Homoleptic Carbonyl Complexes

The number of TM carbonyl complexes for which experimental BDEs are known is relatively large. Theoretical studies of neutral 18-valence electron carbonyl complexes of groups 6, 8 and 10 have been reported [37-42], Table 7.1 lists the calculated and experimental values of De and Do 

BP86 also fails to predict the relative BDEs of Au(CO)+ and Au(CO) . The B3LYP approach produces a higher bond energy of Au(CO)J as compared with Au(CO)+ but this difference is much smaller than that predicted within the CCSD(T) approximation. 

Singly substituted species M(CO)n iL have been investigated [49-51, 54, 55], Table 7.3 lists the calculated BDEs of the group-6 complexes of the type M(CO)5L (M = Cr, Mo, W) with various ligands L, whereas Table 7.4 contains the BDEs for the W-L and the W-CO bonds in W(CO)5L complexes. The latter values are given for the least bonded carbonyl ligand. Note that BDEs for other W(CO)5L complexes with certain particular classes of ligands are discussed in other sections of this chapter. 

---

Hydrogenation of C02 to formic acid could potentially proceed first by reduction to CO, followed by a reaction between CO and water to give formic acid, a reaction which is known (Eq. (3)). It is unlikely that this pathway to formic acid is common because very few homogeneous catalysts (primarily homoleptic carbonyl complexes) [71-73] have been reported for the hydrogenation of COz to CO, and because the few C02 hydrogenation catalysts that have deliberately been exposed to CO, in order to check whether this pathway is operating, have been poisoned as a result [18, 19, 31, 74]. 

Table 7.1 Calculated first BDEs De (Do) (kcal/mol) of neutral homoleptic carbonyl complexes. ...

The structures and properties of the binary (homoleptic) carbonyl complexes of the transition elements, including representative synthetic routes... 

Homoleptic carbonyl complexes of metals in the second and third rows of the transition series form polynuclear carbonyl complexes more often than they form mononuclear carbonyl complexes. For example, Fe(CO)5 is stable and is the most common iron carbonyl, but Os(CO)j is much less stable Os3(CO)jj is more stable. 

Like nickel, iron can also be purified using a carbonyl compound. Iron purified this way is called carbonyl iron, and the iron has an oxidation state of zero. Briefly rationalize why iron(III) does not form a complex with carbonyls whereas Fe(0) does. Based on the EAN rule, speculate on the most likely homoleptic carbonyl complex formed by iron(O). 

Simple Carbonyls, Anions and Cations 6.11.2.1 Homoleptic Carbonyl Complexes... 

Table 1 shows the calculated bond lengths of neutral homoleptic carbonyl complexes at the HF and MP2 levels of theory using basis set 11. Furthermore, DFT results using the BP86 functionals with two different basis sets are shown. One set of DFT calculations using BP86 used relativistic ECPs with TZ quality valence basis sets for the transition... 

The first homoleptic, dinuclear platinum(I) carbonyl complex [Pt2(CO)6]2+ has been prepared by dissolving Pt02 in concentrated sulfuric acid under a CO atmosphere.92,93 The structure is rigid on the NMR time scale at room temperature. DFT studies suggested a staggered structure for the dimer.92,93... 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

Homogeneous wetting, 22 111 Homogenization, 26 699 aluminum alloys, 2 329 Homogenizers, 8 703 10 127 Homoglycans, 4 697, 701 23 62-64 classification by structure, 4 723t Homo-interface, 24 71 Homo-ionic interactions, 8 77 Homojunction BJTs, 22 166 Homojunction devices, LEDs asm 173 Homojunction diode arrays, 29 163 Homojunction laser diode, 24 699 Homoleptic tetranuclear carbonyl complexes, 26 63... 

A recent review has highlighted the extensive and interesting chemistry of metal isocyanide complexes.1 Although synthetic procedures are varied, a vast number are based on substitution in metal carbonyl complexes by isocyanides. Such procedures are, however, not always successful. This is especially so in cases where multiple substitution of CO is required, as in the syntheses of homoleptic isocyanide complexes. Many of the inherent difficulties are illustrated by the reaction of iron pentacarbonyl with isocyanides. 

