Question

Stable Werner type complexes require between 12 and 18 electrons. On the other hand organometallic complexes require 18 electrons for thermodynamic stability. Why the difference? Use molecular orbital considerations in order to explain the difference.

Answer 1

Unlike the main group organometallic compounds, which use mainly ns and np orbitals in chemical bonding, the transition metal compounds regularly use the (n−1)d, ns and np orbitals for chemical bonding. Partial filling of these orbitals thus render these metal centers both electron donor and electron acceptor abilities, thus allowing them to participate in σ-donor/π-acceptor synergic interactions with donor-acceptor ligands like carbonyls, carbenes, arenes, isonitriles and etc,.

The 18 Valence Electron (18 VE) Rule or The Inert Gas Rule or The Effective Atomic Number (EAN) Rule: The 18-valence electron (VE) rule states that thermodynamically stable transition metal compounds contain 18 valence electrons comprising of the metal d electrons plus the electrons supplied by the metal bound ligands. The counting of the 18 valence electrons in transition metal complexes may be obtained by following either of the two methods of electron counting, (i). the ionic method and (ii). the neutral method.

Werner complexes belongs to Class I and Class II type of complexes

In class I complexes, the Δo splitting is small and often applies to 3d metals and σ ligands at lower end of the spectrochemical series. In this case the t2g orbital is nonbonding in nature and may be occupied by 0−6 electrons (Figure 1). The eg* orbital is weakly antibonding and may be occupied by 0−4 electrons. As a consequence, 12−22 valence electron count may be obtained for this class of compounds. Owing to small Δtetr splitting energy, the tetrahedral transition metal complexes also belongs to this class.In class I complexes, the Δo splitting is small and often applies to 3d metals and σ ligands at lower end of the spectrochemical series. In this case the t2g orbital is nonbonding in nature and may be occupied by 0−6 electrons (Figure 2). The eg* orbital is weakly antibonding and may be occupied by 0−4 electrons. As a consequence, 12−22 valence electron count may be obtained for this class of compounds. Owing to small Δtetr splitting energy, the tetrahedral transition metal complexes also belongs to this class.

In class II complexes, the Δo splitting is relatively large and is applicable to 4d and 5d transition metals having high oxidation state and for σ ligands in the intermediate and upper range of the spectrochemical series. In this case, the t2g orbital is essentially nonbonding in nature and can be filled by 0−6 electrons (Figure 2). The eg* orbital is strongly antibonding and is not occupied at all. Consequently, the valence shell electron count of these type of complexes would thus be 18 electrons or less.

Whereas, organomettalics complexes can be explained by the M.O. diagram of Class III type of complexes: In class III complexes, the Δo splitting is the largest and is applicable to good σ donor and π acceptor ligands like CO, PF3, olefins and arenes located at the upper end of the spectrochemical series. The t2gorbital becomes bonding owing to interactions with ligand orbitals and should be occupied by 6 electrons. The eg* orbital is strongly antibonding and therefore remains unoccupied. Follow Fig 3 for M. O. diagram of Cr(CO)6.

Find all figures in attached file.