Carbon monoxide bridge-bonded

In Nb3Cp3(CO)7 the bridging carbon monoxide is bonded to one niobium atom through carbon, and with the other two Nb atoms it forms a Tr-olefm bond (a six-electron ligand). 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

Although we have described terminal and bridging carbon monoxide, it is known to bond to metals in other ways. Some of the other types of linkages are shown as follows ... 

Table III shows the bonding modes that have been established for carbon monoxide in these polynuclear aggregates. In addition to the terminally bonded group 0 ) observed for the mononuclear complexes, a variety of bridging carbonyl species have also been identified, involving two-electron and four-electron donation from the carbonyl group to the cluster unit.

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

The reaction of 36 with carbon monoxide results in the formation of the neutral [Ira CO C/ -COX/ -C Xdppm ] 39, which, according to an X-ray crystallographic analysis, no longer contains an Ir—Ir bond [the M-M separation is 3.388(2) ]. This compound may be the first example of a compound that contains a five-membered metallocycle in which the -02- unit and the carbonyl unit are both bridging. 

Balch ei al. (306) have given a detailed report on novel A-frame-type methylene bridged palladium complexes of composition Pd2(dpm)2( p-CHR)X2 (X = I, Br, Cl R = H, CH3) and [Pd2(dpm)2(joi-CH2)L2]2+ (L = pyridine, methylisocyanide). It was demonstrated that the Pd—C (methylene) bond resists insertion of carbon monoxide, isocyanides, or sulfur dioxide. Protonation of Pd2(dpm)2(/x-CH2)I2 with fluoroboric acid yields the compound [Pd2(dpm)2(ju.-I)(CH3)I]BF4 in which the bridging methylene group of the precursor has been converted into a terminal methyl group, a process that has also been encountered in a previous example (52). 

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