Sodium delocalized bonding

Metals are extreme examples of delocalized bonding. A sodium metal crystal, for example, can be regarded as an array of Na ions surrounded by a sea of electrons (Figure 9.18). The valence, or bonding, electrons are delocalized over the entire metal crystal. The freedom of these electrons to move throughout the crystal is responsible for the electrical conductivity of a metal. 

The concept of electrons not belonging to any particular atom in a molecule brings us back to resonance structures. The electrons in a metal are also delocalized. An electron in a bar of sodium is not associated with any particular atom, just as the electrons in the double bonds of benzene are not associated with any particular atom. 

The pseudohalogen concept can be extended (i) to some special nonplanar anions such as CF3, (ii) to heavier elements (isovalence electronic exchange of O by S, Se, Te of N by P etc. in SCN , SeCN , TeCN , P(CN)2 ) " and (iii) to derivatives such as the five-membered ring anion [CS2N3] which can be obtained in the reaction of carbon disulfide and sodium azide as shown by Sommer as early as 1915. The introduction of heavier elements, however, results in weaker delocalization effects due to the formation of weaker tt-bonds. 

Adducts have also been obtained by the reaction of methylmagnesium iodide with 3,5-dicyanopyridine and related substrates.138 Their formation involves a shift in the IR spectrum from 1563-1575 cm 1 to 1612-1645 cm"1, for C=C bonds, and from 2230-2248 cm 1 to 2125-2225 cm for the C=N bond, the final values being near the absorbance of the dihydro derivatives. Hydrolysis yields the expected dihydro derivatives. In connection with the nature of metal-nitrogen bond, it is of interest that in the sodium adduct 94 the IR spectrum indicates appreciable electron delocalization relative to the corresponding dihydro derivative (shift toward lower frequency), which suggests a substantial ionic character of the bond due to the low electronegativity of sodium. 

This picture, by the way, finds a most important application in the description of bonding in metals. Take, for instance, sodium, with one electron per atom for four orbitals. It is quite clear that here there is a vast excess of orbitals over electron pairs, and electrons in solid sodium, as in all metals, are effectively delocalized over the whole metal crystal. This idea can account satisfactorily for the electrical conductivity of metals. A more detailed discussion of metals would go beyond the space available here. 

If you try to draw an electron-dot structure for a metal, you ll quickly realize that there aren t enough valence electrons available to form an electron-pair bond between every pair of adjacent atoms. Sodium, for example, which has just one valence electron per atom (3s1), crystallizes in a body-centered cubic structure in which each Na atom is surrounded by eight nearest neighbors (Section 10.8). Consequently, the valence electrons can t be localized in a bond between any particular pair of atoms. Instead, they are delocalized and belong to the crystal as a whole. 

A simple alternative model, consistent with band theory, is the electron sea concept illustrated in Fig. 9-22 for sodium. The circles represent the sodium ions which occupy regular lattice positions (the second and fourth lines of atoms are in a plane below the first and third). The eleventh electron from each atom is broadly delocalized so that the space between sodium ions is filled with an electron sea of sufficient density to keep the crystal electrically neutral. The massive ions vibrate about the nominal positions in the electron sea, which holds them in place something like cherries in a bowl of gelatin. This model successfully accounts for the unusual properties of metals, such as the electrical conductivity and mechanical toughness. In many metals, particularly the transition elements, the picture is more complicated, with some electrons participating in local bonding in addition to the delocalized electrons. 

These free electrons give sodium and the other metals their high electrical conductivity. Pump in an electron at one end of a metal wire, and another electron from an almost identical orbital pops out at the other end. The delocalized electrons of the metallic bond ensure that little energy is required for this process, making metals highly conductive and the preferred material for power lines. It also led the renowned materials scientist Sir Alan Cottrell to propose a new definition of a metal Metals, he wrote in a 1960 article, contain free electrons. ... 

The mechanism of dissolving metal reductions depends on the nature of the solvent and the nature of the substrate. The proposed mechanism for the reduction of dialkylacetylenes by sodium in HMPA in the presence of a proton donor is illustrated in equation (18). The addition of an electron to the triple bond of (45) is proposed to produce the rran -sodiovinyl radical (46), or the corresponding radical anion (47), which undergoes protonation by the added alcohol to produce the radical (48). Further reduction of (48) by sodium produces the rrans-sodiovinyl compound (49), which on protonation produces the trans-a -kene (50). In the absence of a proton donor, the reduction of (45) with sodium in HMPA results in the formation of a mixture of cis- and trans-2- and 3-hexenes. Control studies showed that the isomerization products 2- and 3-hexene are not formed by rearrangement of the cis- or frans-3-hexenes. It was concluded that the starting alkyne (45) acts as a reversible proton donor reacting with an intermediate anion or radical anion to produce the delocalized anion (51) which is then protonated to produce the al-lene (52). Reduction of the allene (52), or further rearrangement to the alkyne (53) followed by reduction, then leads to the formation of the mixture of the cis- and trans-2- and 3-hexenes (equation 19). ... 

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