Magnesium isocyanide is a linear radical which is lower in energy than its MgCN isomer. It was observed first as an unidentified species in the circumstellar envelope of the famous carbon star CW Leo, also known as IRC +10216, by
M. Guélin, J. Cernicharo, C. Kahane, E. González-Alfonso,
A New Free Radical in IRC +10216
Astron. Astrophys. 157, L17–L20 (1986).
The authors detected in the 3 mm region both strong fine structure components of the N“ = 6, 7, and 8 transitions. The authors proposed as most likely carriers HSiCC, its isomer HCCSi, or HSCC.

The molecule was identified few years later as MgNC via masurement of its rotational spectrum by:
K. Kawaguchi, E. Kagi, T. Hirano, S. Takano, S. Saito,
Laboratory Spectroscopy of MgNC – The First Radioastronomical Identification of Mg-bearing Molecule
Astrophys. J. 406, L39–L42 (1993).

Besides the main isotope of Mg with the mass number 24, there are two stable isotopes with mass numbers 25 and 26, respectively. The corresponding isotopologs have both been detected toward CW Leo by:
M. Guélin, M. Forstini, P. Valiron, L. M. Ziurys, M. A. Anderson, J. Cernicharo, and C. Kahane,
Nucleosynthesis in AGB Stars: Observation of ²⁵Mg and ²⁶Mg in IRC +10216 and Possible Detection of ²⁶Al
Astron. Astrophys. 297, 183–196 (1995).

More recently, MgNC was also detected toward the protoplanetary nebula (PPN) CRL 2688, the egg nebula:
J. L. Highberger, C. Savage, J. H. Bieging, L. M. Ziurys,
Heavy-Metal Chemistry in Proto-Planetary Nebulae:\\ Detection of MgNC, NaCN, and AlF toward CRL 2688
Astrophys. J. 562, 790–798 (2001).

Sodium cyanide is a nearly T-shaped molecule. It possesses a NaN bond that is slightly shorther than the NaC bond. Therefore, the molecule should be considered more appropriately as sodium isocyanide, NaNC. It was observed first in the circumstellar envelope of CW Leo in four K_a = 0 transitions by
B. E. Turner, T. C. Steimle, and L. Meerts,
Detection of Sodium Cyanide (NaCN) in IRC+10216
Astrophys. J. 426, 97–100 (1994).
The J” = 4, 5, and 6 transitions are in the 3 mm region, the one with J“ = 8 is in the 2 mm region. Note: in that paper the assumed dipole moment was too low by a factor of about 2.5 because an ab initio dipole moment was taken as being in units of Debye whereas it actually was given in atomic units. 1 au equals 2.542 D.

NaCN too was detected toward the PPN CRL 2688:
J. L. Highberger, C. Savage, J. H. Bieging, L. M. Ziurys,
Heavy-Metal Chemistry in Proto-Planetary Nebulae:\\ Detection of MgNC, NaCN, and AlF toward CRL 2688
Astrophys. J. 562, 790–798 (2001).

The magnesium cyanide isomer was detected in three transitions with N” = 8, 9, and 10, all lying in the 3 mm region, by:
L. M. Ziurys, A. J. Apponi, M. Guélin, and J. Cernicharo,
Detection of MgCN in IRC +10216: A New Metal-bearing Free Radical
Astrophys. J. 445, L47–L50 (1995).

Aluminum isocyanide is a linear molecule which is lower in energy than its AlCN isomer. It was observed in five transitions (J“ = 10, 11, and 12 in the 2 mm region and J” = 17 and 20 in the 1 mm region) toward CW Leo by:
L. M. Ziurys, C. Savage, J. L. Highberger, A. J. Apponi, M. Guélin, and J. Cernicharo,
More Metal Cyanide Species: Detection of AlNC (X ¹Σ⁺) toward IRC +10216
Astrophys. J. 564, L45–L48 (2002).

Potassium cyanide is a nearly T-shaped molecule, just as NaCN. It too should be considered as an isocyanide rather than a cyanide. Several transitions in the 3, 2, and 1 mm region were detected toward CW Leo by:
R. L. Pulliam, C. Savage, M. Agúndez, J. Cernicharo, M. Guélin, and L. M. Ziurys,
Identification of KCN in IRC +10216: Evidence for Selective Cyanide Chemistry
Astrophys. J. 725, L181–L185 (2010).

Iron cyanide is a linear molecule. According to laboratory spectroscopic work, it actually is lowein in energy than iron isocyanide, FeNC. The molecule has a ⁴Δ_i ground state with considerable spin-orbit splitting. The Ω = 7/2 spin component is lowest in energy. Three apparently clean transitions with J“ = 9.5, 11.5, and 12.5 as well as two partially overlapped transitions (10.5 and 15.5) were detected with the ARO 12 m radio-telescope toward CW Leo by:
L. N. Zack, D. T. Halfen, and L. M. Ziurys,
Detection of FeCN (//X// ⁴Δ_i) in IRC +10216: A New Interstellar Molecule
Astrophys. J. 733, Art. No. L36 (2011).

Contributor(s): H. S. P. Müller; 09, 2005; 04, 2007; 04, 2011; 05, 2011