Isonitrile ligands

Cyclic Carhene Complexes. The reaction of aziridines with carbonyl, thiocarbonyl, or isonitrile ligands in Mn, Re, Fe, Ru, Pd, or Pt complexes leads to formation of cycHc carhene complexes (324—331). 

The tetrahedral nickel isonitrile cluster Ni4(CNR)7 (R = cyclohexyl or f-Bu) contains three bridging isonitrile ligands and a terminal one at each nickel (52) the bridging ligands are mobile on the cluster surface and a ligand dissociation step occurs (33, 390, 391). 

A similar mechanism was proposed, based on analogous kinetic behavior, for Pt(IV) tetramethyl complexes with isonitrile ligands (47). Isonitrile dissociation preceded the release of ethane. 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

The abundant physical evidence on the molecular structure of isonitriles— namely, infrared and Raman spectra (19), nuclear magnetic resonance (28), electron diffraction studies (9), and dipole moment measurements (3d)—indicates that the isonitrile group is best represented by the polar form, B. When coordinated to a metal, the isonitrile ligand acts as a donor, C, but also as an acceptor of electrons, D (5). 

The problem of to what extent and how the reactivity of the isonitrile ligand is altered by coordination to a metal has an important bearing on the function of transition metals as homogeneous catalysts. It is also of interest from the standpoint of energy transfer in vivo, to determine whether or not there is any correlation between the reactivities of isonitrile complexes and their conductivities. In this paper we summarize some recent work on the syntheses and reactions of iron isonitrile complexes. 

A general method for the preparation of iron isonitrile complexes is the addition of the isonitrile ligand to iron (II) perchlorate (33) ... 

These findings are surprising, since the isonitrile ligand is best represented by the immonium form ... 

It is now of interest to consider the electron-donor capabilities of the isonitrile ligand as influenced by the other ligands in the complex during electrophilic reactions. 

Mixed cyanoisonitrile complexes have been reported to result from the reaction of Ag4[W(CN)8] with RNC (R = Me, Et, Prn, Pr Bu and CPh3).264 The resulting [W(CN)4(CNR)4] complexes are dodecahedral, with the more strongly jt accepting isonitrile ligands occupying the B position. These complexes can also be obtained by the reaction of the silver salt with alkyl iodide.255... 

When an isonitrile ligand containing a suitable hydroxyl group was reacted with AUCI4, it underwent isomerization (147) via a carbene complex (Scheme 1). With CN(CH2)3OH, in the presence of a noncoordinating anion, the bis(carbene)gold(I) complex [Au tO(CH2)3l(jH 2]+BPhJ was formed. 

Spectroscopic data show that the olefin monocyanocarbonylferrates and -ruthenates and their derived isonitrile complexes occur as two isomeric forms, in which the CN or isonitrile ligand occupies either an apical or basal position in a square-pyramidal structure. 

The X-ray structure of the monoclinic CgH8Fe(CO)2CNEt (Fig. 7) (CgHg = 1,3-cyclohexadiene) shows that the isonitrile ligand CNEt, a CO group, and two C atoms of the diene part of the CgHg ring occupy the basal positions of a square pyramid, with the second CO molecule at the apex (158). 

Five-coordinate d4 alkyne derivatives are rare. Addition of acetylene gas to Mo(CNBu )4(SBu )2 in toluene at 30°C forms Mo(HC=CH)(CNBu )2-(SBu )2 as a single acetylene displaces two bulky isonitrile ligands. Slightly higher temperatures (50°C) are required to promote formation of... 

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