Odd-electron ligand

In addition, odd-electron ligands can be substituted to yield diamagnetic complexes (23) ... 

Total valence electrons = Metal group + Electrons donated by all even-and odd-electron ligands - Overall charge on the complex... 

The complexation of unsaturated hydrocarbons by transition metals is a powerful activation method that plays a fundamental role in their stoichiometric and catalytic transformation. The bonds formed are governed by jt backbonding. Olefins, dienes and arenes are usually 2-electron L, 4-electron L2, and 6-electron L3 ligands respectively, and alkynes are 2-electron L or 4-electron L2 depending on the needs of the metal. The odd-electron ligands are the 3-electron LX allyl radical and 5-electron L2X dienyl radicals. Cyclopentadienyl and arene sandwich complexes will be examined in Chap. 11. 

Organotellurium(II) compounds can also contain a three-center, four-electron bond as shown for 39 to 42 in Fig. 19. Typically, these molecules contain an odd number ligands around the central atom and an electronegative atom helps to stabilize a tellurenyl halide or selenenyl halide bond through chelation to form a four, five, or six membered ring. " Such molecules are described as lO-Te-3 and lO-Se-3... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

There is an odd electron partially distributed between the dyz and dzx orbitals, which still retain their degeneracy and their rotational properties with respect to the 2-axis. The orbital angular momentum is thus not quenched by the ligand-field, and values of the magnetic moment as high as 4.1 B.M. are possible. The moment should be temperature dependent, and at low temperatures should drop rapidly towards zero, as the lowest state due to spin-orbit coupling is in fact nonmagnetic (74, 75). 

The electrons courted for the metal atom in each of these complexes are those in its valence s and it orbitals. Metals having odd numbers of electrons obviously cannot satisFy the 18-electron rule by simple addition of CO (or other two-electron) ligands because the resulting moiety will necessarily also have an odd number of electrons. For example, Mn(CO)t and Co(CO)4are both 17-electron species and, consistent with prediction, do not exist as stable molecules. However, their corresponding anions, [MrtCOIjJ- and [Co CO>4] , are stable species and conform to the 18-electron rule. 

Fe (Por)] is formed when Feni(Por) or Feu(Por) is electrochemically reduced or treated with a stoichiometric amount of sodium anthracenide in THF.89 [Fe TTP]" (Na-18-crown-6)+ (THF)2 is a tow-spin square planar complex jitK 2.5 BM, g x 2.1, 1.9, Fe—Npor 2.023(5) A), and shows a similar visible spectrum to that of Fe (TPP) but with a much lower intensity. This species is isoelectronic to Co (Por), the anionic charge, in addition to an odd electron to the dzl orbital, diminishes the ligand affinity of the iron atom. Further reduction generates an Fe"-porphyrin dianion. 

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