Bond multiplicities, direct/indirect

Keywords Bond information probes Bond localization Chemical bonds Chemical reactivity Contra-gradience criterion Covalent/ionic bond components Direct/indirect bond multiplicities Entropic bond indices Fisher information Information theory Molecular information channels Orbital... 

There are several methods available for the electrophilic addition of hydrogen and nitrogen to alkenes, dienes and alkynes. While the direct electrophilic addition of amines to these substrates is not feasible, aminomercuration-demercuration affords a very useful indirect approach to such amines. The addition of amides to C—C multiple bonds can be effected directly through the Ritter reaction or by the less direct, but equally useful, amidomercuration-demercuration process using either nitriles or amides. Similarly, H—N3 addition to alkenes can be carried out directly or via mercuration to produce organic azides. 

Two potentially informative infrared probes in alkyne complexes with ancillary carbonyl ligands are (1) v(C=C) absorptions and (2) v(CO) absorptions. The first provides direct information about the carbon-carbon multiple bond of the bound alkyne ligand, but it is usually weak and has proven particularly difficult to detect in M(CO)(RC=CR)(S2CNEt2)2 complexes (57). The second provides only indirect information about the metal-alkyne moiety, but its intensity and unambiguous assignment as y(CO) have stimulated an extensive literature data bank to assist interpretation. 

These direct bond orders are complemented by following estimates of the resultant multiplicities of the indirect k interactions due to chemical bridges ... 

Before considering with some detail selected examples of compounds containing multiple metal-metal bonds, it is convenient to analyze the criteria usually used for determining the existence of multiple bonds. Most of these criteria are direct or indirectly related with structural data. That is the reason why this chemistry could be developed only when relatively easy access to reliable structural data became possible. 

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