Cyclopentadienyl carbonyl complexes structures

Although the general rule for mixed cyclopentadienyl carbonyl complexes is that metals with odd atomic number form mononuclear complexes whilst those of even atomic number form binuclear complexes, it has been reported that mononuclear cyclopentadienyl rhodium dicarbonyl polymerises to a binuclear complex. The structure of a binuclear complex with an unusual carbonyl bridging group is reported. 

Several other twelve-vertex metallocarborane carbonyl complexes have been prepared (51). In general, the chemistry and structures of these species, when investigated, have been found to parallel the analogous cyclopentadienyl metal carbonyls. 

The formation of the 17-electron paramagnetic vanadium complex is not surprising in view of the known corresponding carbonyl complex, however the 16-electron titanium derivative is unexpected in view of the ready formation of the 18-electron biscarbonyl and bistrifluorophos-phine metal complexes containing the 5-cyclopentadienyl ligand. The solid-state structure of the PF3 adduct of bis[2,4-dimethyl-(pentadienyl)]titanium has recently been determined (111) and is shown in Fig. 20. The corresponding vanadium complex is isomorphous. The metal-PF3 distances are 2.326(Ti) and 2.275(V) A. 

Various examples of bi- and trifurcated-acceptor interactions have been found in supramolecular structures of metal carbonyl complexes. In (/<-CO)(jU-CHCH3)-[(C5H5)Fe(CO)]2, 91, the bridging carbonyl ligand participates in two C-H- -O hydrogen-bonding interactions with C-H units of the cyclopentadienyl ligands. As a result the dinuclear molecules are self-assembled into polymeric chains [152]. 

A very wide variety of w-cyclopentadienyl metal carbonyl complexes is known. The structures of some are illustrated in Figure 43. With very few exceptions these compounds obey the 18-electron rule. This is well exemplified by the series of complexes 7t-C5H5Co(CO)2, ir-CsH5Mn(CO)3 and 77-CsH5V(CO)4. The simplest derivatives of the elements Cr, Fe and Ni are binuclear and held together either by metal-metal bonds or by bridging carbonyl groups and metal-metal bonds. The formation of such binuclear systems by alternate members of the transition series should be compared with the similar formation of dimeric metal carbonyls in the series Cr(CO)e, Mn2(CO)io, Fe(CO)s, Co2(CO)8 and Ni(CO)4. 

Like nickelocene itself, this complex has 2 electrons more than the number expected using the electron-counting approach outlined in this article. Interestingly, the iron analog [(C5H5)sFe2], for which such a tripledecker structure appears appropriate on an electron count, has been detected in mass-spectroscopic studies on ferrocene 179) and on the cyclopentadienyl iron carbonyl tetramer [C5H5FeCO]4 136). 

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