Lecture 26 MO’s of Coordination Compounds MLx (x = 4,6) 1) Octahedral complexes with M-L s-bonds only Consider an example of an octahedral complex ML6 where metal and ligands are s-bound. Group orbitals of 6 L’s suitable for s-bonding transform as a1g, eg and t1u (Lecture 18; p. 4). Look at the options available for the metal complex electron count: If the metal configuration is d5 electrons or less the metal complex will have the electron count less than 18 electrons (TiF62-, [Fe(OH2)6]3+ etc.). If Do is small the antibonding 2eg orbital can be filled so increasing the complex electron count up to 19-22 electrons (Co(NH3)62+, Ni(NH3)62+, Cu(NH3)62+, Zn(NH3)62+).

2) Octahedral complexes with M-L p-bonds. p-Donating ligands The symmetry of metal and ligand group orbitals suitable for M-L p-bonding in octahedral complexes can be found using group theory: Gr(p) = T1g + T2g + T1u + T2u (see Lecture 18). Transition metal atom has orbitals of the t2g (dxy, dyz, dxz) and t1u (px, py, pz) symmetry and no orbitals of the t1g or t2u symmetry (see the character table below). Metal orbitals of the t1u symmetry are already involved in s-bonding with 6 L’s (see the previous diagram). Therefore, p-bonding is only possible between metal and ligand orbitals of the t2g symmetry. Consider the case of the ligand-to-metal p-donation when the ligands t2g orbitals are completely filled (and of low energy) while the metal t2g orbitals are not completely filled. We will get two new t2g-MO’s, 1t2g and 2t2g. The energy gap between the partially filled 2t2g and eg MO’s and thus Do are now smaller. So p-donating ligands are weak field ligands (halogeno ligands, OH-, H2O etc). Oh A1g x2+y2+z2 Eg (2z2-x2-y2, x2-y2) T1g (Rx,Ry,Rz) T2g (xz, yz, xy) T1u (x,y,z) T2u

3) Octahedral complexes with M-L p-bonds. p-Accepting ligands In the case of p-accepting ligands like CO, CN- etc. the ligands group orbitals of t2g symmetry are empty and higher in energy than corresponding t2g metal orbitals. The metal-ligand p-bonding stabilizes the metal complex and increases Do. Therefore the ability of a ligand to be a p-acceptor makes the ligand a stronger field ligand. Increased Do prevents the eg level to be filled and the metal valence shell to be “overfilled” and helps it obey 18 electron rule.

4) Tetrahedral complexes with M-L s- and p-bonds Consider first the case of s-only bonded tetrahedral metal complex ML4. The ligand group orbitals of a1 and t2 symmetry are similar to those considered for 4H GO’s (Lecture 18). The 2tg MO is slightly antibonding and that is why the electron count less than 18 is quite common for ML4 (FeCl4-, CoCl42- etc.). From the other hand, Dt is usually small and the electron count up to 18 electrons for ML4 is possible but rare. The case of tetrahedral metal complex ML4 with M-L p-bonds. The orbitals of e (dz2, dx2-y2), t1 and t2 (px, py, pz; dxz, dyz, dxy) symmetry are suitable for p-bonding with L’s (Lecture 14, p. 5). Two e metal orbitals are only available to form M-L p-bonds since the t2 orbitals are involved in M-L s-bonding. p-acceptors will stabilize MO’s of e-symmetry so increasing Dt (not shown on the diagram).