Cationic metal carbonyls

Nesmeyanov has provided interesting examples of apparent intramolecular nucleophilic attack by amine on carbonyl ligands (37). Angelici (38,39) has demonstrated that amine attack on cationic metal carbonyl complexes is a general reaction resulting in the formation of carbamoyl complexes ... 

Numerous synthetically useful carbon-carbon bond-forming reactions are based on the fact that unsaturated hydrocarbon ligands bound to electrophilic transition metal moieties are activated toward addition of nucleophiles. Normally the metal moiety in such complexes is a neutral or cationic metal carbonyl group. Prominent and well-studied examples include [Cr(arene)(CO)3] complexes (covered in Chapter 2.4, this volume),1 [Fe(dienyl)(CO)3]+ complexes (covered in Chapter 3.4, this volume),2 [FeCp(CO)2(alkene)]+ complexes3 and [M(CO) (diene)] complexes.4... 

The phenomenal growth in the chemistry of metal carbonyl compounds has been heavily concentrated upon the neutral and anionic complexes. Only very recently have the occurrence and usefulness of the cationic metal carbonyls come into real prominence. 

The first recognized member of this species Co(CO)3[(C6H5)3P]2 + [Co(CO)4] was reported (219) as recently as 1958 though it seems likely that cationic metal carbonyls had been made, but unrecognized, at a much earlier date. Thus, for example, Schiitzenburger (222) reported in 1870 the interaction of ammonia and dichloroplatinum(II) dicarbonyl. 

The cationic metal carbonyls are, by definition, electrolytes in polar solvents, and are in general diamagnetic and conform to the rare gas formalism of metal carbonyls (7, 57). 

The nature of bonding in the cationic metal carbonyls has been investigated by both vibrational and electronic spectroscopy, and molecular orbital calculations have been carried out these are consistent with a bonding scheme for carbon monoxide coordinated to a metal, consisting of a dative cr-bond from carbon to metal, augmented by a synergic metal-to-carbonyl dative 7r-bond (7, 57). 

The characteristic high carbonyl frequencies of cationic metal carbonyls are rationalized in terms of a reduced 77-back-donation from the metal (7, 56). Such a proposal implicitly assumes that the carbon-metal cr-bond is independent of charge on the metal, and has been supported by semi-empirical molecular orbital calculations. This assumption of invariance of carbon-metal and carbon-oxygen o--bonds with change in the metal s oxidation state has been invalidated by a Raman and infrared investigation of M(CO)6+, M(CO)6, and M(CO)6 species (3, 4). It appears that the carbon-metal cr-bond increases with rise of positive charge on the metal, with concomitant decrease of metal-carbon 7r-bonding. 

The relative reactivity of cationic metal carbonyls has been predicted in the case of Mn(CO)6+ relative to Cr(CO)6 and V(CO)6 (55). The substitution behavior of Re(CO)6+ relative to W(CO)6 indicates that the cation is not more readily substituted by neutral ligands (3, 6), which correlates with the spectroscopic investigations (3, 4) of the metal-carbonyl bond. 

The high carbon-oxygen bond order in cationic metal carbonyls due to the reduced 77-back-donation is further supported by the intensities of the infrared carbonyl stretching bands (25), estimates of the degree of back-donation from electronic spectra (4), and from calculations of 77-overlap populations (35). 

The methods that have been utilized to prepare the cationic metal carbonyls cover an exceedingly wide range of chemical reactions. An effort has been made to classify these in Sections A to H below. 

The reaction of binuclear metal carbonyls with bases has been recognized (137, 229, 244) as producing cationic metal carbonyl species, although the earlier derivatives defied isolation. This has been classed as a base reaction (122). 

The addition of a proton to a metal carbonyl compound may occur in either of two modes the formation of metal-hydrogen bond, or protonation of a ligand attached to the central metal atom. If the ligand protonated is an organic radical, a carbonium ion is produced, which may be stabilized by suitable delocalization of charge over the complex, including the central metal atom. Consequently, such protonated species may be legitimately considered as examples of cationic metal carbonyl compounds. 

Since, in many cases the metal esters may be prepared from the metal carbonylate anion and a chloroformate, this reaction scheme enables cationic metal carbonyl compounds to be prepared from a neutral carbonyl compound, without recourse to high-pressure carbonylation, as required in Section A,2. Such a synthetic route is indicated as follows ... 

