Each of the metals chromium, molybdenum, and tungsten forms a compound of molecular formula M2(mhp)4, where mhp represents the anion of 2-hydroxy-6-methylpyridine, Hmhp (6-methylpyridone). These compounds, which are yellow, orange, and red, respectively, are all stable toward dry air as solids and can be easily and quickly prepared in large quantities from readily available starting materials. Cr2(mhp)4 is stable toward even humid air and appears to be more chemically and physically stable than any other chromium(II) compound. The Cr and Mo compounds were prepared by reactions of Cr2(02CCH3)4 or Cr2(02CCH3)4(H20)2 and Mo2(02CCH3)4, respectively, with the sodium salt of the ligand; W2(mhp)4 was obtained by reaction of W(CO)8 with Hmhp in refluxing diglyme. All three compounds are volatile and give definitive mass spectra. The Raman spectra have M-M stretching frequencies (±3 cm-1) at 556, 425, and 295 cm-1, for the Cr, Mo, and W compounds, respectively. All three give crystals of composition M2(mhp)4(CH2Cl2) which are isomorphous; each structure has been determined by x-ray crystallography, revealing the M-M distances to be Cr-Cr, 1.889 (1) Á; Mo-Mo, 2.065 (1) Á; W-W, 2.161 (1) A. The compounds form monoclinic crystals (space group P2\/n with Z = 4) with unit cell dimensions at 22 ± 2 °C as follows, where the values for the Cr, Mo, and W compounds are given in that order for each: a (A) = 12.792 (2), 12.959 (1), 12.924 (1); b (A) = 16.985 (3), 16.948 (2), 16.948 (2); c (A) = 12.904 (2), 13.064 (2), 13.010 (2); ß (deg), 104.42 (1), 104.90 (1), 104.78 (1).
