Binuclear Simple Carbonyls

Binuclear Simple Carbonyls. Mechanisms of reactions of the binuclear carbonyls M2(CO)io continue to be a subject of research and discussion. The most recent paper in this field opens with a useful review of current data and opinions, and offers e.s.r. evidence for the intermediacy of radicals in reaction of Mii2(CO)io with tributylphosphine or triethyl-phosphine. Rates of substitution at Mn2(CO)io by poor nucleophiles are decreased by the presence of an excess of carbon monoxide, are independent of nucleophile concentration, and are associated with large positive activation energies. These, and earlier results reported by other workers, can all be accommodated by a mechanism involving reversible loss of carbon monoxide  

While intermediates such as Mn(CO)s and (OC)sMn(CO)Mn(CO)5 might be generated, there is no need to invoke them to explain the kinetic results. However, a comparison of kinetics of substitution at Re2(CO)io in decalin with earlier results on substitution in this carbonyl and in Mn2(CO)io has led to the proposal of a general substitution mechanism for these carbonyls in which metal migration to form an intermediate (OC)6M(CO)M(CO)6 is thought to be the rate-determining step.  

---

Simple carbonyls of the M(C0) type are generally colourless, though Fe(CO)5 is yellow. The hexacarbonyls formed by Gp VI metals are colourless solids, the rest liquids. The binuclear carbonyls with more than one metal atom per molecule are usually coloured solids, though the monoclinic crystals of Re2(CO)jo are colourless. 

The metal carbonyl clusters correspond to situations intermediate between metals and simple mononuclear or binuclear carbonyls. Their existence must be connected either with a delicate thermodynamic balance or with remarkably high activation energies. The last hypothesis is valid for species such as Rh6(CO)j6 which is kineti-cally inert, but in general we are inclined to believe that thermodynamic control is the more significant, especially since reactions, such as that shown in Eq. (1), can be carried out in both directions using mild conditions. 

Rhenium(0) compounds are rare and frequently lie in the realm of the organometallic chemistry. A simple example is decacarbonyldirhenium(0) in which two staggered, square-pyramidal Re(CO)5 fragments are held together by a single rhenium-rhenium bond. Substitution of carbonyl ligands is possible by tertiary phosphines and arsines, silanes and isocyanides, and binuclear Re-Re, Mn-Re, and Co-Re complexes have been studied. " Successive replacement of CO ligands can readily be observed by vibrational spectroscopy. This has been demonstrated... 

The synthesis of pentacarbonyl rhenium(I) halides, Re(CO)5X, succeeded from simple and complex rhenium halides below 200 atm of CO at 200° C. The compounds are extraordinarily stable and form easily, often quantitatively, from carbon monoxide and rhenium metal in the presence of other heavy metal halides or halogen sources such as CC14. Later we prepared the corresponding carbonyl halides of manganese (67) and technetium (68) from their respective carbonyls. It was found that the corresponding binuclear tetracarbonyl halides [M(CO)4X]2 (M = Mn, Re) could be made by heating the mononuclear M(CO)5X complexes (15, 69), as well as by other methods. 

These studies have Indicated that simple rhodium carbonyl complexes, e.g., mono- and binuclear species are Involved in the fragmentation and aggregation reactions of rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen. They indicate that it is possible to write formal equations for such reactions in the case of rhodium carbonyl anionic hydrido clusters (equation 25) and for the more particular situation when there are not hydrides present (equation 26)... 

The relationships between bond enthalpy, bond length and bond order which appear relatively simple in the case of a main group element such as carbon and its compounds, are more difficult to establish when the d-transition metal elements and their compounds are considered. Progress in establishing these relationships for metals is severely hindered by a lack of relevant thermochemical data. This paper reviews some of the more useful information that is available for diatomic molecules, for polynuclear binary carbonyls and for binuclear complexes of the d-transition elements. 

Very few investigations of the electronic spectra of clusters have been made, probably because satisfactory assignment of the spectra of simple binuclear metal-metal-bonded carbonyls has yet to be made. Gray and co-workers 72) have had some success in measuring and assigning polarized electron absorption bands of Mn2(CO)4o and Re2(CO)io by orienting the molecules in a nematic liquid crystal, but this technique would not appear to be capable of application to clusters. 

The simplest mechanism for CO migration in a tetranuclear carbonyl is represented on Figure 25. This process involves a interconversion of the M4(CO)i2 clusters. There is a simple bridge-break, bridge-make mechanism of the type employed in binuclear complexes. 

There have been two reports of CO migrations in binuclear metal complexes. In both cases carbonyl motions are coupled with other ligand movements. Whilst simple W—W bond rotation is responsible for the isomerization in solution of [W2(CO)gCp2], the P and NMR spectra of [WRh(/i-CO)2(CO)(PPhj)2Cp] establish a pseudorotational process that interchanges the inequivalent PPhj groups and all CO ligands. The carbonyl fluxionality of [CH2 (f/ -C5H4)M(CO) 2(At-CO)] (M = Rh, Ir)... 

---