A gas-phase electron-diffraction study on the binuclear complex of chromium is reported. Scattered electron intensities were measured from vapors of Cr2(02CCH3)4 produced in a quartz nozzle system at 200 °C. The measured metal-metal bond length, ra(CrCr) = 1.966 (14) Á, compares remarkably well with the CrCr quadrupole bond value of 1.98 Á calculated by using the maximum valence radii of chromium. Our results differ from the X-ray structural parameters, thus showing that axial coordination and the presence of axial ligands strongly influence the bonding nature of Cr atoms in the binuclear complexes of chromium.In the early 1960s the existence of double,1 triple,1 2 and quad-ruple3 bonds between transition-metal atoms was recognized. Since that time the interest in the chemistry of multiple metalmetal bonds has increased dramatically. There are more than 900 known compounds containing metal-metal bonds of the order 2-4 of which over 300 have been structurally characterized by X-ray crystallography.4 The metal-metal bond lengths in such compounds range from 1.83 to 2.72 Á.Important differences have been noticed in the metal-metal bond lengths of the binuclear complexes of transition metals belonging to the same group in the periodic table. The metalmetal bond length in complexes of molybdenum ranges from 2.04 A in Mo2[pyNC(0)CH3]45 to 2.18 A in Mo2(C3H5)4.6•7 This small variation in the bond lengths suggests that in the case of molybdenum complexes, the presence of axial ligands does not significantly influence the bonding character of the metal atoms. The situation is not so simple in the case of binuclear complexes of chromium. The metal-metal bond length shows a much wider variation, from a super short value of 1.847 A in Cr2(6-Me-2-MeOC6H3)4 to a very long value of 2.541 A in Cr2(02CCF3)4-(Et20)2.8 This wide variation can be explained by the perturbing effects on the axial ligands on the bonding character of Cr atoms.
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