Bond lengths lithium complex

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

Fig. 10.6. 3D structure of the open complex between acrolein and Me(ethynyl)CuLI LiCI, with Me20 coordinated to each lithium atom (B3LYP/631 A). Bond lengths are in angstroms. 

Structural data of phospholide ions themselves are scarce. The lithium salt of the tetramethylphospho-lide ion, which is in fact an y -complex, and the K salt of the 2,4,5-tri-terf-butyl-l,3-diphospholide an-ion have been reported. Also the structure of the Li salt of the 2,5-bis(ferf-butyl)-l,3,4-triphospholide ion has been obtained In all these structures the bond lengths are equalized (CC, 1.396—1.424 A CP, 1.690-1.752 A). 

Figure 2.1 Illustration of the monomeric primary amido complex [Li(NHMes )(tmeda)f showing the distorted trigonal planar geometry at the lithium ion. Selected bond lengths Li-NI 1.895(8) A, Li-N2 2.137(9) A and Li-N3 2.165(9)A...

Very recently, various DHB complexes were analyzed [39].12 The complexes of ammonia and hydronium ions were included in this analysis, in addition to the complexes with acetylene and methane, and their derivatives. Generally, in such complexes, lithium hydride and berylium hydride (and its fluorine derivative) act as the Lewis bases (proton acceptors) while hydronium ion, ammonia ion, methane, acetylene, and their simple derivatives act as the proton donors. Therefore, it was possible to investigate the wide spectrum of DHB interactions, starting from those that possess the covalent character and extending to the systems that are difficult to classify as DHBs (since they rather possess the characteristics of the van der Waals interactions). Figure 12.8 displays the relationship between H—H distance and the electron density at H—H BCP.13 One can observe the H—H distances close to 1 A, (as for the covalent bond lengths) and also the distances of about 2.2—2.5 A, typical for the van der Waals contacts. This also holds for the pc-values - of the order of 0.1 a.u. as for the covalent bonds and much smaller values as for the HBs and weaker interactions. 

Figures 12.2.7(e)-(g) show the core structures of three lithium phosphide complexes. Li(l t20)2(P Bu)2(Ga 2Bu3) has a puckered Ga2P2 four-membered ring, in which one Ga atom is four- and the other three-coordinated, as shown in Fig. 12.2.7(e). The Li-P bond length is 266 pm(av.).

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