Ligands bridging multiplicity

Many organometallic complexes are clusters involving multiple metals that feature metal-metal bonds. The electrons in Me-Me bonds are counted by contributing one electron to each metal connected. Bridging ligands contribute one-half of their electrons to each metal center. Some simple examples in Figure 1.9 illustrate application of the rules. 

If structural information is available, it can be conveyed using the devices already discussed. Bridging ligands are cited as above, unless the symmetry of the system allows a simplification using multiplicative prefixes. Bonding between metal atoms may be indicated in names by italicised element symbols of the appropriate metal atoms, separated by a long dash, and enclosed in parentheses after the list of central atoms and before the ionic charge. 

Multiple bridging ligands between the pair of central atoms are cited in alphabetical order. Italicised element symbols indicating the coordinating atoms in bridging ligands are cited in the order of direction of the CRU and are separated by a colon. 

The observation of a bonding arrangement of the type shown in Fig. 20a in (AlO)2-centred dimers is a recurrent feature of aluminium hydroxides and organooxides of simple, electron rich ligands [44, 99, 110, 125, 134, 204, 205, 214-231]. Those more complex dimers which have been noted take two main forms both polydentate terminal and bridging ligands allow multiple interactions with the Group 13 metal centres. 

The main variables here are the capacity of the ligand to provide single- or multiple-atom bridging and the presence or absence of a monoatomic ligand at the centre of the cube. A central octuply-bridging ligand probably provides mechanical stability. 

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. 

As a monodentate ligand in metal complexes NO behaves in one of two ways (a) as a 12-electron unit (NO ) when it forms a single bond to M with M-N—0 around 125°, and (b) more usually, as a 10-electron unit (NO, when M—N is a multiple bond and M-N-0 is approximately linear. Examples of NO behaving as a bridging ligand are given later. 

Bridging Ligands with Multiple Donor Atoms in a Ring 141... 

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