Molecular orbitals metal-carbonyl complexes

There will always be 36 electrons (6 from each ligand) to place in the molecular orbitals for the complex. In addition, the metal may furnish as many as 6 valence electrons for an octahedral metal carbonyl or cyanide complex. The ground state for a d6 metsl hexacarbonyl or hexacyanide is therefore... 

The molecular orbital theory of complexes (6), developed out of crystal field theory (7) by the inclusion of both a- and sr-covalent interactions, has been successfully applied 8—13) to the octahedral (14. 15) carbonyls of the 4 metals (V i, Cr , Mn+ etc.). The qualitative discussion of this octahedral case is particularly simple because of a clean-cut separation (within the single-configuration approximation) of a- and ji-systems. 

In so far as the decrease in chemical reactivity is an indication of diminished transition metal basicity, it was proposed143 that the thiocarbonyl complex is less basic than its carbonyl analogue. This conclusion is substantiated by the spectral shifts in Table 22 and is also in agreement with molecular orbital calculations which predict the thiocarbonyl complex to be less basic than the carbonyl complex155,156. 

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