With several levels of multireference and restricted open-shell single-reference electronic structure theory, optimum structures, relative energetics, and spectroscopic properties of the low-lying Δ6, Π6, Δ4, Π4, and Σ4 states of linear FeNC and FeCN have been investigated using five contracted Gaussian basis sets ranging from Fe[10s8p3d],C/N[4s2p1d] to Fe[6s8p6d3f2g1h],C/N[6s5p4d3f2g]. Based on multireference configuration interaction (MRCISD+Q) results with a correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis set, appended with core correlation and relativistic corrections, we propose the relative energies: Te(FeNC),Δ6(0)<6Π (2300 cm−1)<4Δ (2700 cm−1)<4Π (4200 cm−1)<4Σ; and Te(FeCN),Δ6(0)<6Π (1800 cm−1)<4Δ (2500 cm−1)<4Π (2900 cm−1)<4Σ. The Δ4 and Π6 states have massive multireference character, arising mostly from 11σ→12σ promotions, whereas the sextet states are dominated by single electronic configurations. The single-reference CCSDT-3 (coupled cluster singles and doubles with iterative partial triples) method appears to significantly overshoot the stabilization of the quartet states provided by both static and dynamical correlation. The Δ4,6 and Π4,6 states of both isomers are rather ionic, and all have dipole moments near 5 D. On the ground Δ6 surface, FeNC is predicted to lie 0.6 kcal mol−1 below FeCN, and the classical barrier for isocyanide/cyanide isomerization is about 6.5 kcal mol−1. Our data support the recent spectroscopic characterization by Lei and Dagdigian [J. Chem. Phys. 114, 2137 (2000)] of linear Δ6FeNC as the first experimentally observed transition-metal monoisocyanide. Their assignments for the ground term symbol, isotopomeric rotational constants, and the Fe–N ω3 stretching frequency are confirmed; however, we find rather different structural parameters for Δ6FeNC:re(Fe–N)=1.940 Å and r(N–C)=1.182 Å at the cc-pVQZ MRCISD+Q level. Our results also reveal that the observed band of FeNC originating at 27 236 cm−1 should have an analog in FeCN near 23 800 cm−1 of almost equal intensity. Therefore, both thermodynamic stability and absorption intensity factors favor the eventual observation of FeCN via a Π66Δ transition in the near-UV.

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