Krypton configurations

To write the full configuration, decompose the krypton configuration ... 

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. 

The Mn atom has 25 electrons. Adding five carbonyl groups would raise the number to 35, leaving the atom one electron short of the krypton configuration. If the single unpaired electron on one manganese atom is then allowed to pair up with an unpaired electron on another to form a metal-metal bond, we have the formula (CO)5Mn-Mn(CO)5 or [Mn(CO)5]2, which is the formula for a manganese carbonyl that obeys the EAN rule. 

There are, however, also numerous multinuclear carbonyl compounds. Since Co(CO)4 would have, compared with Ni(CO)4, one electron less than the above-mentioned krypton configuration, this molecule itself does not exist but the dimer Co2(CO) 8 does, just as two Br atoms form a Br2 molecule with krypton configurations (there are analogous compounds of Ir and Rh). 

K4[Ni(CN)4] also belongs to this group it is produced from potassium in liquid ammonia from normal K2[Ni(GN)4]. The former can perhaps be regarded as a complex, built up from a nickel atom and four CN ions, thus with 10 + 4 X 2 electrons around the central atom in a krypton configuration, in place of 8 + 4 X 2 as in the ordinary complex of Ni2+. One would have to expect tetrahedral sp3 bonding as opposed to the normal dsp2 bonding in K2[Ni(CN)4] the crystal structure is, however, not known. 

In this way the copper atom (with electronic configuration 2, 8, 18, 4s1) effectively acquires seven electrons to give the krypton configuration 2, 8, 18, 4s2, 4p6, and its four bonds may be regarded as sp3 hybrids (of one 4s and three 4p orbitals) with the normal tetrahedral configuration. In terms of the concept of formal charges the atoms must be represented as Cu3 and I3+. 

The concept of the effective atomic number applies particularly well to carbonyl and nitrosyl compounds of the d-block elements. For example, the composition of mononuclear nickel(O) and iron(0) carbonyl complexes may be rationalized in terms of effective atomic number. To attain the krypton configuration, nickel (28 electrons) and iron (26 electrons) need to accept four and five electron pairs, respectively. Thus, [Ni(CO)4] and [FefCOls] are the predicted compositions, linear nitrosyls are three-electron donors and so binding of a [Co(CO)3l fragment (33 electrons) to a single NO ligand to form [Co(NO)(CO)3] would result in the krypton electron configuration. 

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