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Lecture 23 Coordination Chemistry 1) Electron counting in transition metal complexes

Lecture 23 Coordination Chemistry 1) Electron counting in transition metal complexes. “Neutral atom” method . (a) Write down the number of electrons in the neutral atom valence shell, which corresponds to its group number: 3 … 12.

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Lecture 23 Coordination Chemistry 1) Electron counting in transition metal complexes

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  1. Lecture 23 Coordination Chemistry 1) Electron counting in transition metal complexes • “Neutral atom” method. (a) Write down the number of electrons in the neutral atom valence shell, which corresponds to its group number: 3 … 12. (b) Account for the ionic charge if the species is not neutral. This will reduce (positive ion) or increase (negative ion) the number of electrons available in the metal valence shell; (c) Add the number of electrons contributed by ligands (see table on p. 3). Complex (A) Complex (B)

  2. 2) “Oxidation state” method • “Oxidation state” method. (a) Write down the number of electrons in the neutral atom valence shell, which corresponds to its group number: 3 … 12. (b) Determine the formal oxidation state of the metal. This will reduce (positive oxid. state) or increase (negative oxid. state) the number of electrons contributed by the metal (c) Add the number of electrons contributed by ligands. • If you are given a structure with delocalized bonds, counting electrons may be problematic: Ni(0), Ni(I), Ni(II), Ni(III) or Ni(IV) compound?

  3. 3) Counting electrons: ligand contributions • Each ligand is characterized by the number of electrons it can contribute to the metal valence shell. If a ligand is bridging between n metal atoms, the number of electrons it donates to a single atom is n times less. Here you have some data for the “oxidation state” / “neutral atom” methods:

  4. 4) 18 electron rule • For d-elements with 9 orbitals in their valence shell (s, three p and five d orbitals) normally there is a maximum of 18 electrons in the shell (ns2(n-1)d10np6 configuration): [V(CO)6]-; Cr(C6H6)2; [Mn(CO)5]-; [Fe(OH2)6]2+; [Co(CN)6]3-; Ni(CO)4; [Cu(NH3)4]+ • If this number is not reached, the species is coordinatively unsaturated and tend to add more ligands. It is reflected in higher reactivity and therefore the difficulty with which coordinatively unsaturated compounds can be isolated. • There are multiple examples of transition metal compounds with less (group 3, 4 and 10) or more than 18 electrons in the metal shell:. • But these are 18 electron complexes:

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