Final Exam

December 21, 2000

5. J. L. Manson, E. Ressouche, and J. S. Miller (p. 1135-1141) found that the room temperature magnetic moment of Cr(C(CN)₃)₂ was 4.7 bohr-magnetons. Is this complex high spin or low spin? To deduce the charge on tricyanomethide, draw the Lewis dot structure assuming that all atoms maintain octets. Tricyanomethide is found to have a D_3h symmetry. Is this consistent with your Lewis dot structure? Why or why not? If it is not consistent with your Lewis dot structure, draw a Lewis dot structure that is consistent with D_3h symmetry.

Answer:

This is one possible Lewis dot structure that maintains octets on all atoms, implying that the anion has a –1 charge. However, this is not consistent with a D_3h structure; the structure shown would be C_3v. Another Lewis structure (1 of 3 identical resonance forms) consistent with the D_3h structure is:

Since the ligand is a monoanion, then the Cr must be in the +2 oxidation state, which is a d⁴ electron configuration. For a high spin case (4 unpaired spins) the magnetic moment is expected to be ~[4(4+2)]^½ = 4.9 bohr-magnetons; for the low spin case (2 unpaired spins) the magnetic moment is expected to be ~[2(2+2)]^½ = 2.8 bohr-magnetons. The complex must be high spin.