Effective atomic number binary metal carbonyl

The effective atomic number rule (the 18-electron rule) was described briefly in Chapter 16, but we will consider it again here because it is so useful when discussing carbonyl and olefin complexes. The composition of stable binary metal carbonyls is largely predictable by the effective atomic number (EAN) rule, or the "18-electron rule" as it is also known. Stated in the simplest terms, the EAN rule predicts that a metal in the zero or other low oxidation state will gain electrons from a sufficient number of ligands so that the metal will achieve the electron configuration of the next noble gas. For the first-row transition metals, this means the krypton configuration with a total of 36 electrons. 

On the descriptive side, previously known binary carbonyl cations are usually of the [M(C0)6] type with M = Mn, Tc or Re (82), The oxidation state of the metal in these or other ternary cations is 0 or +1, and the ionic charge of the complex does not exceed +1. In addition, far more basic anions are used as counter ions. The effective atomic number rule, which plays an important role in judging stability, structure and reactivity of transition-metal carbonyls, is not valid for the noble-metal carbonyl compounds reported so far. The silver(I) and gold(I) carbonyl derivatives have 14, and the Pt(II) carbonyls have 16 electrons in the metal valence shell. 

In the unusually effective reducing medium of sodium-hexamethylphos-phoramide (HMPA), [Mn(CO)5] underwent facile reduction to form a golden yellow to yellow brown solution. On the basis mainly of derivative chemistry and infrared spectra, the major soluble component was formulated as Na3[Mn(CO)4] (14). Once this materia] was in hand it was evident that similar binary carbonyl trianions should be possible for other rf-block transition elements of odd atomic number. Since 1976, syntheses of mononuclear binary carbonyl trianions of V, Nb, Ta, Re, Co, Rh, and Ir have also been reported from this laboratory (vide infra). Because these substances contain transition metals in their lowest known oxidation states, they have often been referred to as superreduced species (15). 

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