2-electron, 3-center bond

G. Cyclopentadienyl Complexes and Aryl Complexes Containing 3-Center 2-Electron Bonds... 

Bridged silylene complexes are the subject of a recent comprehensive review by Ogino and Tobita338. These complexes can be classified into three types A, B and C (Scheme 9). In type A complexes there is no metal-metal bonding, the silicon is essentially tetravalent, and the bonding is similar to that in mononuclear metal-silyl complexes. In type C complexes, the bonding is best described as /j2-coordination of the Si—H bond to the metal, or alternatively as a metal-hydrogen-silicon 3-center 2-electron bond. 

Fig. 37. The structure of HW2(CO)9(NO) as determined by neutron diffraction (Ref. 23a). Note that the axial ligand-metal vectors do not point at the bridging H atom, but at the center of the W-H-W triangle. This observation was taken as evidence that there is significant metal-metal overlap in a M-H-M 3-center-2-electron bond. The structure shown here illustrates one advantage of using metal complexes to study electron-deficient bonding the ML moiety on each metal atom serves as a convenient coordinate system to pinpoint the direction of the orbital used by tungsten to participate in W-H-W overlap...

XIX), which is consistent with the presence of closed 3-center/2-electron bonding... 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

One of the most significant changes over existing systems is the introduction of a selective use of non-integer bonds directly into the nomenclature. Not only does such an introduction subsume the underlying concepts sometimes expressed as "half-bond" (3 center 2 electron bond) structures in the boranes, as well as "bond and a half (Robinson) ring structures in aromatic compounds, etc., but also this approach points the way... 

Dialkyl-2,3-dihydro-1,3-diboroles attract attention because of their unusual ligand properties. If an H atom is not removed as discussed in the preceding Section the C-2 H moiety can be used in addition to the C=C double bond for a special kind of metal coordination <830M1666, 85AG(E)943, 87ZN(B)186>. As indicated in structures (23)-(25) different 3-center-2-electron bonds can in principle... 

Type (23) represents an agostic M <- H interaction, whereas (24) describes an M - C—H bonding situation using the axial C—H moiety. In (25) a part of a 4-center-4-electron bond is shown, consisting of the 3-center-2-electron bonds B <- H—C and B—M—C, comparable to the bonding situation in BjHs. Some representative examples are mentioned in the following. 

As can be seen from (28) a hydrogen bridge between B-1 and C-2 is formed. It can be described as a 3-center-2-electron bond or, including the Co atom, as a 4-center-4-electron bond, leading to an elongation of the B-1—C-2 bond distance compared with B-3—C-2 and simultaneously to a shortening of the B-1—Co distance. 

Given the orbital diagram in Fig. 3-16c, there are two important electron distributions, namely, 2-electron and 4-electron populations. If the three atoms, between them, supply only two electrons, these will occupy the bonding orbital, 0b. Thus two electrons will serve to unite three atoms. Since the orbital 0b is made up of 0t, 02, and 03, electrons occupying it are spread fairly uniformly over the three atomic centers, thus leading to a basically non-polar distribution. This situation, in which three centers are united by two paired electrons is called 3-center, 2-electron bonding, abbreviated... 

Bridging hydride and alkyl compounds bear a close analogy to other molecules that possess 3-center-2-electron bonds, such as transition metal dihydrogen complexes [M 7] -H2)] and alkane cr-complexes [M(t7 -RH)]. For the... 

The half arrow notation is not restricted to M-H-M interactions, but applies equally well to other complexes that exhibit 3-center-2-electron bonds, such as those involving coordination of (i) G-H and B-H bonds, e.g. agostic alkyl complexes" and borohydride derivatives," and (ii) M-G bonds, e.g. symmetrically bridging methyl ligands. 

M-M antibonding orbital. The resulting interaction has been termed an antibond ,and may be depicted by using a crossed-dashed M - x - -M representation in which the cross is intended to emphasize that the direct interaction is antibonding Despite the presence of an antibond , the overall interaction between the two metal centers is favorable because of the stabilization resulting from the formation of the 3-center-2-electron bond. ... 

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