CO Coordination

The CO molecule can be bonded in a terminal or bridged fashion. The metal-carbonyl bridges are not always symmetric and. in some cases, the CO can be considered a four-electron donor, as in Mn2(CO)s(PhjPCH PPli2)  

On a localized ha carbonyl clusters may behave like metal surfaces with chemisorbed species. Indeed, photoemission spectra of transition metal atoms correspond well with the photoemission spectra of carbon monoxide chemisorbed on the surface of the bulk metal. Such comparisons, for instance, have been made for Ru3(CO)u. Ir4(CO),2. Os (CO)ig [6 although, of course, they bear some difficulties connected with the poor knowledge we have of metallic catalysts. 

Courdinaiion chemists have prepared a variety of clusteis containing CO as a ligand. The withdrawal of electrons by CO seems to stabilize low-valence states. Complexes with up to 38 metal atoms (Pt j(C0)44 units) have been prepared. This number of atoms corresponds to that present in surface crystallites. Small crystallites have stronger surface atom interactions, which could mean that CO adsorption miglit change with crystallite size. 

It is a broadly accepted mechanism that in the Fischcr-Tropsch synthesis, CO dissociation into carbon and oxygen atoms occurs. l his dissociative absorption is the rate-determining step and is a prerequisite for chain growth. In the absence of CO dissociation, Ct species like methanol arc considered to be the main products. Consideration may also be given to the fluxional behavior of carbonyl clusters which has been proved by nuclear magnetic resonance studies. This could mean that the struciutcs deicmuncd in the crystalline state may be dift erent from those in solution. 

especially, metals of Group IV of the periodic table of elements must be quoted. Thus, the formation of methuxy intermediates from CO has been reponed (Equation (2)) [8]. 

---

Steady-state kinetics. Consider the interchange of 02 and CO coordinated to myoglobin, an iron porphyrin represented as PFe ... 

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Frenking s group showed that the Group 11 isocyanides M—NC (M = Cu, Ag and Au) are less well bound compared with the corresponding cyanides M—CN [276]. They also studied CO coordination on Cu, Ag+ and Au with Au(CO)2 being the most stable of all Group 11 dicarbonyl complexes [281]. Vaara et al. demonstrated the importance of relativistic effects in the 13-C NMR nuclear shielding constant in... 

Table 1 Selected reviews and monographs focusing on Co coordination chemistry.

By far the most studied pyrazole-containing ligand is the symmetrical monoanionic tripod hydridotrispyrazolylborate, Tp (3). Substituents introduced at each of the 3-, 4- or 5-positions (3,5-Me2Tp108, 3-z Pr,4-BrTp109) have led to a number of sterically encumbered threefold symmetric complexes and some remarkable Co coordination chemistry has ensued. Hydride substitution at the B atom is also possible by organic groups such as /z-bromophcnyl, which enables further functionalization of the ligand.110... 

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

The oxoanions of these elements represent an extensive set of potential ligands, characterized by a dominant, but not exclusive, tendency towards O bonding. Practically every known oxoanion has been introduced into the Co coordination sphere during the last century. 

The trivalent Co coordination chemistry of amines is simply immense. Amines, both mono- and multidentate, are typically thermally and air stable, often commercially available, easily deriva-tized, and are well matched to the electronic needs of trivalent Co. Space does not permit a discussion of all categories of amines that have been investigated since CCC(1987) nor their limitless permutations when bound to mostly six-coordinate Co. Much of the synthetic and mechanistic work that underpins Co amine chemistry has long been known and will not be restated here. The emphasis here will be on novelty rather than breadth. That is, recent innovative aspects of the structure, reactivity, and applications of selected, but representative, collections of these simple but ever-present compounds will be our focus. 

Although oxime complexes of Co share many of the physical properties of their imine relatives, the presense of an ionizable OH group attached to the coordinated N=C group leads to these ligands binding in their anionic forms. For this reason, the trivalent oxidation state is preferred in the Co coordination chemistry of oximes. 

Amides offer both O and N-donors for Co coordination. N-bound amides are accessible through hydrolysis of their corresponding coordinated nitrile or by linkage isomerization of the O-bound form. The preparation of an extensive series of pentaamminecobalt(III) complexes of (monoden-tate-coordinated) amides of the form RCONH2 (R = H, Me, CF3, CH2C1, CH2F, CH=CH2, Ph,... 

Pentaammineazidocobalt(III) perchlorate, Phenyl isocyanate Burmeister, J. L. et al., Chem. Eng. News, 1968, 46(8), 39 Dining an attempt to introduce phenyl isocyanate into the Co coordination sphere, a mixture of the 3 components exploded violently when stirring was stopped. 

HRh(CO)2(diphosphine) isomers, followed by CO dissociation. For this uphill process, a very low barrier is proposed. As we know, Morokuma and coworkers, using model systems, computed a barrier for ee ea conversion of 40 kJ.mol 1 [17], and a very low barrier for CO dissociation [23], Alkene association is followed by hydride migration (alkene insertion) and CO coordination to obtain either a linear or a branched... 

Another peculiarity of the acyl complex is that it prefers solvent coordination, even when the solvent has weakly coordinating ability such as THF, rather than CO coordination, even in the presence of free CO. This also can contribute to the high activity observed for these systems, as MeOH has not to compete with CO for coordination before the product-forming step. It is noteworthy that the carbonylation of the closely related complex [Pd(Me)(Et20)(dippe)]+ in Et20 or [Pd(OTf)(CH3)(dibpp)] results in the complex [Pd(COMe)(CO)(dippe)]+ containing coordinated CO instead of solvent [133]. 

The tetrapyrrole nucleus frequently holds a co-coordinated metal atom, but it has been found that only diamagnetic metals such as Zn, Pd, In, Sn, and Lu allow the tetrapyrrole to retain its photosensitizing ability, while paramagnetic metals such as Fe, Cu, and Gd do not (Rosenthal et al., 1986). Many of these compounds are lipophilic and some are even insoluble in water. These compounds must either be delivered in an emulsion or else incorporated in liposomes. 

Simultaneously, it was shown that the OH groups of basic surfaces such as magnesia or alumina could behave as nucleophiles toward CO coordinated to... 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

---