Electron-counting procedure

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

Wade s electron-counting procedures (22) start from a closed polyhedron and require that neither removing a vertex from this polyhedron nor capping a face will alter the number of skeletal bonding orbitals. This capping principle has been demonstrated to be general (31). For... 

The anion [Osg(CO)18p has an octahedral arrangement of metal atoms of approximately Oh symmetry, and is crystallographically very similar to the [HRus(CO)w]- ion. This collection of structural data on electron-equivalent systems emphasizes some of the dangers in trying to predict the structure of complexes solely on the basis of electron counting procedures (220). 

It should also be noted that a six-electron rule affords conformity with the electron counting procedures of Rudolph (13)... 

The foregoing examples show the relevance to metal-carbonyl cluster chemistry of the borane-oarborane structural and bonding pattern. Its relevance to other areas of chemistry may be explored readily using a systematic skeletal electron-counting procedure (161, 201). 

For mixed main group-transition element clusters containing but one transition element, modifying the above electron-counting procedure to include the 6 nonbonding electron pairs on the transition metal has been advocated (128) as a way of bringing the electron count into line... 

For a description of the electron-counting procedure as applied to metal clusters, see Ref 37.) The paramagnetism of the nickel cluster, in principle, could be detected directly by neutron diffraction with a polarized beam and an external magnetic field. However, such measurements were not undertaken, and the effects of paramagnetism on the observed diffraction intensities, that are small in the present experiment, were ignored. 

Finally, in applying any electron-counting procedure to orgnnometallic complexes, we must remember that it is merely a formalism—albeit a very useful one We - ... 

Some of the beautiful relationships that exist between r/iuo. nitlti., ind unit lino osmium complexes tire shown in Fig. 16.56.m For Lick of space wc have not touched on many subtleties associated with geometry/bontling/electron counting procedures and the reader is encouraged to consult more advanced sources.144... 

More elaborate electron-counting procedures have been established, and these are usually more appropriate for the rationalisation of high-nuclearity clusters. An approach which was originally applied to complex boranes has been extended to carbonyl clusters and a number of topological theories have been described. Although there is some way to... 

As is obvious from the drawings, the E atoms in these simple molecules retain a lone pair of electrons may be used for a dative bond to another ML fragment (10-13). Data on complexes of types 12 and 13 are summarized in Tables III and IV, respectively. The dilemma that arises is that from the electron counting procedures one would expect two types of bonds— covalent and dative (see Ref. 62 for a discussion of the differences between dative and covalent bonding)—for complexes 11-13, but the ML fragments... 

The X-ray crystal structure of Os02(NBu )2 again shows the expected tetrahedral structure with 0s=0 at 1.744(6) A however, one of the imido groups is essentially linear (Os—N = 1.710(8) A, Os—N—C = 178.9(9)°) and the other substantially bent (Os—N = 1.719(8) A, Os—N— C = 155.1(8) A.301 The presence of one linear and one bent imido group in the complex has been partly rationalized on electron-counting procedures and MO considerations,292,301 although both IR and HNMR data indicate that both the imido groups are equivalent in solution.302... 

A division into Wade s systems, belonging the 2n + 2 rule and non-Wade s clusters has been discussed by Struchkov125b with the aim to extend the electron counting procedure to conjuncto-clusters. An additional concept of structural rules for transition metal clusters has recently been presented by Xu125. Lauher126,127 used extended... 

To use either electron-counting procedure, it is necessary to know how many electrons each ligand in a complex donates to the metal. Table 15.1 gives electron contributions for a variety of ligands for both the neutral atom and oxidation state... 

To facilitate electron counting in composite cluster systems such as these, in which two or more deltahedra share one, two, or three vertex atoms, Jemmis has devised an extended electron counting procedure (his mno rule) that takes account of the number of cages (zzi), vertices (n), and singlevertex shared atoms (o). 

Mingos has recently developed an electron counting procedure (which may be described as Polyhedral Inclusion ) which is based on the formal division of the high nuclearity cluster into an internal (encapsulated) polyhedron and an external (surface) polyhedron128. Three sub-classes of close-packed cluster have been identified ... 

We briefly report here a few definitions coneerning global and local electron counting procedures, remembering also that the number of cluster valence electrons (CVE) strongly determines the stoichiometry and the cluster shape, while the correct local valence electrons (LVE) distribution influences the ligand conformation. For a BMCC ... 

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