While we teach our students in freshman chemistry that the periodic table is filled in the order [Ar]4s23d10, this turns out to be true only for isolated metal atoms. When we put a metal ion into an electronic field (surround it with ligands), the d-orbitals drop in energy and fill first. Therefore, the organometallic chemist considers the transition metal valence electrons to all be d-electrons. There are certain cases where the 4s23dx order does occur, but we can neglect these in our first approximation.
Therefore, when we ask for the d-electron count on a transition metal such as Ti in the zero oxidation state, we call it d4, not d2. For zero-valent metals, we see that the electron count simply corresponds to the column it occupies in the periodic table. Hence, Fe is in the eighth column and is d8 (not d6) and Re3+ is d4 (seventh column for Re, and then add 3 positive charges...or subtract three negative ones). Now that we can assign a d-electron count to a metal center, we are ready to determine the electronic contribution of the surrounding ligands and come up with our overall electron count.
While we teach our students in freshman chemistry that the periodic table is filled in the order [Ar]4s23d10, this turns out to be true only for isolated metal atoms. When we put a metal ion into an electronic field (surround it with ligands), the d-orbitals drop in energy and fill first. Therefore, the organometallic chemist considers the transition metal valence electrons to all be d-electrons. There are certain cases where the 4s23dx order does occur, but we can neglect these in our first approximation.Therefore, when we ask for the d-electron count on a transition metal such as Ti in the zero oxidation state, we call it d4, not d2. For zero-valent metals, we see that the electron count simply corresponds to the column it occupies in the periodic table. Hence, Fe is in the eighth column and is d8 (not d6) and Re3+ is d4 (seventh column for Re, and then add 3 positive charges...or subtract three negative ones). Now that we can assign a d-electron count to a metal center, we are ready to determine the electronic contribution of the surrounding ligands and come up with our overall electron count.
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