Metal-ligand complexes 18-electron rule

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

When the manganese atom (Z = 25) is considered, we see that the addition of five CO molecules would bring the total number of electrons to 35, whereas six CO ligands would bring the total to 37. In neither case is the 18-electron mle obeyed. In accord with these observations, neither Mn(CO)5 nor Mn(CO)6 is a stable complex. What is stable is the complex [Mn(CO)5]2 (sometimes written as Mn2(CO)10) in which there is a metal-metal bond between the manganese atoms, which allows the 18-electron rule to be obeyed. 

The 18-electron rule is especially useful when considering complexes containing ligands such as cyclo-heptatriene, C7H8, abbreviated as cht. This ligand, which has the following structure, can bond to metals in more than one way because each double bond can function by donating two electrons ... 

With its unusual coordination mode, NO forms complexes with a wide variety of metals, especially in cases where the metal can accept the transfer of an electron from the itg orbital. With cobalt having 27 electrons, it is evident that the addition of no integral number of ligands that function as two-electron donors can bring the total to 36. However, when one ligand is an NO molecule, the cobalt has a total of 30 electrons, so three CO ligands can raise the total to 36. Therefore, the stable complex that obeys the 18-electron rule is [Co(CO)3NO]. It should be apparent that complexes such as Mn(CO)4(NO), Fe(CO)2(NO)2, and Mn(CO)(NO)3 also obey the 18-electron rule. 

Comparison of the C-O stretching frequencies for a series of metal carbonyl complexes can reveal interesting trends. The complexes listed below all obey the 18-electron rule, but with different numbers of CO ligands attached, the metal atoms do not have the same increase in electron density on them because the coordination numbers are different. 

For these complexes, the extent of back donation increases as the number of CO ligands increases, which causes the stretching frequencies to be found at lower wave numbers. A similar trend is seen for the following complexes (all of which obey the 18-electron rule), showing the effect of the charge on the metal ion ... 

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 metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

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. 

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. 

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