The optimized geometries and corresponding binding energies of complexes between fluorohalides, FX (X = Cl, Br, and I), and isocyanides, CNY (Y = CN, NC, NO_2, F, CF_3, Cl, Br, H, CCF, CCH, CH_3, SiH_3, Li, and Na), were calculated at the MP2(Full)/aug-cc-pVTZ (aug-cc-pVTZ-PP on I) level of theory, without and with basis set superposition error (BSSE) corrections through the counterpoise (CP) method. The optimized complex geometries were analyzed through the Steiner-Limbach relationship, which can be used to establish correlations between the F-X and X-C bond lengths. For all complexes, the correlations were shown to improve considerably when using optimized geometries including BSSE corrections. It was shown that further improvements can be achieved through the introduction of an extended four-parameter form of the Steiner-Limbach relationship which accounts for all differences between the valences associated with the two bonds involving the halogen in an A-X???B complex. The results indicate that chlorine as a halogen bond donor is affected by the basicity of the isocyanides and forms different types of halogen bonds as the F-Cl bond lengthens in parallel with the shortening of the distance between Cl and the isocyanide carbon. This is not observed for iodine and bromine as halogen-bond donors, which is illustrated by the low levels of correlation obtained when applying the standard and extended Steiner-Limbach relationships to the corresponding complexes.
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