  A model of chemical bonding between ions in manganites involving covalent
one-electron $\sigma$ bonding is suggested. The covalent one-electron $\sigma$
bonding gives rise to a strongly correlated state of electrons resulting from
the exchange interaction between electrons when they are simultaneously at
cation and anion orbitals. The manifestation of the correlatred state is the
spin and spatial ordering of the electrons resulting in the formation of a
spin- ordered electron lattice. The conductivity of manganites in this model is
the consequence of displacement of the electron lattice (or its part) from one
localization site to another and depends on the type of spin ordering of the
electrons in the electron lattice and the localization energy determined by the
energy of the one-electron $\sigma$ bond. The model also assumes a strong
polarization of an anion by cations, which facilitates the 3s2p hybridization
of the anion and transition of one of the pairs of 2p electrons from the
singlet state into the triplet state. This transition leads to formation of the
spin-polarized electron lattice (electron spins are parallel) and ferromagnetic
ordering of manganese ions. In the model, the effect of colossal
magnetoresistance is explained by a change of the conductivity mechanism on
application of an external magnetic field, i.e., transition from the
conductivity mechnism typical of an ionic crystal to the conductivity provided
by the spin-polarized electron lattice.
