9.2 Nomenclature | 317 compounds with six ligands cannot fit the Lewis theory with eight electrons around each atom, and even expanding the shell to 10 or 12 electrons does not work in cases such as [Fe(CN)6]4 - , with a total of 18 electrons to accommodate. The 18-electron rule simply accounts for the bonding in many coordination compounds; the total number of valence electrons around the central atom is counted, with 18 a common result. (This approach is applied to organometallic compounds in Chapter 13.) Pauling6 used his valence bond approach to explain differences in magnetic behavior among coordination compounds by use of either metal ion 3d or 4d orbitals. Griffith and Orgel7 developed ligand field theory, derived from the crystal field theory of Bethe8 and Van Vleck9 on the behavior of metal ions in crystals and from the molecular orbital treatment of Van Vleck.10 Chapter 10 discusses these theories. This chapter describes the different shapes of coordination compounds. It can be difficult to confidently predict shapes with only knowledge of complex formulas; subtle electronic and steric factors often govern these structures. The differences in energy between the observed and unobserved structures of complexes are often small. It is useful to correlate structures with the factors that dictate their shapes. This chapter also describes isomeric possibilities for coordination compounds and experimental methods used to study them. The structures of organometallic complexes (Chapters 13 through 15) are also challenging to predict. 9.2 Nomenclature The nomenclature of coordination chemistry has changed over time. The older literature features multiple nomenclature styles. Contemporary rules used for naming coordination compounds are discussed in this chapter. More complete sources are available to explore classic nomenclature approaches necessary to examine older literature and additional nomenclature schemes not covered in this introductory section.11 Ligands are frequently named using older trivial names rather than the International Union of Pure and Applied Chemistry (IUPAC) names. Tables 9.2, 9.3, and 9.4 list common ligands. Those with two or more points of attachment to metal atoms are called chelating ligands, and their compounds are called chelates (pronounced key ⬘ -lates), a name derived from the Greek khele, the claw of a crab. Ligands such as ammonia are monodentate, with one point of attachment (literally, “one tooth”). Ligands are TABLE 9.2 Classic Monodentate Ligands Common Name IUPAC Name Formula hydrido hydrido H- fluoro fluoro F- chloro chloro Cl - bromo bromo Br- iodo iodo I- nitrido nitrido N3- azido azido N3- oxo oxido O2 - cyano cyano CN- thiocyano thiocyanato-S (S-bonded) SCN- isothiocyano thiocyanato-N (N-bonded) NCS(continues)
