CHM 401

 

Valence Bond Theory

Effective Atomic Number Rule (EAN): complexes with 18 (sometimes 16) electrons are stable (analogous to octet rule for main group elements)

Count the metal valence electrons and the electrons donated by the ligands; if this adds up to 18 (or 16), then the complex is predicted to be stable

EAN is most effective for low oxidation states and organometallic complexes

Organometallic complex: a compound with a metal to carbon bond.

Unlike the main group, d orbital lone pairs are generally not stereochemically active so the geometries of these compounds normally are determined solely by the number of ligands.

Examples

Fe(CO)5

Fe d8 (Note: when in a bonding situation, all of the electrons on a metal move to the orbital with the lowest n quantum number, similar to when ionization happens.)

CO 5(2) = 10

8 + 10 = 18

[Co(NH3)6]3+

Co d6

:NH3 6(2) = 12

6 + 12 = 18

Fe(cp)2

Fe2+ d6

cp 2(6) = 12

6 + 12 = 18

Co(cp)2

Co2+ d7

cp 2(6) = 12

7 + 12 = 19 not stable but Co(cp)2+ is stable

Mn2(CO)10

on each Mn: Mn d7

CO 5(2) = 10

M-M bond, 1

7 + 10 + 1 = 18

[PtCl3(CH2=CH2)]

Pt2+ d8

Cl 3(2) = 6

C2H4, 2

8 + 6 + 2 = 16