Transition metal isocyanide complexes

Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Several thousand isocyanides are known, but the coordination chemistry is dominated by a few ligands.^[2] Common isonitrile ligands are methyl isocyanide, tert-butyl isocyanide, phenyl isocyanide, and cyclohexylisocyanide. Some isocyanide complexes are used in medical imaging.

Technetium (^99mTc) sestamibi is used in nuclear medicine imaging.^[1]

Ligand properties

According to the Covalent bond classification method, isocyanides are classified as L ligands, i.e., charge-neutral Lewis bases. With respect to HSAB theory, it is classified as soft.

Compared to CO, most isocyanides are superior Lewis bases and weaker pi-acceptors. Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO. Isocyanide complexes often mirror the stoichiometry and structures of metal carbonyls. Like CO, isocyanides engage in pi-backbonding. The M-C-N angle provides some measure of the degree of backbonding. In electron-rich complexes, this angle is usually deviates from 180°. Unlike CO, cationic and dicationic complexes are common. RNC ligands are typically terminal, but bridging RNC ligands are common. Bridging isocyanides are always bent. General trends can be appreciated by inspection of the homoleptic complexes of the first row transition metals.

Because the CNC linkage is linear, the cone angle of these ligands is small, so it is easy to prepare polyisocyanide complexes. Many complexes of isocyanides show high coordination numbers, e.g. the eight-coordinate cation [Nb(CNBu−t)₆I₂]⁺.^[3] Very bulky isocyanide ligands are also known, e.g. C₆H₃-2,6-Ar₂-NC (Ar =aryl).^[4]

Di- and triisocyanide ligands

structure of Os₃(CO)₉(CNCH₂)₃CMe.^[5]

Di- and triisocyanide ligands are well developed, e.g., (CH₂)_n(NC)₂. Usually steric factors force these ligands to bind to two separate metals, i.e., they are binucleating ligands.^[6] Chelating diisocyanide ligands require elaborate backbones.^[7]

Synthesis

Structure of Fe(tert-BuNC)₅. Notice that some C-N-C angles strongly deviate from 180°, a characteristic of low-valent isocyanide complexes.^[8]

Because of their low steric profile and high basicity, isocyanide ligands often install easily, e.g. by treating metal halides with the isocyanide. Many metal cyanides can be N-alkylated to give isocyanide complexes.^[9]

Reactions

The first metal carbene complex, Chugaev's red salt, was not recognized as such until decades after its preparation.^[10]

Typically, isocyanides are spectator ligands, but their reduced and oxidized complexes can prove reactive by virtue of the unsaturated nature of the ligand

Cationic isocyanide complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared.

Protonation

Because isocyanides are more basic donors ligands than CO, their complexes are susceptible to oxidation and protonation. Thus, Fe(tBuNC)₅ is easily protonated, whereas its counterpart Fe(CO)₅ is not:^[8]

Fe(CNR)₅ + H⁺ → [HFeL₅]⁺

Fe(CO)₅ + H⁺ → no reaction

Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes:^[11]

L_nM-CNR + H⁺ → [L_nM≡CN(H)R]⁺

Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.^[12]

Redox

Because isocyanides are both acceptors and donors, they exhibit more reversible redox than metal carbonyls. This aspect is illustrated by the isolation of the homoleptic vanadium hexaisocyanide complex in three oxidation states, i.e., [V(CNC₆H₃-2,6-Me₂)₆]ⁿ for n = -1, 0, +1.^[13]

Homoleptic complexes

1st Transition Series

Complex	colour	electron config.	structure	comments
[V(CNC6H3-2,6-Me2)6]−	green	d6, 18e−	octahedral[14]	Cs+ salt
[V(CNC6H3-2,6-Me2)6]0	purple	d5	octahedral[14]
[V(CNC6H3-2,6-Me2)6]+	red	d4	octahedral[14]	PF6− salt
[V(CNC6H3-2,6-Me2)7]+	red	d4, 18e−	monocapped trigonal prism[15]	iodide salt
[Cr(CNPh)6]3+	orange	d3	octahedral[16]
[Cr(CN-t-Bu)7]2+	orange	d4, 18e−	octahedral[17]
[Cr(CNPh)6]0, 18e−		d6	octahedral[18]	many analogues
[Cr(CNMe)6]+OTf−	yellow-brown	d5	octahedral[19]
[Mn(CNPh)6]+	yellow	d6, 18e−	octahedral[20]
[Fe(CNMe)5]0	colourless	d8, 18e−	trigonal bipyramidal
[Fe2(CNEt)9]0	yellow	d8	confacial bioctahedral[21]	see Fe2(CO)9
[Fe(CNMe)6]2+	colourless	d6, 18e−	octahedral
[Co2(CN-t-Bu)8]0	red-orange	d9	pentacoordinated with bridging isocyanides[22]	see Co2(CO)8
[Co(CN-t-Bu)5]+	yellow	d8, 18e−	trigonal bipyramidal[23]
[Co(CNC6H3-2,6-Me2)4]−	red	d6, 18e−	tetrahedral[24]	see Co(CO)4−
[Ni(CNMe)4]0	colourless	d10, 18e−	tetrahedral	see Ni(CO)4
[Ni(CNC6H3-2,6-iPr2)4]2+	yellow	d8	square planar[25]	see [Ni(CN)4]2-
[Ni4(CN-t-Bu)7]0	red	d10	cluster[26]
[Cu(CNMe)4]+	colourless	d10, 18e−	tetrahedral	analogous [Cu(CO)4]+ is unknown

IR spectroscopy

The ν_C≡N band in isocyanides is intense in the range of 2165–2110 cm⁻¹.^[27] The value of ν_C≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, ν_C≡N shifts to higher energies vs free isocyanide. Thus, for [Co(CN−t−Bu)₅]⁺, ν_C≡N = 2152, 2120 cm^−l.^[23] In contrast, for the electron-rich species Fe₂(CNEt)₉, ν_C≡N = 2060, 1920, 1701, 1652 cm^−l.^[28]

See also

• Cyanometalate - coordination compounds containing cyanide ligands (coordinating via C)
• Transition metal nitrile complexes - coordination compounds containing nitrile ligands, which are isomers of isonitriles