Transition antibonding orbital fragments

In the adsorption of ammonia on the surface, the NH3 2p lone-pair molecular orbital interacts with a transition-metal smface atom to form bonding and antibonding orbital fragments. The resulting 4d 2 electron distributions are also shown in Fig. 3.3b. 

Typical three-electron-donor transition-metal fragments (/2c, 25) are the (i)5-C5Hs)Ni and the (CO)3Co groups, which form mononuclear complexes with 3. However, 3 can also hold two metal moieties donating (1 + 2), (2 + 2), (2 + 3), and (3 + 3) electrons because slightly antibonding orbitals in the triple-decker complexes are then occupied. The planar ligands 5, 6, 7, and 8 with the formal acceptor capacities of 4, S, and 6 electrons should only exist in dinuclear complexes when stabilized by a pair of (2 + 2), (2 + 3), and (3 + 3)-electron-donor moieties (7). 

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