Lithium bond lengths

A derivative of 62 has been prepared, as shown in Scheme 2, and an XRD analysis of lithium salt 63 has been carried out.63 The mesityl B,—C bond length is 1.339 A and the mesityl B2—C bond is 1.492 A, while the B —C—B2 linkage is almost linear (1.765(5) A). These findings support the conclusions that the B,—C bond has considerable triple bond character, since it is 0.10 A shorter than that in the Mes2B=CH2 anion (54) and the B2—C bond has significant double bond charracter, being about 0.09 A shorter than a B—C bond. 

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. 

Similar effects were observed in the structures of the lithium salts of ester enolates [43] studied by Seebach et al. (1985). Here too systematic differences in angles are observed compared with amide and ketone enolates, and there is a correlation between the bond angles and the difference in the two C-O bond lengths at the reaction centre for three compounds [43], consistent with incipient elimination of t-butoxide to give the ketene [44] (Ferretti et al., 1991). 

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). 

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