Oxygen-bonded carbonyls isocarbonyls

The reason for isocarbonyl bridging rather than for Al-tm bonding in these complexes can be found in the interaction of the small and therefore hard group 13 metal A1 with the CO group. The A1 is better suited to the harder basic site in the tm compound, and this is the carbonyl oxygen rather than the metal center. 

Isocarbonyls These compounds have one carbonyl ligand that is bonded to two metal centers at the same time, because it has both carbon and an oxygen available for bonding to the metal (iso means two). 

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