Multi-center binding

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The concept of the multi-center bond contains the most extensive definition of the atomic bond (covalent bond). Electrons are not only assigned to two atomic centers, as is done for the classic covalent bond, but also bind several atoms to one another - you get multi-center orbitals. Strictly speaking, it follows from the orbital theory that this is the case for every polyatomic substance. The LCAO-MO method (currently the most widely used quantum chemical method, which is used to construct molecular orbitals from atomic orbitals ) always delivers multi-center orbitals that are delocalized over all atoms of the substance, so-called canonical orbitals . However, for most simple substances, the (delocalized) canonical orbitals obtained in this way can be converted into (localized) one- and two-center orbitals using mathematical tricks (see for example NBO analysis ). These are closer to most chemists, since they can be related to the free electrons (pair) and binding electrons (pair), which are represented as dots (lines) in the chemical structural formula. A well-known quantum chemical method that exclusively uses these localized orbitals and that has experienced a renaissance in recent years is valence bond theory.

However, for many substances these methods do not give satisfactory unambiguous solutions with orbitals located on one or two atoms. Instead, several resulting structures must be considered together in order to describe the actual electronic structure. The phenomenon is called mesomerism , and the structures are called resonance structures . If one nevertheless wants to describe these systems with largely "localized" electrons, one needs a new concept, the concept of multi-center bonds.

Special types

All covalent bonds can ultimately be subsumed under multi-center bonds - these include:

  • The two-center two-electron bond (two-center bond, normal covalent bond ) - Abbreviation: 2c-2e
  • The three-center-two-electron bond ( three -center bond ) - abbreviation 3c-2e - is formed when there is a lack of electrons
  • The three-center-four-electron bond ( three -center bond ) - abbreviation 3c-4e - is formed when there is an excess of electrons ( hypervalence )
  • The metallic bond - here all atomic cores of the entire substance are bound to one another via shared electrons. An older, alternative (but worse) interpretation of this type of bond uses classical electrostatic forces between positive atomic cores ( cations ) and a surrounding electron gas .

Orbital schemes

The energetic position and relative arrangement of the molecular orbitals, which arise from a multicenter interaction, can be estimated based on the symmetry of the molecule (or in the case of ordered polymeric substances: based on the symmetry of the crystal). Ultimately, however, detailed information can only be obtained from quantum chemical calculations or from experimental spectroscopic data. A distinction is made between bonding orbitals, the occupation of which with electrons strengthens the bond, non-bonding orbitals, the occupation of which has no significant influence on the strength of the bond, but which can have a significant influence on the most stable atomic arrangement, and antibonding orbitals, the occupancy of which is the bond weakens.

In the case of multi-center bonds that extend over all atoms of a polymeric substance, so-called orbital bands are obtained , i.e. H. Groups of very many, energetically extremely close together orbitals. The "shape" of these bands and the energetic distance between different bands are used in particular to interpret or, if necessary, predict the electrical conductivity of the substances and their temperature dependence.