Discussion
The electronic ground state term for the chromium ion in [Cr(CN)6]4- is
Answer: B
The ground state term is the triplet \(^{3}H\).
In [Cr(CN)6]4- chromium is Cr(II), a d4 ion. Cyanide is a strong-field ligand, so the complex is low spin: all four electrons go into the t2g set, t2g4, with two unpaired electrons, a spin triplet (S = 1).
On the d4 Tanabe-Sugano diagram, the high-spin ground state \(^{5}E_{g}\) comes from the free-ion \(^{5}D\) term, but beyond the spin-crossover point the ground state becomes \(^{3}T_{1g}\), which derives from \(^{3}H\), the lowest triplet term of the d4 configuration (largest L among the triplets, by Hund's rules).
\(^{5}D\) is the term of the weak-field, high-spin complex, which strong-field cyanide does not give. \(^{3}F\) and \(^{3}G\) are higher-lying triplet terms, not the parent of the ground state.
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