Electron counting complexes

The homoleptic complex Co( -C3H5)3 (12) obeys the 18-electron rule (see Eighteen Electron Compound and could be expected to be stable by analogy to the abundant allyl chemistry of Ni. The compound has been prepared from Co(acac)3 and CsHsMgCl (equation 40) or, alternatively, from C0CI2 and CsHsMgCl/CsHsCl (equation 41) in about 50% yield. The latter method has also furnished a number of substituted allyl derivatives. Despite its favored electron count, complex (12) decomposes in solution around —60°C, and the pure crystalline solid decomposes spontaneously in an inert atmosphere at —20°C. The solid ignites spontaneously in air (cf. Co (allyl) 2 (ethylene) Li, Section 5.1.2). 

Examples of odd-electron-count (i.e., paramagnetic) intermediates are also known but are very rare and much less understood. Therefore, our focus will continue to be the even-electron-count complexes considered so far. 

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... 

The first closo metaHaborane complexes prepared (159) were the nickelaboranes [< /(9j 0-( q -C H )Ni(B22H22)] and closo-l]l- r]-Q ]) -l]l-53i] pri Q [55266-88-1] (Fig. 13). These species are equivalent to closo-C ]]ri ][ i closo-Q, p5 2 by tbe electron-counting formaUsm. The mixed bimetallic anion [ /(9j (9-(Tj -C H )2CoNi(B2QH2Q)] and other related species were reported later (160). These metallaboranes display remarkable hydrolytic, oxidative, and thermal stabiUty. 

For tetranuclear cluster complexes, three stmcture types are observed tetrahedral open tetrahedral (butterfly) or square planar, for typical total valence electron counts of 60, 62, and 64, respectively. The earliest tetracarbonyl cluster complexes known were Co4(CO)22, and the rhodium and iridium analogues. The... 

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... 

In addition, complexes like 11 are also capable of catalyzing [2 - - 2 - - 2] cycloadditions of alkyne moieties resulting in the formation of substituted benzenes. Furthermore, Fe(I) catalysts like 22 with an odd electron count (17-electron species) have been studied in this context (Fig. 12) and the initial results demonstrate that they are catalytically relevant, uncovering a previously largely unrecognized aspect. 

From the inspection of the data in Table 2.4, it is clear that NO changes its original molecular character after adsorption. In general, coordination of nitric oxide leads to a pronounced redistribution of the electron and spin densities, accompanied by modification of the N-0 bond order and its polarization. Thus, in the case of the (MNO 7 10 and ZnNO 11 species, slender shortening of the N-0 bond is observed, whereas for the MNO 6 and CuNO 11 complexes it is distinctly elongated. Interestingly, polarization of the bound nitric oxide assumes its extreme values in the complexes of the same formal electron count ( NiNO 10 and CuNO 10) exhibiting however different valence. 

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. 

As exemplified above, among the various heteroleptic Cp M(dt)m complexes described so far, only a few series have been isolated in a radical state these are collected in Scheme 4 and finally concern only three classes, according to the formal electron count on the metal center ... 

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 ... 

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). 

This field has developed at a rapid pace since 1968, and a wide range of heteronuclear complexes of the tri- and tetranuclear variety has been established. It will be convenient to discuss the compounds in the first instance on the basis of nuclearity and, for the tetranuclear species, to subdivide the discussion on the basis of the carbonyl stoichiometry and cluster electron count. We have excluded from the discussion the interaction with nontransition elements, such as Hg, Tl, and Cd, which form a wide range of compounds. 

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