Unsymmetric coordination

In most cases, it has been proposed that the cationic intermediate resulting from electrophilic attack by the mercuric ion is, in fact, a mercurinium species 109 in which the mercury cation is unsymmetrically coordinated to the two alkynyl carbon atoms. Hence, these reactions typically afford Markovnikov products. In the presence of excess Hg(OAc)2, nucleophilic attack of the mercurinium intermediate is intermolecular and occurs in an anti-fashion leading to the (ft)-isomer. When no excess of Hg(OAc)2 is present, nucleophilic attack of the mercurinium intermediate is intramolecular and occurs in a ry -fashion leading to the (Z)-isomer.1... 

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-15-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyI acetoacetate enolate showing unsymmetrical coordination of K by 8 oxygen atoms, and (c) the RbNCS ion pair coordinated by dibenzo-18-crown-6 to give seven-fold coordination about Rb.

In another reaction with intramolecular C,H activation of a Cp methyl group, iron complex 232 was treated with azidodicarboxylates, resulting in the formation of unsaturated chelate complexes 236—238 in 48%, 54%, and 48% yield with loss of 1 equiv of dimethylamine (Scheme 44). The authors discuss 233—235 as intermediates in this remarkable process. The constitution of 236 was confirmed by an X-ray structure analysis. An interesting feature is the unsymmetrical coordination of the cyclopentadienyl system, with the carbon atom bearing the phosphane... 

The shapes of the spin density distribution are similar to those of RAMOs in both the 0u and tTu oxidized states, respectively, indicating that the SOMO is dominant to the mixed-valence Cu - +-Cu - + character of the Cua site. While an unpaired electron is localized on one iron ion in the Fe " -Fe mixed-valence active site of uteroferrin [29], due to the unsymmetrical coordination (class I mixed-valence state), it is fully delocalized over the CU2S2 core of the models of the Cua site in both the Ou and ttu states. This means that the oxidized Cua site is a completely delocalized (class 111) mixed-valence Cu " -Cu species even in the low-symmetry environment [42], because the direct bonding character of the copper ions is caused by the strong orbital interaction, leading to the strong delocalization over the CU2S2 core in the Cua site. 

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