Three-centre bonds atomic properties

Three versions of Modified Intermediate Neglect of Differential Overlap (MINDO) models exist, MINDO/1, MlNDO/2 and MlNDO/3.The first two attempts at parameterizing INDO gave quite poor results, but MINDO/3, introduced in 1975, produced the first general purpose quantum chemical method that could successfully predict molecular properties at a relatively low computational cost. The parameterization of MINDO contains diatomic variables in the two-centre one-electron term, thus the )3ab parameters must be derived for all pmrs of bonded atoms. The parameters are ionization potentials. 

Chirality is a property of the entire molecule and there are molecules that are chiral but do not contain a chiral centre. For example, 1,3-dimethylallene is a chiral molecule since it has no plane of symmetry, and it can exist as a pair of mirror images (Figure 20.73). The molecule has a chiral axis rather than a chiral centre. The presence of two rigid 7t bonds prevents free rotation around the three central carbon atoms. 

The majority of elemental substances, and a large number of compounds, have metallic properties (see Section 3.3). Metallic elemental substances are characterised by three-dimensional structures with high coordination numbers. For example, Na(s) has a body-centred cubic (bcc) structure in which each atom is surrounded by eight others at the corners of a cube, each at a distance of 371.6pm from the atom at the centre. The Na atom also has six next-nearest neighbours in the form of an octahedron, with Na-Na distances of 429.1pm. A fragment of this lattice is shown in Fig. 7.14. These distances may be compared with the Na-Na bond length of 307.6 pm in the Na2 molecule, which can be studied in the gas phase by vaporisation of sodium metal. 

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