Naturally, the ideal source of starting materials for homoleptic metal isocyanide compounds is via metal carbonyl complexes, but previously only with the two carbonyls Ni(CO)4 (24) and Co2(CO)g (25) has direct substitution of all carbonyl groups been effected. Recently, however, remarkable discoveries by Coville and co-workers (26-31) on the transition-metal-catalyzed substitution of carbonyl groups in monomeric and cluster compounds have shown that Fe(CNR)s, Mo(CNR)6, and Ir4(CO)5(CNR)7 (32) can be prepared in high yield by stepwise substitution from the parent carbonyl. 

There are few other examples of complete substitution of carbonyl groups from a homoleptic metal-carbonyl complex by isocyanide ligands [cf. Ni(CO)4 (24), Fe(CO)s (26), and Mo(CO)6 (124)]. The corresponding butyl isocyanide derivative Co CNBuOg was formed by reduction of [Co(CNBu )5]PF6 with potassium amalgam (19). 

The classical protocol for synthesis of iron-diene complexes starts from the homoleptic pentacarbonyliron complex. In a stepwise fashion, via a dissociative mechanism, two carbonyl ligands are displaced by the diene system. However, thermal dissociation of the first CO ligand requires rather harsh conditions (ca. 140 °C). For acyclic 1,3-dienes, the diene ligand adopts an s-cis conformation to form stable q4-complexes (Scheme 1.18). 

Homoleptic carbonyl ligands, in palladium complexes, 8, 197 Homoleptic chromium alkyl compounds, preparation,... 

Homoleptic platinum carbonyl anions, characteristics, 8, 410 Homoleptic tantalum complexes, preparation and characteristics, 5, 108... 

Photochemical reactions of the homoleptic carbonyls of manganese and rhenium with donor ligands have been thoroughly studied in recent years (2-4). The nature of the primary photoproducts of the complexes M2(CO)10 (M = Mn, Re) has been investigated by different techniques (151-165). The... 

In this work the ultrafast photophysical processes associated with photoinduced CO loss will be described with particular regard to the mononuclear homoleptic carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4. These complexes were the earliest metal carbonyls to be synthesized, indeed Fe(CO)5 and Ni(CO)4 have been known since the latter part of the nineteenth century. The carbonylation of nickel provided an important route to high purity nickel which was required by many important industrial processes. However, it is only within the last few decades that adequate models have been developed to explain the mechanisms of photoinduced CO loss from metal carbonyl complexes. 

Isocyanides (RNC) are better a donors and poorer xr acceptors than CO, as indicated by the observation that typical homoleptic isonitrile complexes of many metals are in higher oxidation states than the typical carbonyl complexes of the same metal. Some metal isocyanide complexes are given in Table 7-4. It should be pointed out that there are no carbonyl analogues of those compounds in the higher oxidation... 

The simplest homoleptic carbonyl species is Fe(CO)5. The 18 electron complex is easily made by direct combination of highly dispersed metal and CO at high tempera-... 

The majority of these contain CO ligands. The neutral homoleptic carbonyls have not been isolated, but anionic M(CO)6 as well as highly reduced M(CO) species are known. The original synthesis of the -1 species required elevated temperatures and high pressure but recently two simple, atmospheric pressure methods have been developed. They involve reduction of pentahalides in pyridine with Zn/Mg or in dimethoxyethane with sodium naphthalenide under an atmosphere of CO. These yellow salts contain discrete M(CO)6 anions. The facile syntheses of the octahedral hexacarbonyl anions allowed systematic exploration of the previously difficult to access area of low-valent complexes of Nb and Ta. Since M(CO)s are rather inert towards displacement of CO the substitution products of general formula M(CO)6- L have to be obtained by other routes, for example, by reduction of MX(CO)6- L compounds. The monosubstituted derivatives are conveniently prepared via the following method87 ... 

A recent review gives a chronological survey of the syntheses and characterizations of homoleptic mononuclear metal-carbonyl (see Carbonyl Complexes of the Transition Metals) anions. [M(CO)6] is a key precursor to niobium and tantalum carbonyls. More reliable synthesis of [Ta(CO)6] ... 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

Isonitriles are isoelectronic with carbon monoxide bnt homoleptic see Homoleptic Compound) isonitrile complexes are more difficnlt to prepare than carbonyl complexes. The reduction of OsXg with ethanol in the presence of methylisonitrile gives OsX2(CNMe)4. The osmium(II) isonitrile complex [Os(CNMe)6] + results from the alkylation of [Os(CN)6]" with MeOS02CF3. More generally, [Os(CNR)6] + can be prepared by reaction of 0s2(02CMe)4Cl2 with alkyl isonitriles. 

---