Silicon-Transition-Metal Bond

II. Organometallic Complexes with Silicon-Transition Metal Bonds.. 254... 

ORGANOMETALLIC COMPLEXES WITH SILICON-TRANSITION METAL BONDS... 

No new silicon-transition metal bonds are formed in these reactions, although new compounds containing such bonds are produced. Details are in Section II,C,l-3. 

It has been noted (Section II,B,1) that reactions between transition metal carbonyl anions and silicon halides often fail to produce species containing silicon-transition metal bonds, and that such failure has been ascribed to nucleophilic attack by carbonyl oxygen. It is therefore interesting that compounds containing Si—O—C—transition metal linkages have recently been isolated from such reactions [Eqs. (105) (R = Me, Ph) 183) and (106)... 

The chromium complexes are proved to be silanediyl complexes, as shown by the silicon-transition metal bond lengths (Table 5) and by the extreme low field shift of the 29Si NMR signals (124.9 and 121.2) at 22 °C for R = H and CH2NMe2, respectively (Table 6). The 29Si NMR shifts of these complexes are temperature-dependent due to the hindered rotation of the phenyl ring and dynamic coordination of the nitrogens to the Si atom. 

It is possible to selectively cleave a Si—C(Cp )bond even in the presence of silicon-transition metal bonds as shown in reactions with nucleophiles, electrophiles and chlorinated hydrocarbons (equations 41 and 42)92,93. 

Compounds with silicon-transition-metal bonds fall into two distinct classes, which until recently showed no points of connection. The first class comprises the metal silicides typical binary examples are FeSi,... 

The second class consists of molecular compounds containing one or more silicon-transition-metal bonds. The first example, Me3SiFe(CO)2-(tjs-CbHb), was prepared in 1956 (359), but nearly 10 years then elapsed before other compounds were described (26, 94). These heralded many more, and now examples are known in which silicon is bonded to almost every transition metal (Fig. 1). Curiously, no Si-Ag compounds have been described there are also no reports of derivatives of lanthanides or actinides. Most work has involved Fe, Co, Pt, Mn, Re, Mo, Ru, and Ni, in roughly decreasing order of frequency. Almost all well-characterized molecular silicon-transition-metal compounds known at present are diamagnetic some possible exceptions are noted in Section II,F. The most recent comprehensive reviews of the area were published in 1973 (134) and 1974 (235), covering the literature until 1971 and 1972, respectively these contain details of earlier reviews. Other surveys of certain aspects have also appeared, two of them very recently (24, 25, 201). 

The second harmful effect of tetrahydrofuran is that it may undergo ring opening and insert into the silicon-transition-metal bond, again with the formation of a siloxy derivative (252, 336, 337) e.g ... 

Table VI lists various ways in which the elimination of small molecules has been used to produce silicon-transition-metal bonds most can be pictured as proceeding via consecutive processes of oxidative addition and reductive elimination. Dihydrogen may result from reaction between compounds with M-H and Si-H bonds (entries 1-10).

Breaking of silicon-transition-metal bonds by mercury compounds is one aspect of the well-known "conversion series in organosilicon chemistry (22, 24, 154). It was established at an early stage that equilibrium in the system... 

When the silicon-transition-metal bond is reasonably strong, hydrogen attached to the metal may be replaced (mode 4b in Fig. 2) by halogens (entries 26,27, and 30) or deuterium (entry 29). In the case of the ruthenium example, halogenation can be followed by reductive elimination of RsSiH (226). 

Up to the end of 1972, more than 350 compounds containing silicon-transition-metal bonds had been synthesized, but their distribution among the d-block elements is by no means uniform. It can be inferred that the stability of such complexes decreases from right to left in a transition series and with increasing oxidation state of the transition metal. Nevertheless, it is remarkable that no silyl derivatives of the vanadium group have been prepared and that there are few nickel or palladium derivatives, as can be seen in the complete list of compounds given in Section 6. 

There have been numerous spectroscopic and structural studies on the properties of the silicon-transitions metal bond. Infrared studies on the substituents of the Si and M centers were the usual starting point, so these will be the first to be discussed. 

A variety of metallic silicides are obtained by melting silicon with a transition metal. Condensation of a transition-metal vapor with a silicon compound, or silicon vapor with a transition-metal complex has not so far yielded a species with a discrete silicon-transition-metal bond. 

This route provides a clean synthesis of silicon-transition-metal bonds because dihydrogen is eliminated as the sole side product. 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

Scheme 60. Possible pathways for the electrophilic cleavage of silicon-transition metal bonds.

Reviews, which include coverage of diffraction studies of organometallic complexes containing silicon-transition metal bonds, have appeared in 1973380 and 1982381. The vast majority of structural characterizations are for tne later transition metals (groups... 

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