Coordination 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... 

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... 

For example, in Ni(CO) nickel metal having 28 electrons coordinates four CO molecules to achieve a total of 36 electrons, the configuration of the inert gas krypton. Nearly every metal forming a carbonyl obeys the 18-electron rule. An exception is vanadium, forming a hexacarbonyl in which the number of electrons is 35. This carbonyl, which has a paramagnetism equivalent to one unpaired electron, however, readily adds one electron to form a closed valence shell complex containing the V(CO)(, anion. 

Structure. The CO molecule coordinates in the ways shown diagrammaticaHy in Figure 1. Terminal carbonyls are the most common. Bridging carbonyls are common in most polynuclear metal carbonyls. As depicted, metal—metal bonds also play an important role in polynuclear metal carbonyls. The metal atoms in carbonyl complexes show a strong tendency to use ak their valence orbitals in forming bonds. These include the n + 1)5 and the n + l)p orbitals. As a result, use of the 18-electron rule is successflil in predicting the stmcture of most metal carbonyls. 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

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. 

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. 

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. 

As a simple example of non-d coordination, let us consider the hexaammine-zinc(II) cation [Zn(NH3)6]2+, whose optimized structure is shown in Fig. 4.51. Each ammine ligand serves as a formal two-electron sigma donor, and the total electron count atZn therefore corresponds to a 22e system, again violating the 18-electron rule. Each ammine ligand is bound to the Zn2+ cation by about 60.7 kcal mol-1, which is in part attributable to classical electrostatic interactions of ion-dipole type. 

The rules in overall decreasing order of importance essentially state that the ideal structures for carboranes will be based on most spherical deltahedra (rule 1) the BE hydrogens will tend to be placed in the lowest possible coordination environments (rule 2) when elements to the right of boron in the periodic table are incorporated into the deltahedron or deltahedral fragment, they will tend to preempt low-coordination sites (e.g., carbon) or, if electron-deficient, high coordination sites (rule 3) and, lastly, boron will eschew seven-coordinate BH or six-coordinate... 

The highly covalent nature of transition metal carbonyls and their derivatives leads to the 18-electron rule being closely followed. The mononuclear species Ni(CO)4, Fe(CO)5, Ru(CO)5, Os(CO)5, Cr(CO)6, Mo(CO)6 and W(CO)6 obey this well and, if the formalized rules of electron counting are applied, so do the metal—metal bonded and carbonyl bridged species. Such compounds are therefore coordinately saturated and the normal (but by no means unique) mode of substitution is dissociative (a 16-electron valence shell being less difficult to achieve than one with 20 electrons).94... 

Finally it should be noted that formation of the perfluorobicyclo[3.3.0]-octa-2,7-diene-4,6-diyl ligand allows pyramidalization of four fluorinated carbons and may reflect a thermodynamic preference for sp2-hybridized carbon atoms in coordinated OFCOT to undergo rehybridization to sp3, provided that the ancillary ligands present on the metal can support an increase in the formal oxidation state and that the constraints of the 18-electron rule are obeyed. The origins of this thermodynamic effect for uncoordinated fluorinated alkenes have been discussed in detail (2). Extensions to nickel, palladium, and platinum systems are described in Section IX. 

The most typical oxidative addition occurs with square coplanar complexes where the central atom has the d8 configuration, especially Rh(I) or Ir(I). These conform to a 16-electron rule (Section 8.6) oxidative addition leads to an octahedral six-coordinate complex obeying the 18-electron rule, with a low-spin d6 central atom, e.g. ... 

Note, however, that one CO ligand is lost. The oxidation state of the Os increases by two units, from 0 to II but the coordination number only increases by one. Both species obey the 18-electron rule. Oxidative addition is exhibited by some d10 systems, notably complexes of Pd(0) and Pt(0). A reaction such as ... 

Four molecules of CO coordinate to Ni to form Ni(CO)4, but Ni(CO)5 is never formed. The stoichiometry of complex formation can be understood by the 18-electron rule. According to this rule, a stable complex with an electron configuration of the next highest noble gas is obtained when the sum of d electrons of metals and electrons donated from ligands equals 18. Complexes that obey the 18-electron rule are said to... 

Well-known complexes that obey the 18-electron rule are shown below. Typical ligands, such as CO, phosphine and alkenes, donate two electrons each. The total number of d electrons of Ni(CO)4 can be calculated as 10 + (2 x 4) = 18, and hence Ni(CO)5 cannot be formed. In Co2(CO)8, the number of d electrons from Co(0) is nine and four CO molecules donate eight electrons. Furthermore, a Co-Co bond is formed by donating one electron each. Therefore, the total is 9 + 8 + 1 = 18 electrons, to satisfy the 18-electron rule. The relationship between the coordination numbers and numbers of d electrons of metal carbonyls is shown in Tables 2.2 and 2.3. 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

A convenient tool for understanding organometallic catalysis mechanisms is the 16 and 18 electron rule, whereby valence electrons are counted in order to ascertain whether or not complexes are coordinatively unsaturated. An 18 electron complex possesses an inert gas configuration and must first undergo dissociation to achieve the coordinative unsaturation necessary for reactivity. The number of valence electrons for various transition metals is readily seen from their position in the periodic table (e.g., Mn has 7, Fe has 8). The number counted for a particular metal is independent of its oxidation state. 

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