Synthesis, chromatographic separation, and stereophysical analysis of the homologous [Ni12-x(PMe)x(CO)24-3x]2- series (x = 2, 3, 4) containing noncentered Ni12-xPx icosahedral cages and the [Ni10(.mu.5-PMe)2(.mu.4-PMe)5(CO)10]2- dianion containing a structurally unprecedented heptacapped pentagonal prismatic metal cage: structural, spectroscopic, and electrochemical consequences due to replacement of Ni(CO)3 fragments with electronically equivalent (isolobal) PMe fragments