Preparation and Structural Characterization of the Intermediate Complex [Er{H2C8H16N4(Ch2Coo)(3) (Ch2(Ph)Po2)}(H2O)(2)](2)Cl-2 Center Dot Xh(2)O in the Reaction of Er3+ and the Dota-Type Ligand. An Interesting Example of Two Stereoforms of a Lanthanide Complex