metal-/u--allenyl complexes were developed. The difference between this approach and that of Stone and co-workers (89,90) is that the unsaturation is contained entirely within the ligand, which, as a result of this, often rearranges. Complex 28 was obtained in high yields from the reaction of the iron-propargyl precursor and diiron nonacarbonyl (111). 

The complexes NMe4[M(CO)4(COMe)(COPh)] (M = Mn, Re) decompose thermally to give CO and PhCOMe initial decarbonylation gives NMe4[M(CO)4 (Ph)(COMe)] followed by reductive elimination. The coordinatively unsaturated metal fragment is then trapped by phosphine present in the reaction system... 

The reverse reaction will be discussed in section 8.3. An a-carbon of vinyli-dene ligand, generated from 1,2-hydrogen shift of terminal alkyne upon contact with coordinatively unsaturated metal fragment, is also susceptible to the nucleophilic attack (Scheme 8.17). This attack is believed to constitute a catalytic cycle for anti-Markownikov addition of NuH (Nu = RO, RCOO, etc.) to alkynes [4c]. Schrock carbenes are sensitive to electrophiles (see 8.3.1), but not to nucleophiles. 

Monomeric sulfur diimides have an extensive coordination chemistry as might be anticipated from the availability of three potential donor sites and two r-bonds. In addition, they are prone to fragmentation to produce thionitroso and, subsequently, sulfido and imido ligands. Under mild conditions with suitable coordinatively unsaturated metal... 

In fact, it is just the unsaturated central double bond of the pyracylene unit which commonly becomes the reactive site for coordination to metal fragments. This is the case not only for [( 2-C6O) (K20,0)-Os(O4)(4-terf-butylpyridine) ], but for example also for C60[Pt(PPh3)2]25 and [Ir(CO)( 2-C6o)Cl(PPh3)2].26 In both compounds the C1-C2 bond length ( 1.50 A) is considerably longer than the value (1.38 A) that the 6 6 C = C bond should have in the absence of coordination (in the strictly related [Pd( 2-C60)(PPh3)2] the C1-C2 distance is 1.45 A27). At variance with [O72-C6o) (K20,0)-Os(O4)(4-terr-butylpyridine) ], in the latter cases the metal atom is directly linked to the fullerene unit. 

As seen, fullerenes behave as unsaturated ligands. Hence it is expected that coordination to metal fragments might saturate (at least partially) their electron-affinity. Hence it is expected that the electron-withdrawing ability of fullerenes is lowered after complexation with respect to that in their free state. 

A coordination chemist might prefer a formula like B and consider the X—Y metal fragment. However, someone engaged in the chemistry of either element X or Y might look at it in a different way. Formula B stresses the chemical origin of most of these compounds, which are usually obtained by addition of X—Y to a coordina-tively unsaturated metal fragment [Eq. (1)]. 

Thiolate donors in [M(S )] fragments with coordinatively unsaturated metal centers may stabilize these metal centers by either n donation or formation of M—S—M bridges. 

Some coordinatively unsaturated metal dimers are not proposed to contain multiple bonds, for example, CoRh(CO)7. This complex can be viewed as an analog of Co2(CO)g in which one CO ligand has dissociated from the complex and the heavier congener Rh has replaced one of the Co atoms. The degree of interaction between the two metals is not clear in spite of the known stracture. This complex can fragment, but can also undergo addition reactions at the metal-metal bond, as shown in equations (21) and (22) respectively. 

A premiere area of organotransition metal chemistry is the scission and formation of C—C, C—H, and H—H bonds of organic substrates. These transformations have been demonstrated to be accessible through the use of coordinatively unsaturated organometallic fragments as shown in Eq. (1), where R, R = alkyl, aryl, H, etc. Mechanistic aspects of this reaction have still not been fully appreciated. 

Many transition metal cluster compounds are metal carbonyl derivatives. Many metal carbonyl derivatives lose carbon monoxide upon pyrolysis or photolysis. If these decomposition processes are controlled, a reactive coordinatively unsaturated metal carbonyl fragment which oligomerizes to a metal carbonyl cluster may be obtained, as exemplified by the following syntheses of metal carbonyl cluster compounds discussed later in this chapter, on pages 399, 396, and 375, respectively. 

Mixed metal carbonyl clusters may sometimes be obtained by cooligomerization of different coordinatively unsaturated metal carbonyl fragments, for example,... 

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