Metals hydrido carbonyl complexes

In general, trivalent phosphorus compounds, arsines, stibenes and several amines improve the thermal stability of hydrido metal-carbonyl complexes because of superior o-donor and weaker it-acceptor properties [18]. This feature enhances the electron density at the metal center, and hence the metal-CO bond is strengthened as a result of enhanced electron backdonation. However, the special effect of a ligand on the activity and selectivity may be entirely different from one metal to another, and therefore conclusions should be drawn only in close relation to the metal that is used. Only some selected observations will be detailed here, showing the uniqueness of each catalytic system. 

Nitroarenes are reduced to anilines (>85%) under the influence of metal carbonyl complexes. In a two-phase system, the complex hydridoiron complex [HFe,(CO)u]2-is produced from tri-iron dodecacarbonyl at the interface between the organic phase and the basic aqueous phase [7], The generation of the active hydridoiron complex is catalysed by a range of quaternary ammonium salts and an analogous hydrido-manganese complex is obtained from dimanganese decacarbonyl under similar conditions [8], Virtually no reduction occurs in the absence of the quaternary ammonium salt, and the reduction is also suppressed by the presence of carbon monoxide [9], In contrast, dicobalt octacarbonyl reacts with quaternary ammonium fluorides to form complexes which do not reduce nitroarenes. 

Transition metal carbonyl complexes can be suitable precursors, for example, treatment of hydrido complexes or their alkali salts with PF3 under UV irradiation followed if necessary by acidification of the reaction mixture (method E). Interestingly, vanadium forms the hydrido trifluorophosphine complex [VH(PF3)6], whereas with CO, the paramagnetic [V(CO)6] is formed. 

Another salient feature of these hydrido metal carbonyl systems is the underestimation, at the SCF level, of the d - back donation which we have already mentioned. This is not peculiar to the hydrido carbonyl complexes and has already been noted in many other carbonyl complexes (7-10,25,26). It is best evidenced by the orbital occupation number of the d orbitals (Table V) which range between 1.91 and 1.94e, with the noticeable exception of the HCr(CO)4(NO) system... 

From formal retrosynthetic considerations and a mechanistic point of view these carbonylations are closely related. The reactions are thought to proceed as shown in Scheme 1. The salient points of the mechanism include ligand dissociation to the active catalyst (eq. (2)), oxidative addition (eq. (3)) of the organic compound to a metal(O) complex (e. g., M = Pd, Co, Ni, Pt L = PR3), CO insertion forming a metal(II) acyl complex (eq. (4)), either reductive elimination (eq. (5)) or reaction of this complex with nueleophiles yielding the carbonylated product and an X(hydrido)metal(II) complex (eq. (6)) and elimination of HX to regenerate the original catalyst (eq. (7)). 

Organometallic probe molecules possessing bifunctional character (i.e., both basic and acidic functional groups) provide means of probing bifunctional surface sites see Table l-I. Hydrido-metal carbonyls are an example of such bifxinctional complexes which also provide convenient spectroscopic means of characterization. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

The chemistry of the metal carbonyl hydrides and metal carbonylates remained the principal research topic for Hieber until the 1960s. He mentioned in his account [25], that it was a particular pleasure for him that in his laboratory the first hydrido carbonyl complexes of the manganese group, HMn(CO)5 and HRe(CO)5, were prepared by careful addition of concentrated phosphoric acid to solid samples of the sodium salts of the [M(CO)5] anions, giving the highly volatile hydrido derivatives in nearly quantitative yield [45, 46]. In contrast to HCo(CO)4 and its rhodium and iridium analogues, the pentacarbonyl hydrido compounds of manganese and rhenium are thermally remarkably stable, and in... 

For many of these compounds the method of preparation is to react an MX2 compound with a hydrido complex or a transition metal carbonylate anion, as in the following reactions ... 

An interesting feature of hydrido transition metal-PF3 complexes is that apart from a few dinuclear systems (Section VI) only mononuclear systems are so far known and there is as yet no corresponding chemistry analogous to that of polynuclear carbonyl hydrido compounds. The trifluorophosphine metal hydrido compounds are usually highly acidic and can readily form metallate ions such as [M(PF3)m]x and [MH(PF3) r. 

An important class of metal-metal bonded dimers is the hydrido-bridged carbonyls (see Hydrides Solid State Transition Metal Complexes). These can be obtained through a number of reactions, most commonly, through protonation and direct reaction with hydrogen. Protonation of the metal carbonyl dianion Mo2(CO)io yields the complex shown in equation (10). 

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