Orbital organolithium compounds

One of the most important features of dipole-stabilized a-amino-organolithium compounds is the fact that the carbon-lithium bond is in the nodal plane of the Jt orbitals... 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

Organolithium compounds typify a general class of systems for which the Mulliken procedure is grossly inadequate. Any system that has a region that is poorly described will use functions from other areas to supplement its Hilbert space. Carbanions seek diffuse space and use the lithium p orbitals to assist in the description of this space. 

Additional aspects of the chemistry of lithium that have facilitated research are (1) the ability to acquire lithium NMR data, and (2) the possibility of performing high-level theoretical calculations of organolithium compounds (see Molecular Orbital Theor. Lithium exists naturally as two isotopes, Li (7.4% natural abundance) having a nuclear spin of 1, and Li (92.6% natural abundance) having a nuclear spin of... 

Association of organolithium compounds occurs because lithium can acquire multivalent character by hybridization of its 2s and 2p orbitals. Hybridization is also responsible for the formation of complexes such as Na+[ oLi (0Et2)] and Li+(< Li) (Wittig et al, 1951 Wittig, 1958). 

Molecular orbital modeling of the reaction of organolithium compounds with carbonyl groups has examined the interaction of formaldehyde with the dimer of methyllithium. The reaction is predicted to proceed by initial complexation of the carbonyl group at lithium, followed by a rate-determining step involving formation of the new carbon-carbon bond. The cluster then reorganizes to incorporate the newly formed alkoxide ion. ... 

The highly polar character of organolithium compounds causes strong association. The geometry of the coordination sphere is determined essentially by steric effects, as in ionic structures, rather than by interaction of electron pairs. Even where lithium may appear to possess an octet configuration, it is not envisaged that the electrons are strongly held by the valence orbitals of the metal in covalent bonds. 

The dissociation energy of alkyllithium is very large. In the case of MeLi (dimer, trimer and tetramer) they are —42, —82 and — 128kcaI/mole, respectively [42]. Organolithium compounds are non-transition metal compounds but they can form t-bond structures. The elements of non-transition metal compounds which can form the 7t-bond, are Na, Be, Mg, Ca, B, Al, Ga, In, Tl, Ge, Sn, Pb, P, As, Sb, S, Se, Te, etc., besides Li [44]. The olefinic rr-bond with transition metals is well-known the coordination of the Tt-bond is such that the electrons of the olefinic n bond are donated to the vacant d orbitals, and the backdonation of the rr-bond is such that the electrons of the metal d-orbitals are donated to the antibonding n orbital of the olefin. However, as non-transition metals have no vacant d orbitals, the r-electrons of olefins only partially move to the s- or p-orbital of the metal. Then, the electrons largely remain in a non-bonding orbital, and the backdonation is therefore almost none [44]. 

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