AbstractThe structures of complexes [MⅡ2Cl4L2] and [MⅢ2Cl7L]- (M = Mo, Re; L = Ph2Ppy, (Ph2P)2py) were calculated by using density functional theory (DFT) PBE0 method. Based on the optimized geometries, the natural bond orbital (NBO) analyses were carried out to study the nature of Re–Re and Mo–Mo bonds. The conclusions are as follows: the M–M distances in two-Ph2Ppy or (Ph2P)2py complexes [MⅡ2Cl4L2] are shorter than those in mono-Ph2Ppy or (Ph2P)2py complexes [MⅢ2Cl7L]– due to the double bridged N–C–P interactions. For singlet of all complexes, there is ReⅢ–ReⅢ or MoⅡ–MoⅡ quadruply bond in complex [Re2Cl7L]– or [Mo2Cl4L2], while only ReⅡ–ReⅡ or MoⅢ–MoⅢ triply bond in complex [Re2Cl4L2] or [Mo2Cl7L]–. The most stable spin state of 2 and 6, triplet, only contains triple ReⅢ–ReⅢ bond. Because the LPCl → BD*Re–Re delocalizations weaken the Re–Re bond, the distance of ReⅢ–ReⅢ quadruple bonds in [Re2Cl7L]– is slightly longer than that of ReⅡ–ReⅡ triple bonds in [Re2Cl4L2]. Moreover, due to the delocalizations from the lone pair electrons of the remaining P’ atom to the M–M antibonding orbitals, the M–M distance in (Ph2P)2py complexes is slightly longer than that in Ph2Ppy complexes.