For the rational design of new complexes the concept of the isolobal analogy has been found useful. Roald Hoffmann was awarded with the Nobel Prize in chemistry for the development of the concept. The concept describes metal carbonyl fragments of M(CO)"n" as parts of octahedral building blocks in analogy to the tetrahedral CH3–, CH2– or CH– fragments in organic chemistry. In example Dimanganese decacarbonyl is formed in terms of the isolobal analogy of two "d"7 Mn(CO)5 fragments, that are isolobal to the methyl radical . In analogy to how methyl radicals combine to form ethane, these can combine to dimanganese decacarbonyl. The presence of isolobal analog fragments does not mean that the desired structures can be synthezied. In his Nobel Prize lecture Hoffmann emphasized that the isolobal analogy is a useful but simple model, and in some cases does not lead to success.