Metal-ligand multiple bonds

Classical complexes are identified [1112] as those species in which the central metal ion possesses a well-defined oxidation number and a set of ligands with a discrete electron population. Non-classical complexes , in contrast, involve highly covalent and/or multiple metal-ligand bonding resulting in indistinct oxidation numbers for both participants. 

The mutual influence of ligands in transition metal coordination compounds with multiple metal-ligand bonds. E. M. Shustorovich, M. A. Porai-Koshits and Y. A. Buslaev, Coord. Chem. Rev., 1975,17,1-98 (345). 

Late transition metals have a marked affinity towards coordinating carbon monoxide and carbon-carbon multiple bonds. If there is another suitable ligand on the metal centre, this coordination might be followed by the insertion of a carbonyl group or the carbon-carbon moiety into the metal-ligand bond. Both types of attachment reactions are commonly exploited in catalytic processes and their characteristics will be discussed separately. 

Two additional points that are self-evident are discussed below for the sake of completeness. The coordination number of a metal ion counting all ligands other than the adjacent metal involved in multiple metal bonding is less than the maximum coordination number possible for that metal center. Furthermore, the number of d-electrons on the metal is nearly equal to the number of metal valence orbitals not involved in metal-ligand bonding to optimize metal-metal bond formation by filled bonding MOs. 

Multiple Metal Ligand Bonds. Metal Methvlidene Ions. Recently, we completed measurements for the first row MCH2 bond energies including reevaluations of several previously measured values (Fisher, E. R. Armentrout, P. B. J. Phvs. Chem. accepted for publication. Sunderlin, L. S. Armentrout, P. B. J. Phvs. Chem. submitted for publication). When these values. Table II, are plotted vs. Ep(double) the result is shown in Figure 1 (Armentrout,... 

Fig. 3 Development of mechanoluminescence as a stress probe within polymeric materials (a) in multiple network elastomers (b) in swollen gels supramolecularly cross-linked with transient metal-ligand bonds and (c) in thermoplastic elastomers. Reprinted with permission from [32] (Copyright AAAS 2014), [33] (Copyright Wiley VCH 2014) and [34] (Copyright American Chemical Society 2014)...

As a result of the multiple orbitals available for metal-ligand bonding, alkynes readily coordinate with multiple metal centres [12]. Where two metals coordinate to a single alkyne (or acetylide) moiety, a number of different arrangements have been characterised (Fig. 2). An acetylide moiety may coordinate in an end-on fashion via two metal-carbon o-bonds (structure A) with the C = C axis perpendicular to the metal-metal axis (p—r r ). The alkyne can also coordinate parallel to the metal-metal axis (structure B), with each alkyne carbon forming a o-bond to a different... 

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