Bridging Ligands with Participating Heteroatoms

Complexes with cr-coordinated heteroatoms and rr-coordinated hydrocarbons have been synthesized and studied extensively, but they are not included in this article. On the other hand, complexes with cr-bonded carbon atoms and 77-bonded CX groups (X = heteroatom), with the exception of species with metal-metal bonds and Tj rTj -coordinated carbonyl or isocyanide ligands, have been very rarely reported in the literature. In most cases heteroatoms preferentially coordinate by lone pair electrons of the heteroatom and not through the adjacent unsaturated bond. 

Shaw and co-workers 152) observed broad and poorly defined resonances in the P NMR spectrum of (CO)3Mo(/i-dppm)2Rh(CO)Br, 

The bimetallic fulvalene complex CpZr(/u,-i7 V-C5H4—C5H4)(jit-Cl)2ZrCp reacts with tert-buty isocyanide to give Cp(Cl)Zr jLt-CioHgKju,-T/ T7 -C=N Bu ZrCp(Cl) (136) (344,345). During a sequence of insertion 

The coordination chemistry of compounds containing phosphorus-carbon multiple bonds, such as phosphaalkynes, phosphaalkenes, phos-phaallenes, phosphaalkenyls, and phosphaallyls, has been studied extensively (349,350). The chemistry is dominated by the donor properties of the phosphorus atom, and, as far as we are aware, no examples of bimetallic or trimetallic compounds with bridging ligands which link metal centers by a carbon cr bond and a C=P or C P n bond have been recorded. Once the lone pair on the phosphorus atom is involved in bonding, the unsaturated bond becomes a possible site for further coordination. 

In the cobalt and rhodium sandwich complexes MCp Tj -CRCR-BR CHR BR the acidic proton of the diborole ring was removed and 

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The methylene or heteroatom bridges in V or VII can be replaced with methyne carbon atoms. In this case (VII), we have two systems of conjugated electrons with aromatic features. A 26-electron n = 6) aromatic system is formed by the outer double bonds, whereas the inner double bonds and M-atoms form another conjugated system, M-N-C-C-N-M. The inner system can exist in two forms. For four-valent metals, we have structure VIII and for two-valent metal we have structure VII. In chelate VII, two d-electrons of M are required for the formation of an aromatic conjugated six-electron ring. In chelate VIII, empty d-orbitals participate in the ring formation. Quantum calculations may answer the question which structure is more favorable thermodynamically. Apparently, the answer depends on the nature of the metal atoms and their axial ligands. 

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