Coordination Modes for Carbon Monoxide

Coordination modes for CO in metal complexes are summarized in Fig. 15. The first row shows the well-known terminal, edge-bridging, and face-bridging carbonyl arrays. In either case, CO is bonded exclusively through carbon. No example has been reported of CO being bonded solely through oxygen. 

The vibrational spectra, particularly the infrared spectra are useful in deducing the coordination mode of the carbonyl ligand in metal complexes. The terminal CO generally absorbs at high frequencies (1850-2125 cm while the edge-bridging and face-bridging CO 

All other entries in Fig. 15 represent some degree of oxygen bonding as well. 

Compounds (iv)-(vi) in the second row in Fig. 15 contain a CO ligand of which the carbon atom is terminally bonded to a metal atom or bridging two or three metal atoms, and the oxygen atom is further bonded to another metal atom M.  

Modem NMR spectrometers are routinely equipped with temperature-controlled probes. 

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There are two principal modes of coordination of the carbon monoxide ligand in transition metal carbonyls terminal coordination, to a single metal atom, and bridging coordination, to two or more metal atoms. The Ols spectrum of Co4(CO)12, shown in Fig. 5, can be readily deconvoluted into two peaks corresponding to these two types of carbonyl groups109. This spectrum is useful for determining the relative chemical shifts for the two types because... 

From the standpoint of the present review, we will focus on compounds in which unusual and potentially reactive features of CO coordination are revealed, and on certain classes of reactions in which the conversion of CO to other chemical entities is achieved. For the purposes of determining what a reactive mode of CO coordination is and in what way the CO ligand can be activated, it is first instructive to briefly review the primary modes of carbon monoxide coordination. These are illustrated as Lewis structures... 

The methylene isomer 15 eliminates carbon monoxide when heated at 110°C (toluene, xylene 24 h), and yet another carbon-metal hydrogen shift takes place (75). The methylidyne species 16formed in this irreversible reaction has been characterized by its NMR spectra [S(CH) 9.36, quartet (76) 8(CH) 118.4 (CD3)2CO (77)] which implies a structure analogous to the previously known ethylidyne derivative (/i.-H)3(/i.-CCH3)-Os3(CO)io (138). NMR data obtained for partially deuterated samples of the methyl complex 14 have yielded evidence that the methyl ligand occupies an unsymmetrical bridging position between two metal centers, a coordination mode which implies significant C H Os interaction... 

Second, the stoichiometry of adsorption must be known in order to calculate surface concentrations. This is extremely difhcult to establish. Hydrogen, for example, adsorbs as M-H species over crystalline planes. This is conhrmed with parallel hydrogen chemisorption and BET measurements on nonsupported nickel. However, the stoichiometry of the bond M Hn appears to increase for low coordination sites (see Chapter 3) so that overall values for very small crystallites may be greater than one. Carbon monoxide is even more troublesome. Several modes coexist linear Ni-CO, bridged, Ni CO, and subcarbonyl, Ni(CO), so that some assumptions are inherent in its use. ... 

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