O-End-On Bonded Complexes

The bonding model foresees an oxidized original U(III) to U(IV) with resonance structures represented by U =0=C-0 U -0=C-0 which justify the short and long C-0 bonds. Magnetic moment (/ eff) measurements have clearly established that U was oxidized to with the transfer of virtually one electron to CO2. This transfer does not bring the electron in the empty LUMO centred on C and, therefore, the molecule is not bent as in ti -C or r -C,0 complexes. 

An interesting example of a p-q complex in which both C=0 bonds of a single cumulene molecule are bonded to a metal centre was reported by Sadighi et al. in 2007 [31]. The p-q q -C02 coordination has been demonstrated to exist in a dinickel complex having the core [(IPr)Ni]2(p-C0)(p-q, q -C02) 6, where IPr is l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. The CO originates from CO2 and the two CO2 ligands bridge two nickel atoms via two Ni-CO bonds in a side-on way (Structure 2.4). 

The bridged CO2 is bent at 133.4° and the CO bond lengths are 1.255 and 1.257 A, much longer than for free CO2 (1.16 A) but comparable to the length found in similar side-on complexes. Similar Ni-biscarbene complexes, (IMe)2Ni(0), form analogous complexes (IMe= l,3-dimesitylimidazol-2-ylidene). 

2 CO2 Coordination to Metal Centres Modes of Bonding and Reactivity 

Structure 2.4 Reprinted with permission from [31]. Copyright (2007) American Chemical Society 

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