Tetrameric units

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

The structure and bonding in lithium methyl have been particularly fully studied. The crystal structure consists of interconnected tetrameric units (LiMe)4 as shown in Fig. 4.17 the individual Li4C4 clusters consist of a tetrahedron... 

Figure 16.13 Structures of some tetrahalides of Se and Te (a) Sep4 (gas), (b) crystalline Sep4, and schematic representation of the association of the pseudo-tbp molecules (see text), (c) coordination environment of Te in crystalline Tep4 and schematic representation of the polymerized square pyramidal units, (d) the tetrameric unit in crystalline (TeCl4)4, and (e) two representations of the tetrameric molecules in Te4li6 showing the shared edges of the Telg octahedral subunits.

Some immunoglobulins such as immune IgG exist only in the basic tetrameric structure, while others such as IgA and IgM can exist as higher order polymers of two, three (IgA), or five (IgM) tetrameric units (Figure 50-10). 

The bulky silanediol TsiSi(0H)202CCF3 also forms tetrameric units... 

The mononuclear hexahalochalcogenate anion EX62 (E = Se, Te X = Cl, Br, I) usually adopts a nearly ideal octahedral geometry, and has been widely reported (formally it can be considered as the end product of the nucleophilic disintegration of the tetrameric units of (EX4)4, see Figure 8). Also commonly studied are the dinuclear and trinuclear chloro- and bromo-chalcogenates(IV). Attention here will therefore be focused on the relatively rare multinuclear... 

Sn—C is normal, RSnR = 125°-135°, Sn-Cl distances are 236-240 pm while Sn... Cl is around 350 pm. Both coordinations are linked into zigzag chains which are linear at Sn (Cl—Sn... Cl 170-175°) and bent at Cl (Sn—Cl... Sn around 105°). The cyclohexyl-bromide shows similar features (Sn-Br = 250 pm Sn- -Br = 377 pm). While the long Sn- -Cl or Br distances are only 10% less than the van der Waals radii, the interpretation in terms of very weak additional coordination seems justified. By comparison, in truly 4-coordinate (biphenylyl)2SnCl2, Sn-Cl = 238.6 pm and all the intermolecular Sn/Cl distances exceed 600 pm, and in Et2Snl2 the extramolecular Sn/I is 428 pm, distinctly outside the van der Waals limit. In the end the matter is one of interpretation175. When R2 = Ph2 or MePh, tetrameric units are found containing both 5- and 6-coordinate Sn. 

The RuF5-t q)e elucidated by Holloway, Peacock and SmaU (150) is a distorted version of the MoFs-stracture. The tetramere units Me4F2o are maintained the 4-membered ring of Ru-atoms is distorted to form a rhombus, however, and the Ru—F—Ru-bridges are no longer hnear (127° and 137° resp.). Similarly tilted rings are found in the stracture of the trifluoride RuFa (139), only that they are further poljmierized to form a three-dimensional framework (page 39). 

Fig. 6.15 The structure of solid XcF. Each Xe atom sits at We base of a square pyramid of five fluorine atoms. The bridging fluorine atoms are shown as larger circles, (a) The tetrameric unit with the Xe atoms forming a tetrahedron, (b) the bexamenc unit with the Xe atoms forming an octahedron, (c) the Cu Au structure The shaded circles represent the octahedral dusters and the open cirdes the tetrahedral dusters, tin part from Burbank R. E>. Jones. O. R. J. Am. Chem. See. 1974.96.43-48. Used with permission.]...

Fig. 7.7 Stereoview of the tetrameric unit of RhoFju. The rhodium atoms are at the centers of Ihe octahedra of fluorine atoms. Note bridging fluorine atoms. [From Morrell, B. K. Zalkin. A. Tressaud, A. Bartlett, N. Ircrg. Chem. 1973, 12, 2640-2644. Reproduced with permission.]...

A related complex is the dithioacetate [(AuS2CMe)4] prepared from NafAuCLt] and MeCS2H. This forms a tetrameric unit (22) in which all four Au—Au bond distances are 301 pm.282... 

The benzyl and o-xylyl compounds [NaCH2Ph.TMEDA]432 and [NaCH2 C6H4Me.TMEDA]430 form tetrameric units in the solid state. A tetrameric structure (Fig. 2) has also been found for the compound with stoichiometry... 

Fio. 1. (a) A view of the influenza virus hemagglutinin trimer showing jV-acetylneuraminic acid (3, in CPK form) bound, (b) The tetrameric unit of influenza A virus sialidase. The figures were generated using the PyMOL Molecular Graphics System (Delano, W.L. (2002) at http // www.pymol.org). 

Methyllithium, (CH3Li)4, also crystallizes in space group /43m with a = 724 pm and Z = 2. The tetrameric unit may be described as a tetrahedral array of Li atoms with a methyl C atom located above the center of each face of the Li4 tetrahedron or, alternately, as a distorted cubic arrangement of Li atoms and CH3 groups. The locations of the (CH3Li)4 unit and its constituent atoms are listed in the following table. 

The crystal structure of methyllithium is not built of discrete (CH3Li)4 molecules. Such tetrameric units are connected together to give a three-dimensional polymeric network, as illustrated in Fig. 9.6.28. The Li-C bond length within a tetrameric unit is 231 pm, which is comparable to the intertetramer Li-C bond distance of 236 pm. 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8, 10,10,12,12-decamethylcyclohexasiloxane have very different hydrogen bonded structures. The cis isomer contains an intramolecular hydrogen bond and two further intermolecular hydrogen bonds link the molecules into cyclic dimers. The tram isomer forms intermolecular hydrogen bonds to give tetrameric units, which are further hydrogen bonded to form infinite sheets256. 

The intermolecular (interionic) distances must be regular. This "mixed valency" requires that there be only one crystallographically unique molecular site, which must share its partial valency with the nearest neighbor sites along the stack. The many "complex stoichiometry" TCNQ salts—for example, Cs2(TCNQ)32 or triethylammonium(TCNQ)2-, which exhibit "trimeric" or "tetrameric" units of several crystallographically distinct TCNQ molecules and TCNQ- anions held at van der Waals separations—do not conduct well. 

Figure 19 (a) Two tetrameric units of the same chirality type are necessary to form a closed decagon ring (b) a fragment of the 3-D chiral framework. 

Similar dimeric structures are also encountered with bromides. In the case of Gd compound, tetramers have been reported containing two non-equivalent Gd3+ ions with formal coordination numbers of 8 and 9. These tetrameric units form the basic building blocks of compounds like [CpiGdBr] resulting in polymeric infinite double chains. The dysprosium compound [Cp2DyCl]oo has also the polymeric structure. The structure [62] of tetrameric [Cp2GdCIU is shown in Fig. 6.5. 

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