The geometrical and electronic structures of heterometal string complexes [CuCuM(npa)₄Cl]⁺ (1:M=Pt, 2:M=Pd, 3:M=Ni) were investigated theoretically by density functional theory UBP86 method with incorporating the external electric field along the metal chain in Cl→M(r) and M(r)→Cl two directions. In general, under the electric field, the spin densities of the high potential end decrease while that of the low potential end increase, and the negative charge transfers to the high potential end. When the electric field enhances, the molecular energy decreases and the dipole moment increases linearly. With the increase of the electric field, the spatial distribution of frontier orbitals changes regularly, and the energies of frontier occupied orbitals increase in the sensitive order of π_nb>σ^*>δ^*_M(r)-N(r), which leads to the frontier occupied orbital level crossing. When the Cl→M(r) electric field is applied, the spin densities delocalize from Cu to M(r), and the positive charge transfers from Cu to M(r). Moreover, the molecular energy decreases more sensitively. Also in the Cl→M(r) electric field, the contractions of Cu-Cu and Cu-M(r) distances and the decrease of the frontier orbital energy gap are beneficial to the electron transport of the metal string complexes. However, under the M(r)→Cl electric field, the distances of Cu-Cu and Cu-M increase. The effect of M(r)→Cl electric field on the molecular energy, spin density and charge density of metal atoms are less obvious than that of Cl→M(r) electric field. When the electric field increases to some extent, the spin densities of the metal atoms remain stable values, and the changes of them are obviously asymmetric in Cl→M(r) and M(r)→Cl directions. Therefore, 1～3 may have the potential application of molecular rectifier.

Key words: metal string complex, electronic structure, geometrical structure, DFT, electric field