Question

The anion C(CN)32 has a trigonal planar geometry about the central carbon atom. Draw Lewis structures for C(CN)32, including resonance forms, and determine which structure contributes the most to the bonding.

Answer 1

The room temperature magnetic moment of Cr(C(CN)₃)₂ was 4.7 bohr-magnetons (BM). Tricyanomethide is found to have a D_3h symmetry (trigonal planar geometry about the central carbon atom). 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 BM; for the low spin case (2 unpaired spins) the magnetic moment is expected to be ~[2(2+2)]^½ = 2.8 BM. The complex must be high spin.

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