Coordination compounds 18-electron rule

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... 

Stability of the examined complexes is usually explained by the 18-electron rule (details in Sec. 1.2.3) [3,4,21b, 184h,221-223], It has been observed that a large number of coordination compounds are formed by metal-centered radicals containing from seven to 21 electrons [223]. Also important is that many 19-electron compounds are among them, such as, for instance, [FeCp(r 6-arene)]+(PF6)A [CoCp F BR)], and [Co(C5H4BR)2], in which the fragments 135 137 are contained in due order [223] ... 

The structures of two transition-metal complexes that contain a Mo=Mo quadruple bond are shown in Fig. 19.2.3. In the compound [M02(O2CMe)2(NCMe)6](BF4)2, each Mo atom is six-coordinate, and all nine valence AO s are used for bonding according to the 18-electron rule. In the cation [M02(O2CMe)2(NCMe)6]2+,... 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

In order to demonstrate the practical importance of the 18-electron rules, we consider formation complexes in aqueous salt systems. Since many uranyl compounds can be S5mthesized by crystallization from aqueous solutions, let us first discuss the coordination of uranyl ions by water molecules in aqueous... 

In tiddition to the monoeyeiopentadienyl compounds just de.scribed. there are many compounds which contain a single ring per metal atom with a variety of additioniil ligands, such as carbon monoxide, to complete the coordination sphere. These include monomers and dimers, vvith iind without metal-metal bonds as necessary to obey the 18-electron rule. Some examples are shown in Fig. 15.38. 

---