The pentacoordinate complex ZnS4(PMDETA) (1) is formed by treatment of ZnS6(TMEDA) (2) with the tridentate ligand pentamethyldiethylenetriamine (PMDETA). H-1 NMR studies show that samples of 1 react with elemental sulfur to produce a significant (<30%) amount of a second polysulfido complex, assigned as ZnS5(PMDETA). Crystallographic studies reveal that 1 has a ZnN3S2 coordination sphere with a distorted trigonal bipyramidal structure. The inversion of the stereochemistry at Zn in 1 has been observed by variable-temperature H-1 NMR spectroscopy. Neither free PMDETA, elemental sulfur, nor ZnS5(PMDETA) affects the coalescence behavior of 1. The isomerization mechanism proposed for this process involves the dissociation of the central amine, formation of a tetrahedral ZnN2S2 intermediate, and inversion at the central nitrogen, followed by recoordination of the central amine. Other PMDETA complexes studied in this work are subject to similar dynamics. Reactivity studies indicate that complex 1 is significantly more nucleophilic than the corresponding ZnS6(TMEDA), the enhanced nucleophilicity being attributed to the influence of the third amine ligand. Complex 1 reacts rapidly at room with dimethyl acetylenedicarboxylate (DMAD) to give the dithiolene complex ZnS2C2(CO2Me)(2)(PMDETA) (3). The rate of the DMAD reaction was > 100 times faster higher than that for the reaction of ZnS6(TMEDA) with DMAD, indicative of the enhanced nucleophilicity of the sulfur ligands in 1. As further evidence for the enhanced nucleophilicity of its polysulfido ligand, 1 reacts with other unsaturated species such as CS2 and the dithiocarbonate C-2(S2CO)(2) to give ZnS3CS(PMDETA) and Zn(S2C2S2CO)(PMDETA), respectively. The perthiocarbonate reacts further with PPh3 to give ZnS2CS(PMDETA).