“…Experimental techniques (single crystal X-ray analysis, 135/137 NMR and NQR spectroscopy) 25,60,129–132,137 and computation (MEP surface, QTAIM/NCIPlot, and Hirshfeld surface analyses) 25,60,129,130,133 consistently prove the σ-hole character and the attractive nature of the short contacts observed in the crystals. In isolated MaO 4 − anions there are four identical σ-holes where the electrostatic potential is negative but less negative than in other regions.…”