Crystal structures and spectroscopic characterization of MBr(CNXyl) (M = Fe and Co, n = 4; M = Ni, n = 2; Xyl = 2,6-dimethylphenyl), and of formally zero-valent iron as a cocrystal of Fe(CNXyl) and Fe(CNXyl).

Structures and spectroscopic characterization of the divalent complexes cis-dibromidotetrakis(2,6-dimethylphenyl isocyanide)iron(II) dichloromethane 0.771-solvate, [FeBr(CHN)]·0.771CHCl or cis-FeBr(CNXyl)·0.771CHCl (Xyl = 2,6-dimethylphenyl), trans-dibromidotetrakis(2,6-dimethylphenyl isocyanide)iron(II), [FeBr(CHN)] or trans-FeBr(CNXyl), trans-dibromidotetrakis(2,6-dimethylphenyl isocyanide)cobalt(II), [CoBr(CHN)] or trans-CoBr(CNXyl), and trans-dibromidobis(2,6-dimethylphenyl isocyanide)nickel(II), [NiBr(CHN)] or trans-NiBr(CNXyl), are presented. Additionally, crystals grown from a cold diethyl ether solution of zero-valent Fe(CNXyl) produced a structure containing a cocrystallization of mononuclear Fe(CNXyl) and the previously unknown dinuclear Fe(CNXyl), namely pentakis(2,6-dimethylphenyl isocyanide)iron(0) tris(μ-2,6-dimethylphenyl isocyanide)bis[tris(2,6-dimethylphenyl isocyanide)iron(0)], [Fe(CHN)][Fe(CHN)]. The (M)C-N-C(Xyl) angles of the isocyanide ligand are nearly linear for the metals in the +2 oxidation state, for which the ligands function essentially as pure donors. The νCN stretching frequencies for these divalent metal isocyanides are at or above that of the free ligand. Relative to Fe, in the structure containing iron in the formally zero-valent oxidation state, the Fe-C bond lengths have shortened, the C[triple-bond]N bond lengths have elongated, the (M)C-N-C(Xyl) angles of the terminal CNXyl ligands are more bent, and the νCN stretching frequencies have shifted to lower energies, all indicative of substantial M(dπ)→π* backbonding.