Metallic coordination compounds can be classified in various ways.
The Electron number (EN): Treat the metal and ligands as neutral entities. Classify the ligands by the number of electrons they provide, l. The number of electrons the metal provides, m, is equivalent to its periodic group. Allow for any charge of the complex (q). Then EN=m+l-q.
Oxidation State: Remove all ligands will filled valence shells, and then the charge remaining on the metal is the oxidation state.
Number of d-electrons on the metal: This can be calculated from the metal’s group number (GN) and the oxidation state (OS), and is GN-OS.
The Coordination number: This is often difficult to define. For example in ferrocene, Fe(Cp)2, where Cp denotes the cyclopentadienyl group, there are 2 ligands but 10 equidistant carbon atoms around the Fe, and so the coordination number does not have a unique value.
Steric Constraints: The ligands are often discussed in term of their size, and this can be defined by Tolman’s cone angle. This is the solid angle subtended at the metal when a cone is drawn which contains all the atoms in the ligand, whose sizes are defined by their van der Waals’ radii.
The stability of coordination complexes depends upon two factors, the thermodynamic and the kinetic. Thermodynamic stability is largely conferred by the presence of large metal to ligand bonds. Kinetic stability is conferred by the complex having a large HOMO–LUMO gap, so that activation of the complex towards reaction is difficult.
A compound can be described as electronically saturated when it has all of its bonding orbitals filled (which gives thermodynamic stability), all of its non-bonding orbitals filled (which gives thermodynamic and kinetic stability), and when its antibonding orbitals are empty (which also gives thermodynamic and kinetic stability). Electronic saturation confers stability on a complex.
A complex can also be sterically saturated. The compound TiR4 has a stability which varies with R: when R is the methyl (-CH3) group, the compound decomposes at -40oC, but when R is the -CH2Si(CH3)3 group, it is stable to 100oC, reflecting the much larger size of the ligand which prevents reactive species from getting close to the metal center, which they need to do in order to react.
The 18 Electron Rule
In the formation of compounds of elements of period 2, the stoichiometries and reactions are such that the elements obtain the electronic configuration of neon, or they complete their octet. Similarly, in the formation of coordination compounds of the firt row transition metals, compounds are often formed such that the metals obtain the electronic configuration of Argon. This is known as the 18 Electron Rule. Its origins are in the fact that the metals have 9 valence orbitals (1 s, 3 p, and 5 d), and to fill these orbitals 18 electrons are required.
In complexes MLn, n orbitals are used in forming bonds to the ligands (which have one bonding orbital each), and the remaining 9-n orbitals are non-bonding in the complex. The molecular orbital diagram for the complex therefore has n bonding orbitals, (9 – n) non-bonding orbitals and n antibonding orbitals.
The stability of compounds with the full valence shell means that reactions often proceed in such a way as to complete, or retain, the 18 electron requirement.
Some exceptions to the 18 electron rule exist, just as the octet rule is not universally obeyed for period 2 elements. In square planar d8 (ie. there are 8 d electrons on the metal) complexes, the 5 d-orbitals on the metal are not degenerate, and the d(x2-y2) orbital is higher in energy, is not involved in bonding and is empty. This means that only 16 electrons are required to generate stability. This is known as the 16 electron rule.
The 18 electron rule can explain the number of carbonyl ligands in the metal-carbonyl complexes: Fe has 8 valence electrons (s2d6), and so requires 10 electrons to fulfill the 18 electron requirement, and forms the compound Fe(CO)5, with each CO ligand providing two electrons, whereas Ni has 10 valence electrons (s2d8), and so requires 8 electrons to fulfill the 18 electron requirement, and forms the compound Ni(CO)4.