= Solution
\b[The ordering $\mathrm{SCN^-<CNS^-\ll NSC^-}$ 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 $574.0\,\mathrm{kJ\,mol^{-1}}$, respectively.
For $\mathrm{S-C-N}$, carbon can make strong C–N multiple bonding with good 2p–2p overlap while keeping its octet. The contributors $\mathrm{^-S-C\equiv N}$ and $\mathrm{S=C=N^-}$ avoid a compulsory positive central atom and place the excess negative charge on terminal heteroatoms.
For $\mathrm{C-N-S}$, an octet description with four bond-order units forces the central nitrogen to formal charge $+1$. A representative contributor is $\mathrm{^-C\equiv N^+-S^-}$, with both charge separation and negative charge on less electronegative carbon. The alternative $\mathrm{C^{2-}=N^+=S}$ does not remove that disadvantage. Connectivity prevents simply transferring the favourable thiocyanate charge pattern to this isomer.
For $\mathrm{N-S-C}$, a central-sulfur octet with four bond-order units gives sulfur formal charge $+2$. One illustrative contributor is $\mathrm{^-N=S^{2+}=C^{2-}}$, 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.
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