Full structural characterization of the first homologous series of ditungsten paddlewheel compounds having an electronic configuration of the type σ 2 π 4 δ x ,x=2,1,0, and W-W bond orders of 4, 3.5 and 3, respectively, has been accomplished with the guanidinate-type ligand hpp (hpp=the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine); the lengthening in bond distances from W 2 (hpp) 4 .2NaHBEt 3 to W 2 (hpp) 4 Cl to W 2 (hpp) 4 Cl 2 is 0.05 Å for each step.