Divalent multiple bonds

Iodomethylzinc iodide is often refened to as a carbenoid, meaning that it resembles a carbene in its chemical reactions. Caibenes are neutral molecules in which one of the caibon atoms has six valence electrons. Such caibons aie divalent they are directly bonded to only two other atoms and have no multiple bonds. Iodomethylzinc iodide reacts as if it were a source of the caibene H—C—H. 

The five-membered rings in 122 and 124 are planar, while the ring in 123 is necessarily puckered. The N—C distances in the ring are shorter in 122 and 124 than in 123 by 10 pm. The Si—N bond lengths are similar to the normal Si—N single bond distances ( 172-174 pm). However, bonds to a divalent silicon atom are predicted to be longer, by 8 pm, than those to a tetravalent Si. The bond lengths in the silylenes mentioned above are, therefore, consistent with some multiple bond character in the Si—N bonds. 

The reluctance of silicon to form multiple bonds, its high divalent state stabilization, its preference for bridged bonding and other factors result in the existence of compounds which have no stable carbon analogues. 

We have not yet completed our overview of multiple bonding, however. Carbon and hydrogen can form yet another compound, acetylene, in which each carbon is connected to only two other atoms a carbon and a hydrogen. This can be regarded as an example of divalent carbon, but is usually rationalized by writing a triple bond between the two carbon atoms. 

Equation (19) is related to the 1,4-addition reactions reported for divalent Group IV analogs of carhene in the preparation of five-membered heterocycles containing a multiple-bond j8 to the heteroatom [see Eq. (7)]. If a method is developed for the generation of organometallic carhene analogs, reactions with trienes may provide a convenient route to seven-membered ring dienes. 

With the valence electron configuration s p2 the nonmetallic elements of group 14 can form compounds with four tetrahedrally directed covalent bonds. Only carbon forms strong multiple bonds, and its compounds show many differences in structure and properties from those of Si and Ge. Like the metallic elements of the group (Sn and Pb), germanium has some stable divalent compounds. 

The organic compound ethyne, C2H2, combines hybridisation with multiple bond formation. The formula is conventionally drawn as HC=CH, in which three bonds from each quadrivalent carbon atom link to another carbon atom and the other bond links to hydrogen. Because there are two p electrons available on carbon it would be possible to write down a bonding scheme involving only cr bonds, one between the two carbons and one between a carbon and a hydrogen atom. However, this is not in accord with the properties of the molecule. First, the carbon atom so described would be divalent not quadrivalent. Second, experiments show that the... 

TM=tetrel complexes (1) W. Petz, Chem. Rea, 86, 1019 (1986). Transition Metal Complexes with Derivatives of Divalent Silicon, Germanium, Tin, and Lead as Ligands, (m) W. A. Herrmann, Angew. Chem., Int. Ed. Engl., 25, 56 (1986). Multiple Bonds Between Transition Metals and Bare Main Group Elements Links Between Inorganic Solid State Chemistry and Organometallic Chemistry. 

The relative high stabilities of the pyramidal, bridged and silylenic structures in the case of SisHs result from several electronic effects typical of silicon, which were already discussed in detail in this review, such as the reluctance to form multiple bonds, high divalent state stabilization, preference for bridged bonding etc. These effects lead in the case of Si5H5+, as in many other examples discussed in this chapter, to stable three-dimensional structures which are not minima on the PES of the analogous carbon compounds. 

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