The unpredictable structural features of chromium(II) pyrrolyls: synthesis and x-ray structures of monomeric square-planar (.eta.1-2,5-Me2C4H2N)2Cr(py)2, square-pyramidal (.eta.1-C4H4N)2Cr(py)3, dimeric (7-azaindolyl)2Cr(DMF)2, and polymeric (.eta.1-2,5-Me2C4N2)4CrNa2(THF)2(Et2O)n. An aborted Cr-Cr quadruple bond formation?