D electron count

The d electron count is correct (8), and the filling order follows both the Pauli principle and Hund s rule. 

C20-0040. Determine the oxidation states and d electron counts for the metal ions in the following... 

Compounds and complexes of the early transition metals are oxophilic because the low d-electron count invites the stabilization of metal-oxo bonds by 7T-bond formation. To a substantial extent, their reactivity is typical of complexes of metals other than rhenium. That is particularly the case insofar as activation of hydrogen peroxide is concerned. Catalysis by d° metals - not only Revn, but also CrVI, WVI, MoVI, Vv, ZrIV and HfIV - has been noted. The parent forms of these compounds have at least one oxo group. Again the issue is the coordination of the oxygen donating substrate, HOOH, to the metal, usually by condensation ... 

Compounds containing M=C bonds can undergo [2+2] cycloadditions, and this reaction allows olefin metathesis to occur. The Mo=C bond [2+2] cycloadds to the C4=C5 bond to give a metallacyclobutane A retro [2+2] cycloaddition cleaves the C4=C5 bond and makes a Mo=C4 bond. This new bond cycloadds across another C4-C5 bond to make a new C4-C5 bond retro [2+2] cycloaddition cleaves the C4=C5 bond and completes the formation of the C4=C5 bond. The process repeats itself many times over to make the polymer. No change in Mo s oxidation state or d electron count occurs in any step. 

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

It makes sense to use the terms low-spin and high-spin only with metal ions having certain numbers of d-electrons. For the other d-electron counts, only one electron configuration is possible. For what d-electron counts are both high-spin and low-spin octahedral complexes possible ... 

Studying the plots in Fig. 4.3.6, we can see that for d electron counts of 1 and 2, the preferred structure is hep. For n = 3 or 4, the bcp structure is the most stable. These results are in agreement with the observation as listed in Table 4.3.2. For d electron counts of 5 or more, the hep and ccp structures have comparable energies. However, the hep structure is correctly predicted to be more stable for metals with six d electrons, and the ccp for later transition elements. These calculations show how the structures of metallic elements are determined by rather subtle differences in the density of states, which in turn are controlled by the different types of bonding interaction present. 

The formal oxidation state and d-electron count of the metal in the following complexes (a) (h6C6H6)2 Mo, (b) cp2ZrCl (OMe)... 

Fig. 6. AS, the difference between free atom and metallic binding energies of 2s or 2p core levels in the elements Ti through Zn. Estimates by Ley et al. [Ref. (79)], shown as filled circles, are based on dnsm configurations which were taken to differ between atom and metal for all elements except Cr and Zn. The open circles are values based on free atom binding energies for atom configurations d s most closely corresponding to the metallic d electron counts. (See text.)...

Note that in both interstitial transition metal carbides and the molecular clusters with interstitial atoms, the octahedral metal arrangement changes to trigonal prismatic as the d-electron count increases. That is, octahedral carbides are found for d4 or d5 metals as exemplified by ZrC and NbC and trigonal prismatic carbides are found for d6 metals as in WC. 

The stoichiometric pyrochlore transition metal oxides exhibit a wide range of magnetic and electronic transport properties. These properties are, of course, dependent on the d electron count of the B cation. Electrical conductivity may be insulating... 

In some cases, secondary forces may exert a sizable influence on the coordination environment as well. For example, it was seen earlier how the second-order JT effect frequently manifests itself as a displacement of transition metals from the center of an octahedron. The phenomenon is only observed for metals with low d electron counts. This could be used advantageously in synthetic strategies where the goal is to selectively place cations in specific sites within a stmcture. 

Figure 3.27. The transition metal cations in the outer octahedral layers of Na2La2Ti3 xRUxOio are displaced from the centers of their octahedra. Only transition metal cations with a low d electron count (i.e. Ti +, d°) can readily accommodate this distortion. Hence, the Ru + cations (d ) are found mostly in the central undistorted layer.

The implications are that conduction electrons confined to the inner-layer slab, in oxides with low d electron counts, may be more spatially screened from electron localizing effects such as chemical or structural disorder in the rock-salt-like slabs, as compared with conduction electrons in single-layer slabs. 

Metal Effects on Metal-carbonyl Reactivity. The effect of the metal center on organometaUic reactivity is not as clearly defined as for coordination complexes. We will examine (a) the effect of charge, (b) first-row, second-row, and third-row effects and (c) the effect of d-electron count. As will be discussed in Section 4.1, the enhanced reactivity of odd-electron complexes is a major effect of the metal center. 

The d electron count of the metal is calculated by subtracting the metal s oxidation state from the number of valence electrons (including the two s electrons) in its elemental state. The d electron count is an inorganic chemistry term for unshared valence electrons. The d electron count of a metal has important ramifications for reactivity. For example, metallocyclopropane resonance structures cannot be drawn for alkene complexes of d° metals. 

Common error alert Do not confuse total electron count, d electron count, and oxidation state with one another. All three characteristics are important to the reactivity of the metal. 

The d electron count is written as a superscript (e.g., a d2 complex). The two valence s electrons are always counted toward the d electron count. Thus, Pd(0) is said to be d10, even though it has eight 3d electrons and two 4s electrons. 

Oxidative addition and reductive elimination are the microscopic reverse of each other. In oxidative addition, a metal inserts itself into an X-Y bond (i.e., M + X-Y-> X-M-Y). The X-Y bond is broken, and M-X and M-Y bonds are formed. The reaction is an oxidation, because the metal s oxidation state increases by 2 (and its d electron count decreases by 2), but the metal also increases its total electron count by 2, so it becomes less electron-deficient. The apparent paradox that the oxidation of a metal results in a larger electron count is an artifact of the language with which compounds are described. 

Any metal with a d electron count of d2 or greater can undergo oxidative addition, but 18-electron complexes do not undergo oxidative addition reactions. Oxidative addition is very common for late metals like Pd, Pt, Ir, Rh, and the like. 

Insertions and (3-eliminations are also the microscopic reverse of each other. In an insertion, an A=B 77 bond inserts into an M-X bond (M-X + A=B —> M-A-B-X). The M-X and A=B bonds are broken, and M-A and B-X bonds are formed. Insertion is usually preceded by coordination of the A=B 77 bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M-X bond is replaced with an M-A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a 77 bond. The nature of the reaction requires that the new C-M and C-H bonds form to the same face of the A=B 77 bond, resulting in syn addition. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

Elimination is the microscopic reverse of insertion. Just as insertion does not, a / -elimination causes no change in the oxidation state, d electron count, or total electron count of the metal. By far the most common /3-elimination is the (3-hydride elimination, in which M-A-B-H -h> M-H + A=B. The /3-hydride elimination is the bane of the organometallic chemist s existence, as it causes many metal-alkyl bonds to be extremely labile. /3-Alkoxy and /3-halide eliminations are also known, as in the reaction of BrCH2CH2Br with Mg. 

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