Bridging electron counting

Halet )-F, Saillard )-Y (1997) Electron Count Versus Structural Arrangement in Clusters Based on a Cubic Transition Metal Core with Bridging Main Group Elements. 87 81-110 Hall DI, Ling JH, Nyholm RS (1973) Metal Complexes of Chelating Olefin-Group V Ligands. 15 3-51... 

The location of electrons linking more than three atoms cannot be illustrated as easily. The simple, descriptive models must give way to the theoretical treatment by molecular orbital theory. With its aid, however, certain electron counting rules have been deduced for cluster compounds that set up relations between the structure and the number of valence electrons. A bridge between molecular-orbital theory and vividness is offered by the electron-localization function (cf p. 89). 

There is a series of metallaboranes of the type [SBgHjoM], [S2B7H7M], etc., where M seems to contribute two cluster electrons giving a nido-electron count. Nevertheless, these metallaboranes must be designated as arachno-dusters. Though the principal structure of the 10-vertex nido- and arachno-skeleton is the same, they differ structurally by the position of the bridging H atoms (which are the gunwale positions 5-10 and 7-8 in the arachno-case) and, moreover, in their physical properties, particularly in the sequence of the 11B NMR shift values. We will discuss the ar-10 clusters with formal ni-10 electron count later. 

At first glance, electron count in these clusters gives a remarkable analogy to the carbaboranes. Each aluminum atom contributes two electrons to the cluster, three electrons are from each carbon atom, and one electron is added by the bridging hydrogen atom. A total count of 32 electrons or 16 electron pairs results, which in accordance with the Wade rules [6] gives an arachno-type 13-vertex cluster. Indeed,... 

The molecular structure of the manganese dimer clearly reveals that there is an Mn—Mn bond (Fig. I5.la. b). In the cobalt structure, two of the CO ligands are bridging, i.e.. they are simultaneously bound to both Co atoms (Fig. IS.Ic, d). This does not affect the electron count, however, because CO and other neutral ligands donate two electrons to a complex whether they are terminal or bridging (Table IS. I) ... 

The highly covalent nature of transition metal carbonyls and their derivatives leads to the 18-electron rule being closely followed. The mononuclear species Ni(CO)4, Fe(CO)5, Ru(CO)5, Os(CO)5, Cr(CO)6, Mo(CO)6 and W(CO)6 obey this well and, if the formalized rules of electron counting are applied, so do the metal—metal bonded and carbonyl bridged species. Such compounds are therefore coordinately saturated and the normal (but by no means unique) mode of substitution is dissociative (a 16-electron valence shell being less difficult to achieve than one with 20 electrons).94... 

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

Whereas in ligand bridged dinuclear complexes, removal or addition of two electrons makes or breaks one metal-metal bond (15) this does not seem to be the case for clusters, presumably because of their delocalized bonding. At least for one case, however, two-electron reduction can induce a significant change in cluster shape (18,42) the 84-electron cluster Os6(CO),g with framework 1 is easily reduced to the 86-electron anion Os6(CO) g with framework 2, in accordance with skeletal electron counting rules. 

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