Systems Using Inner d Orbitals

The participation of inner d orbitals in bonding may be observed with transition metal atoms of the third or higher rows of the periodic table. If these transition metal atoms or ions are coordinated by heteroatoms of ligands which are themselves connected by conjugated multiple bonds, one obtains chelate rings of different ring sizes which may form cyclic (pd)n systems. Some examples of such compounds with different coordinating heteroatoms are collected in Fig. 2. 

The five- and six-membered transition metal chelate rings contain examples of compounds which have been most extensively studied, theoretically as well as experimentally. The tris-(gIyoxalbis-N-me thy limine )iron (II) chelate 23 synthesized by Krumholz 68 is the structurally simplest example of a vast number of intensely coloured chelates containing different 1,2-diimine ligand systems which have been recently reviewed 69 . (pd)n -Bonding was thought to be so important for this type of chelates that it could explain their properties by an aromatic 70 or quasiaromatic 71-72) behaviour. These types of chelates will be considered specifically in Chapt. V. 

Bis-duroquinone-nickel(0) 731 24 is one of the few examples of five-mem-bered ring chelates coordinated by oxygen. Coordination by sulphur leads to 

The bis-malodianil-copper(II) 78 26 is one example of six-membered ring chelates coordinated by nitrogen for which Daltrozzo 79 showed by light-absorption spectroscopy and NMR spectroscopy that no cyclic delocalization is observable across the transition metal. 

The acetylacetonate chelates 8°1 27 form an extensively studied class of complexes for which the suggestion that cyclic conjugation should lead to aromatic stability was applied historically for the first time 81.82), However, Musso etal. 83.84) showed by analysis of vibration spectra that the ti bonds in the chelated ligands are completely delocalized and the use of a mesityl substituent in position 3 as an indicator for diamagnetic ring currents showed no diamagnetic anisotropy comparable to that in benzene. They therefore discarded the concept of cyclic delocalization and aromatic character in these compounds. 

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