Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Because isocyanides are relatively basic, but also good pi-acceptors, a wide range of complexes are known. Some isocyanide complexes are used in medical imaging.
Several thousand isocyanides are known, but the coordination chemistry is dominated by a few ligands. [3] Common isonitrile ligands are methyl isocyanide, tert-butyl isocyanide, phenyl isocyanide, and cyclohexylisocyanide.
Isocyanides are electronically similar to CO, but for most R groups, isocyanides are superior Lewis bases and weaker pi-acceptors. Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO.
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]+. [4] Very bulky isocyanide ligands are also known, e.g. C6H3-2,6-Ar2-NC (Ar =aryl). [5]
Di- and triisocyanide ligands are well developed, e.g., (CH2)n(NC)2. (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]
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]
Typically, isocyanides are spectator ligands, but their reduced and oxidized complexes can prove reactive by virtue of the unsaturated nature of the ligand
Cationic complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared. Because isocyanides are both acceptors and donors, they stabilize a broader range of oxidation states than does CO. This advantage 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. [11]
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]
Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes: [12]
Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls. [13]
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.
| 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 |
The νC≡N band in isocyanides is intense in the range of 2165–2110 cm−1. [27] The value of νC≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, νC≡N shifts to higher energies vs free isocyanide. Thus, for [Co(CN−t−Bu)5]+, νC≡N = 2152, 2120 cm−l. [23] In contrast, for the electron-rich species Fe2(CNEt)9, νC≡N = 2060, 1920, 1701, 1652 cm−l. [28]
In chemistry, π backbonding is a π-bonding interaction between a filled (or half filled) orbital of a transition metal atom and a vacant orbital on an adjacent ion or molecule. In this type of interaction, electrons from the metal are used to bond to the ligand, which dissipates excess negative charge and stabilizes the metal. It is common in transition metals with low oxidation states that have ligands such as carbon monoxide, olefins, or phosphines. The ligands involved in π backbonding can be broken into three groups: carbonyls and nitrogen analogs, alkenes and alkynes, and phosphines. Compounds where π backbonding is prominent include Ni(CO)4, Zeise's salt, and molybdenym and iron dinitrogen complexes.
An isocyanide is an organic compound with the functional group –N+≡C−. It is the isomer of the related nitrile (–C≡N), hence the prefix is isocyano. The organic fragment is connected to the isocyanide group through the nitrogen atom, not via the carbon. They are used as building blocks for the synthesis of other compounds.
Chromium hexacarbonyl is a chromium(0) organometallic compound with the formula Cr(CO)6. It is a homoleptic complex, which means that all the ligands are identical. It is a colorless crystalline air-stable solid, with a high vapor pressure.
The Reformatsky reaction is an organic reaction which condenses aldehydes or ketones with α-halo esters using metallic zinc to form β-hydroxy-esters:
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes.
tert-Butyl isocyanide is an organic compound with the formula Me3CNC (Me = methyl, CH3). It is an isocyanide, commonly called isonitrile or carbylamine, as defined by the functional group C≡N-R. tert-Butyl isocyanide, like most alkyl isocyanides, is a reactive colorless liquid with an extremely unpleasant odor. It forms stable complexes with transition metals and can insert into metal-carbon bonds.
Salcomine is a coordination complex derived from the salen ligand and cobalt. The complex, which is planar, and a variety of its derivatives are carriers for O2 as well as oxidation catalysts.
Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
Peter Michael Maitlis, FRS was a British organometallic chemist.
Cyanometallates or cyanometalates are a class of coordination compounds, most often consisting only of cyanide ligands. Most are anions. Cyanide is a highly basic and small ligand, hence it readily saturates the coordination sphere of metal ions. The resulting cyanometallate anions are often used as building blocks for more complex structures called coordination polymers, the best known example of which is Prussian blue, a common dyestuff.
A metal salen complex is a coordination compound between a metal cation and a ligand derived from N,N′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt Co2+, usually denoted as Co(salen). These complexes are widely investigated as catalysts and enzyme mimics.
Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples include cyclobutadieneiron tricarbonyl and (C5H5)TiCl3. Commercially useful examples include (C5H5)Co(CO)2, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.
Transition metal thiolate complexes are metal complexes containing thiolate ligands. Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as opposed to those that behave as hard Lewis acids. Most complexes contain other ligands in addition to thiolate, but many homoleptic complexes are known with only thiolate ligands. The amino acid cysteine has a thiol functional group, consequently many cofactors in proteins and enzymes feature cysteinate-metal cofactors.
Transition metal alkyl complexes are coordination complexes that contain a bond between a transition metal and an alkyl ligand. Such complexes are not only pervasive but are of practical and theoretical interest.
Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile.
Transition metal pyridine complexes encompass many coordination complexes that contain pyridine as a ligand. Most examples are mixed-ligand complexes. Many variants of pyridine are also known to coordinate to metal ions, such as the methylpyridines, quinolines, and more complex rings.
Sodium naphthalene is an organic salt with the chemical formula Na+[C10H8]−. In the research laboratory, it is used as a reductant in the synthesis of organic, organometallic, and inorganic chemistry. It is usually generated in situ. When isolated, it invariably crystallizes as a solvate with ligands bound to Na+.
In chemistry, a transition metal ether complex is a coordination complex consisting of a transition metal bonded to one or more ether ligand. The inventory of complexes is extensive. Common ether ligands are diethyl ether and tetrahydrofuran. Common chelating ether ligands include the glymes, dimethoxyethane (dme) and diglyme, and the crown ethers. Being lipophilic, metal-ether complexes often exhibit solubility in organic solvents, a property of interest in synthetic chemistry. In contrast, the di-ether 1,4-dioxane is generally a bridging ligand.
Bismuthinidenes are a class of organobismuth compounds, analogous to carbenes. These compounds have the general form R-Bi, with two lone pairs of electrons on the central bismuth(I) atom. Due to the unusually low valency and oxidation state of +1, most bismuthinidenes are reactive and unstable, though in recent decades, both transition metals and polydentate chelating Lewis base ligands have been employed to stabilize the low-valent bismuth(I) center through steric protection and π donation either in solution or in crystal structures. Lewis base-stabilized bismuthinidenes adopt a singlet ground state with an inert lone pair of electrons in the 6s orbital. A second lone pair in a 6p orbital and a single empty 6p orbital make Lewis base-stabilized bismuthinidenes ambiphilic.
Transition metal complexes of thiocyanate describes coordination complexes containing one or more thiocyanate (SCN-) ligands. The topic also includes transition metal complexes of isothiocyanate. These complexes have few applications but played significant role in the development of coordination chemistry.
{{cite journal}}: CS1 maint: multiple names: authors list (link)