D Block Chemistry & Coordination Complexes of Cobalt

[ampakine]

I'm a bit confused about how the d block elements form coordination complexes. Cobalt for example. It has 7 d electrons. In the +3 oxidation state it has 4 d electrons left (unless it loses its 4s electrons first) so I don't understand why it coordinates 6 ligands. The 6 ligands acting as Lewis bases donate 2 electrons each so wouldn't the cobalt atom end up with 16 electrons in its 3d orbital? I thought d orbitals can only hold 10 electrons.

 

[SpectraCat]

I'm a bit confused about how the d block elements form coordination complexes. Cobalt for example. It has 7 d electrons. In the +3 oxidation state it has 4 d electrons left (unless it loses its 4s electrons first) so I don't understand why it coordinates 6 ligands. The 6 ligands acting as Lewis bases donate 2 electrons each so wouldn't the cobalt atom end up with 16 electrons in its 3d orbital? I thought d orbitals can only hold 10 electrons.

Good question! However first a correction .. Co³⁺ actually has 6 d electrons not 4 (outer shell s-electrons are removed first when ionizing). So the complex has 18 electrons, not 16. Anyway, regarding your question, the point is that you need to consider all of the valence orbitals ... remember that you can have higher hybridizations (up to sp³d²)? So there are really 9 orbitals involved in the full coordination scheme .. e.g. for Co there are 5 3d-orbitals, 3 4p orbitals and 1 4s orbital. When Co³⁺ coordinates 6 ligands, it stores its own 6 d-electrons in the 3d_xz, 3d_yz and 3d_xy orbitals. The 3d_z² and 3d_x²-y² are then hybridized with the 4p and 4s orbitals for form an octahedral arrangement of empty orbitals that accept the 6 donor pairs from the ligands.

Another relevant point is that the coordinating ligands don't really "donate" their electrons completely to the metal ion in the way that you implied in your post. They share them in a kind of weaker covalent interaction called a dative bond. So the Co ion doesn't really have "own" 18 electrons in its valence orbitals ... if it did it would have a charge of -9, right? A useful (but not strictly correct) way to think about it is that the ligands "own" 12 of the electrons, and only share them "a little bit" with the Co ion.

 

[ampakine]

Extremely concise and informative answer. Thanks! Don't even have any follow up questions. Yeah donate was the wrong word, I was wondering what the word for these kinda bonds is though, thanks.