O S-form

A key concept in LCAO-MO bonding theory is the formation of the same number of molecular orbitals as the number of atomic orbitals that are combined (e.g., there are 12 M.O.s formed when 4s and 3d atomic orbitals combine in the Ti2 molecule, see Figure 2.68a). As the number of atoms increases to infinity within a crystal lattice, the AE 0 between energy levels within bonding and antibonding regions (Figure 2.68b). This is an application of the Pauli exclusion principle, which states for N electrons, there must be N/2 available states to house the electron density. ... 

The M.O.s formed by the jr-radial- or 7i-tangential-orbitals are not necessarily equally involved in bonding to the metal and one consequence of this would be that the M-CO system would be non-linear, which is frequently found [129u]. The deviation from non-linearity would obviously depend on the effects of other ligands attached to the metal. 

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