Hydrido transition metal carbonyls

There seems to be some confusion about the polarization of the metal-hydrogen bond in certain hydrido-transition metal-carbonyl compounds, in particular HCo(CO)4. This and some related hydrido-carbonyl compounds are slightly soluble in water, and such solutions are clearly acidic In the gas phase or in non-polar solvents, however, these complexes exhibit spectroscopic and chemical properties similar to those of other non-acidic hydride complexes. A MO calculation of the charge densities in HCo(CO)4 indicated that 1.6 electrons are associated with the hydrogen atom, and the question was discussed as to how a molecule with a negative-... 

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. 

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. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

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. 

Several examples of direct ring phosphorus interaction with transition metals are now known [279-287]. For example, reaction of N3P3CI6 with Na2Fe2(CO)g affords a novel spirocyclic diiron octa carbonyl derivative (Scheme 25) [282, 283]. The diiron spiro derivative acts as a template for the construction of several transition metal clusters. Some of the other examples where ring phosphorus atom is involved in interaction with transition metals are summarized in Scheme 25. The ring phosphorus atoms in hydrido phos-phazenes, N3P3R4R H also coordinates to transition metals. This has been discussed in an earlier section (vide supra). 

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). 

It has been shown that in a number of transition-metal hydrido complexes H atoms are directly bonded to M and occupy definite coordination positions (as in K2ReH9, HPtBr[P(C2Hs)3] 2, etc., for which see Chapter 27). This would also appear to be true in certain carbonyl hydrides, though the H atoms have been directly located by n.d. in only one form of Mn(CO)5H. In Mn(CO)5H five CO are situated at five of the vertices of an octahedron (Mn-C, 1 -84 A) and the H atom is situated at the sixth vertex, with Mn-H, 1-60 (C-0, M34 A). 

This chapter is largely concerned with the structural chemistry of Fe, Co, Ni, Pd, and Pt, for the most part in finite complexes. The structures of the simpler compounds of these and other transition metals have been described under Halides, Oxides, etc. in the appropriate chapters. Other groups of compounds described in earlier chapters include hydrido compounds, oxo-, peroxo-, and superoxo-compounds, carbonyls, and nitrosyls, in Chapters 11, 12, and 18, respectively. We note here a few general points. 

There are only limited pKj, scales for transition metal hydride complexes compared to the extensive scales for organic and inorganic acids, despite the fact that hydrides often mediate organometallic reactions these were reviewed in 1991 [42]. Most of these complexes contain carbonyl and/or phosphine ligands and are usually insoluble in water. The for about 20 neutral hydrido-carbonyl... 

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

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