Arachno-pentagonal bipyramid: myth or reality?

Methods of synthesis, geometry and electronic structure of polynuclear clusters, related earlier to arachno-pentagonal bipyramid type (including Ru-complexes) are analyzed. Impact of steric and electron effects on direction of connection to clusters with 'butterfly' and tetragonal structure is discussed. The conclusion is made that referral of the above-mentioned compounds to the arachno-pentagonal bipyramid type is just formal and has no profound grounds. From the chemical viewpoint the compounds under consideration are derivatives of the 'butterfly' structure clusters. 43 refs.; 3 figs.; 2 tabs