Hypervalent bridges

The azide Me3PbN3 forms a linear chain polymer with a p2-N atom symmetrically hypervalent bridging the pentacoordinate lead centers (N—Pb 2.54 A). The N—Pb—N angle (178.6°) deviates only slightly from the ideal value of 180°77,78. The carbodiimide Me3SnNCNSnMe3 forms an infinite helical network, too79. 

Neutral compounds of heavy group 14 elements having donor atoms in monodentate ligands can be tetrahedral in the solution. However, in this case the environment for the element in the solid state can be five-coordinate due to hypervalent bridges. Selfassociation can be also observed for pentacoordinate species when the donor atom forms a unidentate bridge across the six-coordinate centers. The self-association is also characteristic of anionic species (Sections IV.A and IX.A). For a tin atom the intermolecular coordination is observed very frequently . As a rule it results in a polymeric structure mainly via the oxygen atom . However, dimers and other simple cyclic hypervalent structures are formed only seldom. 

Although most of the fundamental studies of cycl[3.2.2]azines were reported in CHEC(1984) (see Section 12.16.6.3 for leading references), there is continuing interest - synthetic, spectroscopic, and theoretical - in these and their benzo- and dibenzo-fused analogues, all of which may be considered as bridged analogues of [10]-, [14]-, and [18]-annulenes, respectively. The same level of theoretical interest continues to apply to those />m -fused systems with a hypervalent sulfur or selenium at the 5 5 ring junction (Section 12.16.6.6). 

Chloro(diethylamino)dimethyltin consists of discrete dimer molecules 1, with Sn atoms linked by bridging diethylamino groups. The coordination geometry about the metal atom is a distorted trigonal bipyramid with two C atoms and one N atom in the equatorial plane, and Cl and the second N atom in the axial hypervalent bonding80. 

TABLE 3. Selected bond distances and angles of the bridge hypervalent unit for pyridoxine (PN) tin dimers131... 

Compounds having the bridged tetrahedal structural type are listed in Table XVII. Also listed in the table are EM4 molecules that have a similar edge-bridged tetrahedral core. The examples known are tetrametal systems with a PR group (50). Like the E2M3 described previously, compounds 50 have eight skeletal electron pairs. Their electron distributions obey the 18-electron rule at the metals however, to do this the phosphorus atoms become 10-electron centers and are formally hypervalent. 

Hypervalent Cl Sn—Cl bridges are also typical for pentacoordinate tin atoms. For example, the crystal structure of 20 and 21 comprise dimeric pairs of molecules bridged by weak intermolecular Cl Sn interactions. The Cl —Sn distances in 20 (2.475 A ), 21 (average 3.869 A ) and in MeSn(Ar)Cl2 (Ar = 3-methyl-4-nitropyridine-N-oxide) (3.934 A ) are longer than in most comparable structures The Sn NMR resonance (—285.7 ppm) for 21 suggests that it possesses a five-coordinate structure in solution . 

Hydrogen bonds contain linear 4e,3-center E H M bridge bonds with hypervalent hydrogen, while sigma complexes contain bent 2e,3-center E-H-M bridge bonds with electron-deficient hydrogen. In the former, the metal acts as a weak base, while in the latter it acts as a weak acid. In principle, intermediate situations are possible where both interactions compete but information is still sparse in this area. 

Three-center Three-center bonds are necessary for the description of some molecules. Bridge bonds in bonding diborane are of the three-center two-electron type, whereas three-center four-electron bonding provides an explanation of some hypervalent molecules. 

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