The alkylation of an unsaturated atom or moiety bonded to a metal carbonyl produces an onium salt. This complex may be considered to be a further example of a cationic metal carbonyl, provided that the formal charge on the alkylated center can interact with the atomic orbitals of the metal. This may be justified on the basis of electronegativity differences between the metal atom and the alkylated center. It seems reasonable to argue that the location of the formal positive charge is on the electropositive metal atom, although the metal is probably only fractionally charged. 

Hydride abstraction of a hydrogen directly bonded to a metal atom has been used to synthesize cationic metal carbonyls. This may be accomplished by protonation, as outlined in Section C,l,f, or with a Lewis acid, such as boron trifluoride, in the presence of carbon monoxide (98). 

A variety of unusual cationic metal carbonyl derivatives displaying uncommon oxidation states, geometries, and magnetic properties have been obtained by the controlled oxidation of some metal carbonyl compounds. 

Several cationic metal carbonyls have been prepared by carbonylation of cationic metal complexes. The metal remains in the same oxidation state in this process, but can either change or retain its coordination number. 

The replacement of ligands of a metal carbonyl cation, either carbon monoxide or another ligand, serves as a valuable route to substituted cationic metal carbonyls. 

The displacement of carbon monoxide from a cationic metal carbonyl has been reported in only a few instances [v-triars = tris-1,1,1-(dimethyl-arsinomethyl)ethane]. 

The general classification outlined in Sections A to G covers the vast majority of the types of reaction whereby cationic metal carbonyl compounds are prepared. There remain, nevertheless, a few further reactions which, for the sake of completeness, are included in this survey, but which do not belong to a specific section. 

The substitution of a neutral ligand, including carbon monoxide, attached to a metal carbonyl complex by the nitrosyl cation, produces a cationic metal carbonyl nitrosyl. 

SURVEY OF THE CATIONIC METAL CARBONYLS AND THEIR PROPERTIES BY GROUPS... 

The first successful attempt to prepare a tropylium cationic metal carbonyl was by hydride abstraction from cycloheptatrienylmolybdenum tricarbonyl using triphenylmethyl tetraduoroborate to produce [C7H7... 

This reaction requires the existence of a stable neutral or cationic metal-carbonyl complex, thus confining its applicability to the Cr, Fe, Mn, Co, Ni triads. The preparation of acyl complexes of these metals is presented here and the relative stability and reactivity discussed. The literature is reviewed to September 1988, although most of the work appeared in 1975 or before. 

Carbamoyl-metal complexes are prepared by the reaction between a cationic metal-carbonyl complex and NHj, or a primary or secondary alkyl (not aryl) amine ... 

Table 1. Reactions of Cationic Metal Carbonyls with Amines and Ammonia...

In reactions of [Mn(NOXCO)2Cp- / ][PFg] and derivatives with alkoxides the products possess a chiral metal center. The reaction sequence illustrates the application of a cationic metal carbonyl-alkoxide reaction to resolve optical isomers of [Mn(NOXCOXPPh3XCp-i/ )][PFg] using menthoxide Na[OC,oH,g]... 

Carborane-metal complexes, 8, 87-113 Catalysis by organometallics carbonylation, 6, 158-163 hydroformylation, 6, 128-136 olefin reactions, 7, 199-202 organic syntheses by nickel compounds, 8, 48-83 reviews, 10, 331-336 Cationic metal carbonyls, 8, 117-159 Chromium carbonyls, 8, 133-159, see also Benchrotrenes Cobaloximes, 7, 161, 203 Cobalt carbonyl, 6, 119-163 8, 152-155 Cotton-Kraihanzel method, 10, 213-214 Coupling reactions, on nickel, 8, 30-39, 82-83... 

This base reaction does not depend on hydroxide and proceeds under mild conditions, thus replacing the often less-clean photoelimination of carbon monoxide. Certain cationic metal carbonyls are attacked even by water, a reaction that... 

An alternative preparation method for metal TjHCl-COa complexes is the deprotonation of hydroxycarbonyl complexes 25). A general route for the preparation of hydroxycarbonyl complexes is a nucleophilic attack of OH" or H2O on cationic metal carbonyl species (52) ... 

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