 Solution

The ordering $\mathrm{SC}{\mathrm{N}}^{-}<\mathrm{CN}{\mathrm{S}}^{-}\ll \mathrm{NS}{\mathrm{C}}^{-}$ reflects different connectivities, bond strengths and charge distributions, rather than different elemental composition. Relative to thiocyanate, the supplied formation enthalpies are higher by $154.8$ and $574.0\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, respectively.

For $\mathrm{S}-\mathrm{C}-\mathrm{N}$, carbon can make strong C–N multiple bonding with good 2p–2p overlap while keeping its octet. The contributors ${}^{-}\mathrm{S}-\mathrm{C}\equiv \mathrm{N}$ and $\mathrm{S}=\mathrm{C}={\mathrm{N}}^{-}$ avoid a compulsory positive central atom and place the excess negative charge on terminal heteroatoms.

For $\mathrm{C}-\mathrm{N}-\mathrm{S}$, an octet description with four bond-order units forces the central nitrogen to formal charge $+1$. A representative contributor is ${}^{-}\mathrm{C}\equiv {\mathrm{N}}^{+}-{\mathrm{S}}^{-}$, with both charge separation and negative charge on less electronegative carbon. The alternative ${\mathrm{C}}^{2-}={\mathrm{N}}^{+}=\mathrm{S}$ does not remove that disadvantage. Connectivity prevents simply transferring the favourable thiocyanate charge pattern to this isomer.

For $\mathrm{N}-\mathrm{S}-\mathrm{C}$, a central-sulfur octet with four bond-order units gives sulfur formal charge $+2$. One illustrative contributor is ${}^{-}\mathrm{N}={\mathrm{S}}^{2+}={\mathrm{C}}^{2-}$, which has severe charge separation. Attempts to avoid it by drawing strong expanded-octet multiple bonding at sulfur do not supply the strong, well-matched second-row pi overlap available in C–N bonds. Sulfur 3p functions are larger and less well matched to 2p functions, and high-energy 3d orbitals are not a remedy for this deficiency.

These arguments rationalize the large differences but do not quantitatively predict the supplied enthalpies from electronegativity alone. Real charge delocalization and geometry relaxation matter; formal charges are bookkeeping, not measured atomic charges.