Abstract: The bonding and spin filtering properties of metal string complexes CoMCo(dpa)4(NCS)2(M=Co, Ni, Pd, Pt; dpa=dipyridylamide) have been investigated with density functional theory B3LYP method. The results show that: the ground state of complex 1 is doublet, there is a 3-center-3-electron bond delocalized over the Co36+ chain. However, the ground states of complexes 2~4 are antiferromagnetic(AF) singlet, corresponded to the high spin configurations with two unpaired, six unpaired and six unpaired electrons, respectively. The unpaired electrons of complexes 2~4 largely localize on the termimal CoII ions and there is a 3-center-4-electron bond delocalized over the CoMCo6+ chain. The molecular orbital energy level diagrams and PDOS diagrams show that the spin- components of singly nb orbital are the dominant transport channel. Complexes 1~4 all possess spin filtering effect. The distance between spin- components of nb orbital and Fermi levels is 1234. Under the electric field, the Co2―N4 bond lengths at the high potential side increase, while the Co3―N7 distances at the low potential side tend to be shortened. The average Co―M distances, the spin densities and NPA charge of metal atoms are less affected by electric field, indicating the electromagnetic properties are stable. Under the electric field, the nb orbital still keeps delocalized along the metal chain and the LUMO-HOMO gaps decrease. This is beneficial for electron transfer.