Transition metal complexes electron counting

Ru and Os tetranuclear clusters, 6, 874 in transition metal complex electron counting, 1, 2-3 Eight-membered rings, via ring-closing diene metathesis,... 

Neutral ligand method, for transition metal complex electron counting, 1, 4... 

Oxidative additions typically occur on transition metal complexes with counts of 16 electrons or fewer. Addition is possible to 18-electron complexes however, loss of a ligand (dissociation) must occur first (equation 7.26).55... 

Because the electron-counting paradigm incorporates the 18-electron rule when appHed to transition-metal complexes, exceptions can be expected as found for classical coordination complexes. Relatively minor exceptions are found in (Tj -C H )2Fe2C2BgHg [54854-86-3] (52) and [Ni(B2QH22)2] A [11141-32-5] (53). The former Q,n electrons) is noticeably distorted from an idealized stmcture, and the latter is reminiscent of the and complexes discussed above. An extremely deficient electron count is obtained for complexes such as P7036-06-9] which have essentially undistorted... 

Electron-beam heating, in metal vapor synthesis, 1, 232 Electron-beam vaporization, in metal vapor synthesis, 1, 224 Electron correlation, and computational chemistry, 1, 642 Electron counting, in transition metal complexes bridging ligands, 1, 14 examples, 1, 9 ligand electrons, 1, 5/... 

Electron counting, in transition metal complexes (continued) and MLX plots, 1, 36 via neutral ligand formalism, 1, 4 overview, 1, 2... 

Eighteen-electron rule — An electron-counting rule to which an overwhelming majority of stable diamagnetic transition metal complexes adhere. The number of non-bonded electrons at the metal plus the number of electrons in the metal-ligand bonds should be 18. The 18-electron rule in transition metal chemistry is a full analogue of the Lewis octet rule . 

Finally, many complexes that participate in homogeneous catalytic reactions have electron counts less than 16. This is especially true for high-oxidation-state early-transition-metal complexes such as (C2H5)TiCl3, Ti(OPr )4, etc. Cat-alytically active, late-transition-element complexes with electron counts less than sixteen are also known. An important example is RhCl(PPh3)2, a 14-electron complex that plays a crucial role in homogeneous hydrogenation reactions (see Section 7.3.1). 

Finally, note that a very similar situation obtains for transition-metal complexes -the favored electron count of tetracoordinate square-planar complexes is 18 — 2 = 16 and the favored electron count of dicoordinate linear complexes is 18 — 4 = 14. 

Electron counting rules have been developed to explain the observed geometries of many cluster compounds (see Electronic Structure of Clusters). Interesting, heavy main group element-transition metal complexes tend to violate... 

We have emphasized this point elsewhere - in connection, not only with the allene geometry, but also in connection with the geometries of both main group and transition metal complexes. In terms of moments it is those walks of the type shown in 12 and 15 which allow one ligand orbital to see another via a central atom orbital which destabilize the busy orbital geometry at the half filled point. For other electron counts however, the busy/idle orbital combination may well be stable as indicated both by Schleyer s electron deficient allenes and the transition metal oxo examples. 20 and 21... 

Electron counting in transition metal complexes (18 e rule) is an useful tool to understand their stability and structure, although it does not apply to all transition metal complexes—only for a majority of compounds containing 7i-acceptor ligands. The 18 electron rule is an extension of the idea of the octet rule, which applies to atoms having only s and p orbitals. The idea is that the molecule will be stable when the central atom has the same electronic structure as noble gases of the same row. A similar concept can be applied to transition metal complexes having d electrons. The compound is considered most stable when the total number of electrons around the atom becomes the... 

A Few Examples of Electron Counting of Transition Metal Complexes. A (Covalent Bond of M-X). B (Ionic Bond of M -X )... 

SCHEME 11.32 Some possible mechanisms for C—H activation by transition metal complexes with d-electron counts greater than zero. 

Some metals prefer to change oxidation state, electron count, and coordination number by one unit instead of two. Eq. 2.8 shows an example of binuclear oxidative addition, a reaction that brings about these changes and therefore can be favored by such metals. This typically occurs for a paramagnetic first row 17e transition metal complex or an 18e M-M bonded dimer that can dissociate to give... 

The formal electron count, as it is usually performed for transition metal complexes, (Chapter 1, 1.1), takes account of the electrons involved in the a interactions and the n nonbonding electrons on the metal which do not participate in these interactions (d" electronic configuration). But it ignores the consequences of the n interactions, even though these are accompanied by transfers of electron density involving some d-block orbitals. 

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