Lithium coordination number

The general intransigence of the crystalline iminolithium hexamers toward further interaction with Lewis bases is not shown by the amorphous diaryliminolithiums. This may reflect their extensively stacked nature (Section II,A Fig. 10), which, while raising the lithium coordination number to four in all but the outer rings of the polymer, will presumably weaken individual N—Li bonds. These materials dissolve quite readily in several polar solvents, e.g., THF (66), pyridine (66, 78, 85), and HMPA (86). Crystalline complexes can be recovered from these solutions. Two of these, both derivatives of (Ph2C=NLi) , have been characterized structurally in the solid state. The tetrameric cubane (Ph2C=NLipyridine)4 (7) is depicted in Fig. 14 (78, 85). 

Self-organization as a result of the formation of amide-bridged dimers is not limited to rfiorganoamides of the alkali metals. Occasionally alkah metal derivatives of primary amines have been shown to be dimeric in the solid state. Structurally characterized examples are [LiNHPh(thf)2]2, 79a [77a], [LiNHC6F5(thf)2]2, 79b, and [K NHC6H2(CF3)3-2,4,6 (thf)3]2, 105 [77b]. These three examples nicely illustrate how the composition of the solvates depends on the size of the alkali metal ion. In 79a and 79b two additional THF ligands are sufficient to saturate the coordination sphere of lithium (coordination number 4), whereas the potassium ions in 105 are coordinated by three THF ligands and by two weak K- - F interactions. 

The structure of LiTa02F2, as reported by Vlasse et al. [218], is similar to a ReC>3 type structure and consists of triple layers of octahedrons linked together through their vertexes. The layers are perpendicular to the c axis, and each layer is shifted, relative to the layer below, by half a cell in the direction (110). Lithium atoms are situated in the centers of the tetragonal pyramids (coordination number = 5). The other lithium atoms are statistically distributed along with tantalum atoms (coordination number = 6) at a ratio of 1 3. The sequence of the metal atoms in alternating layers is (Ta-Li) - Ta - (Ta-Li). Positions of oxygen and fluorine atoms were not determined. The main interatomic distances are (in A) Ta-(0, F) - 1.845-2.114 Li-(0, F) - 2.087-2.048 (O, F)-(0,F) - 2.717-2.844. 

Other alkali ions (except lithium) also probably have the coordination Number 8 as a rule, and should similarly have a tendency to a 1 1 ratio with aluminum this is shown in NaAlSi3Os, albite, H2Na2Al2Si30i2, natro-lite, H2Cs4Al4Si9027, pollucite, etc. 

Interestingly, the nonpolyether-type polymer electrolyte 7 showed a relatively high lithium transference number of 0.47 in the presence of LiTFSI. This is possibly due to the absence of strong binding of ether oxygen to the lithium cation. Moreover, anion trapping of the boron atom is not retarded by coordination of oxygen to the... 

Four equivalents of lithium phenylacetylide reacted with bis bis(trimethylsilyl)amido zinc (Scheme 51) to form the ion-paired dilithiotetra(phenylacetylido)zincate 65, whose structure is shown in Figure 33.121 The zinc-carbon bonds (2.05 A) are somewhat longer than those observed in the tri(phenylacetylido)zincate 62a, due to the increase in both the coordination number and the negative charge on the ion. Each lithium ion is associated with the zincate ion through coordination to two of the triple bonds. 

For example, atoms of both the alkaline-earth family (ZAval = 2) and the chalcogen family (ZAval = 6) correspond to FAemp = 2, and their stoichiometric proportionality (or coordination number) to monovalent atoms is therefore commonly two (AH2, ALi2, AF2, etc.). It is a remarkable and characteristic feature of chemical periodicity that the empirical valency FAemp applies both to covalent and to ionic limits of bonding, so that, e.g., the monovalency of lithium (Vuemp = 1) correctly predicts the stoichiometry and coordination number of covalent (e.g., Li2), polar covalent (e.g., LiH), and extreme ionic (e.g., LiF) molecules. Following Musher,132 we can therefore describe hypervalency as referring to cases in which the apparent valency FA exceeds the normal empirical valency (3.184),... 

Although much of the V NMR has been performed on model systems or catalytic materials containing vanadium, 29 >30 compounds such as V2O5 or VOPO4 are used in both the catalysis and lithium battery fields, and many of the results can be used to help elucidate the structures of vanadium-containing cathode materials. V NMR spectra are sensitive to changes in the vanadium coordination number and distortions of the vanadium local environments from regular tetrahedra or octahedra. >33 5>V isotropic chemical shifts of between —400 and —800 ppm are seen for vanadium oxides, and unfortunately, unlike... 

The 778 present lithium organic structures in the CSD contain 3228 Li—C contacts. This means that every lithium atom either forms a multiple contact to the related carbanion or that most of the structures dimerize around the metal or both. On average, every lithium shows four Li—C contacts and certainly it is not just coincidence that this number is identical to the favorite coordination number of lithium in a molecular environment. 

The coordination number is deduced from the fact that six lithium atoms are required to dissolve one nitrogen. 

With such high coordination numbers it is quite clear that there can be no possibility of covalency, because there are insufficient numbers of electrons. The difficulty is shown in the case of metallic lithium, with its body-centred cubic structure and coordination number of 14. Each lithium atom has one valency electron and for each atom to participate in 14 covalent bonds is quite impossible. 

There are, however, other exceptions that are difficult to attribute to directional covalent bonds. The heavier lithium halides only marginally obey the rule, and perhaps a case couk) be made for C.N. — 4 for lil (Fig. 4.18). Much more serious, however, is the prohlcm of coordination number 6 versus 8- The relative lack of eight-coordinate structures—CsQ, CsBr, and Csl being the only known alkali metal examples—is commonly found, if hard to explain. There are no eight-coordinate... 

Rg. 4.18 Actual crystal structures of the alkali halides (as shown by the symbols) contrasted with the predictions cl the radius ratio rule. Tie figure is divided into three regions by the lines rjr. 0.414 and r+/h- a 0.732, predicting coordination number 4 (wurizite or zinc blende, upper left), coordination number 6 (rock salt, NaCl, middle), and coordination number 8 (CsCI, lower right). The crystal radius of lithium, and to a lesser extent that of sodium, changes with coordination number, so both ihe radii with C.N. 4 (left) and C.N = 6 Iright) have been plotted. 

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