Metal-ligand complexes electron counting

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

Ans 2.13-2.17, 2.19,2.20, 2.22,2.23, 2.25,2.44,2.45 are all 18-electron, 2.18 and 231 are 16-electron, and 2.29 and 2.36 are 14-electron complexes. 2.33-2.35 are 12-, 8-, and 10-electron complexes, respectively. Complexes with metals in lower oxidation states and with acceptor ligands have electron counts of 18 and 16, but complexes with metals in high oxidation states and mainly donor ligands often have electron counts less than 14. Electron counts of 14 are encountered sometimes when bulky ligands are present. 

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... 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Many organometallic complexes are clusters involving multiple metals that feature metal-metal bonds. The electrons in Me-Me bonds are counted by contributing one electron to each metal connected. Bridging ligands contribute one-half of their electrons to each metal center. Some simple examples in Figure 1.9 illustrate application of the rules. 

Hence, these models are somehow of complementary characters and efforts should be directed toward their developments and wider applications, including those situations in which the additivity fails. Extensions of the Pickett s and Lever s models have already been proposed to overcome their limitations, as discussed above, and in some cases the dependency of the ligand parameters on the metal centers and generalizations to types (with different electron-counts, structures, compositions, etc.) of complexes, metal centers and ligands not included in the original proposals, have already been successfully treated. 

Ruthenium and osmium carbene complexes possess metal centers that are formally in the +2 oxidation state, have an electron count of 16 and are penta-coordinated. Ruthenium complexes exhibit a higher catalytic activity when an imidazole carbene ligand is coordinated to the ruthenium metal center (21). 

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