23.1B: Structure and bonding

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Learning Objectives

In this lecture you will learn the following

Solid state structures of nickel-arsenide, alkyl lithium and alkyl aluminium compounds.

Structure and bonding

The slight differences that arise between organometallic compounds and binary hydrogen compounds are mainly due to the tendency of alkyl groups to avoid ionic bonding. The molecular structures of AlMe₃and MeLi differ from AlH₃and LiH. Even the more ionic MeK crystallizes in the nickel-arsenide structure rather than the rock-salt structure adopted by KCl.

Nickel-arsenide structure is typical of soft-cation, soft-anion combinations.

Electron deficient compounds such as AlMe₃contain 3c-2e bonds analogous to the B—H—B bridges in diborane.

The Nickel-Arsenide, NiAs, Structure

MeLi in nonpolar solvents consists of tetrahedron of Li atoms with each face bridged by a methyl group. Similar to Al₂Me₆, the bonding in MeLi consists of a set of localized molecular orbitals. The symmetric combination of three Li 2s orbitals on each face of the Li₄ tetrahedron and one sp³ hybrid orbital from CH₃ gives an orbital that can accommodate a pair of electron to form a 4c-2e bond.

The lower energy of the C orbital compared with the Li orbitals indicates that the bonding pair of electrons will be associated primarily with the CH₃group, thus supporting the carbanionic character of the molecule. Some analysis has indicated that about 90% ionic character for the Li-CH₃ interaction.

The interaction between an sp³ orbital from a methyl group and the three 2s orbitals of the Li atoms in a triangular face of Li₄(CH₃)₄to form a totally symmetric 4c,2e bonding orbital. The next higher orbital is non-bonding and the uppermost is antibonding.

Me₂Be and Me₂Mg exist in a polymeric structure with two 3c,2e-bonding CH₃ bridges between each metal atom.

Contributors and Attributions

http://nptel.ac.in/courses/104101006/6