Domain: Thermal analysis / materials

A TG run in inert gas on an unknown crystalline salt shows two main mass-loss steps: 80-130 °C: 40.9% mass loss; 130-300 °C: 5.5% mass loss. Evolved-gas MS during the first step shows a dominant signal at m/z=79. Powder XRD of the post-TG residue (room temperature) shows reflections indexed to metallic silver and beta-MnO_2. What is the compound?

Grading rubric:
criterion: Explicitly rules out Bromine (Br) as a candidate for the m/z=79 signal by noting the absence of a corresponding isotopic peak at m/z=81 (due to the ~1:1 natural abundance of 79Br/81Br).
weight: 10
criterion: Identifies the Step 1 mass loss (80-130 °C) specifically as the cleavage of dative Ag-N bonds, contrasting the lability of the neutral ligands with the higher thermal stability of the ionic salt lattice.
weight: 10
criterion: Proposes the linear coordination geometry [N-Ag-N]+ for the cation, consistent with the d10 electronic configuration of Silver(I) and the stoichiometry n=2.
weight: 10
criterion: Analyzes the Step 2 decomposition mechanism as a 'dual-reduction' event where both metal centers (Ag+ -> Ag0 and Mn+7 -> Mn+4) are reduced simultaneously by the oxidation of oxide ligands.
weight: 10
criterion: Justifies the discrepancy between the theoretical oxygen mass loss (~8.3%) and the experimental value (5.5%) by attributing it to kinetic limitations (e.g., slow solid-state diffusion, surface passivation) or incomplete decomposition within the specific TGA ramp timeframe.
weight: 10
criterion: Verifies the charge balance of the proposed formula, confirming that the +1 charge of the [Ag(C5H5N)2]+ complex is exactly neutralized by the -1 charge of the uncoordinated Permanganate anion.
weight: 10
criterion: Correctly calculates the theoretical mass of the AgMnO4 intermediate (approx. 226.8 g/mol) to validate the baseline mass remaining after Step 1.
weight: 10
criterion: Synthesizes all data to state the final structure as Bis(pyridine)silver(I) permanganate, formatted as [Ag(C_5H_5N)_2]MnO_4.
weight: 30

Answer:
bis(pyridine)silver(I) permanganate, [Ag(C5H5N)2]MnO4