Ligand-to-metal a bonding

The current understanding of the nature of the metal-( 72-H-X) bonding interaction for various types of cr-complexes is based on the traditional Dewar-Chatt-Duncanson model for the well-known 7r-olefin complexes which emphasizes both ligand-to-metal a bonding and metal(d7r)-to-ligand(a )... 

This assumes the sign of A to be positive. This is so for the model described but is not necessarily so for complexes where ligand-to-metal back bonding might occur [e.g., in hexa-cyanide complexes). Such situations have not yet been investigated by neutron diffraction. 

A trisimido complex possessing partial ligand-to-metal ir-bonding due to symmetry considerations. Adapted with permission from Sohrock, R. R. J.Am. Chem.Soc. 1990, 112,1642. 

The [Cr KNPr a)s] molecule contains a chromium atom co-ordinated by a trigonal-planar array of nitrogen atoms, and is one of only rarely found examples of this type of co-ordination geometry. The Cr(NC2)a unit has Z>3 symmetry. The Cr-N distances of ca. 1.87 A and the planarity of the CrNCa system for each of the amide ligands are considered indicative of ligand-to-metal w-bonding. 

A ligand must have a lone pair of electrons that it donates to form a bond to the metal. For example, the Ni— bonds in [Ni (NH3)g form by overlap of an empty valence orbital on the metal with the lone pair. s orbital on the nitrogen atom. The water ligands in [Ni (H2 0) ] " coordinate to the metal in a similar manner, with the oxygen atoms donating lone pairs to form Ni—O bonds. 

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