Relativistic effects in PtF$_6$ and isoelectronic octahedral groups: Magnetic susceptibility, $^{19}$F NMR, and $ab$ $initio$ calculations

The temperature-independent paramagnetism and high $O_h$ symmetry of PtF$_6$ molecules in solids (at $77$ K) and of the octahedral molecular complexes [IrF$_6$] in KIrF$_6$ and [OsF$_6]^2$ in K$_2$OsF$_6$ are shown to be consistent with the model of strong spin-orbit coupling of $5d^4$ electrons.