LINUS PAULING 



ferrous ion, [Fe(OH 2 ) 6 ] + + , whereas oxyhaemoglobin and carbon- 

 monoxyhaemoglobin are diamagnetic, and are thus similar in magnetic 



properties to the ferrocyanide ion, [Fe(CN) 6 ] . The bipositive 



iron ion Fe + + has 24 extranuclear electrons, permitting it to form a 

 completed argon shell plus six outer electrons. There are nine stable 

 orbitals outside of the argon shell, the five 3d orbitals, one 4s orbital, 

 and three 4p orbitals, of which the 3d orbitals are somewhat more 

 stable for unshared electrons than the other two. In the isolated 

 ferrous ion the six electrons occupy the five 3d orbitals, and give rise 

 to a normal state in which four of the electron spins are unpaired. It 

 is assumed that in the hydrated ferrous ion and in ferrohaemoglobin 

 the bonds between the iron atom and the ligated atoms do not make 

 use of more than four of the nine outer orbitals, permitting the six 

 electrons to occupy the remaining five orbitals, with four electron spins 

 unpaired. The magnetic moment expected for four electron spins, 

 without contribution from orbital moments, is 4-90 Bohr magnetons, 

 and the values observed for the hydrated ferrous ion, 5-3 magnetons, 

 and the ferrohaemoglobin molecule, 5-44 magnetons per iron atom 

 (assuming that the moments of the four iron atoms in the molecule 

 orient themselves independently in the applied magnetic field), are in 

 approximate agreement with this value. In the ferrocyanide ion, on 

 the other hand, all of the nine outer orbitals are indicated by the 

 observed diamagnetism to be used in the formation of covalent bonds 

 or for occupancy by unshared pairs of electrons. The same sort of 

 structure was proposed for oxyhaemoglobin and carbonmonoxyhaemo- 



globin. 



It has been customary to refer to the structure indicated by the large 

 magnetic moment of ferrohaemoglobin as involving essentially ionic 

 bonds between the iron atoms and surrounding atoms, and to the 

 structure in oxyhaemoglobin and carbonmonoxyhaemoglobin as 

 involving octahedral coordination with essentially covalent bonds. 



The magnetic properties of ferrihaemoglobin (methaemoglobin) and 

 its derivatives show that a similar change in electronic structure occurs 

 on chemical reaction of this molecule. Ferrihaemoglobin itself has 

 magnetic moment 5-46 in acid solutions (below pH 5), and the moment 

 changes to 5-77 when the solution is made more basic (pU 6-0-7-0). 

 These values are slightly smaller than the value 5-92 corresponding to 

 five unpaired electron spins, which would be expected in case that the 

 five 3d orbitals were all available for occupancy by five outer electrons 

 of the Fe + + + ion. Values close to the theoretical are observed for 

 ferrihaemoglobin fluoride (5-90) and the ferrihaemoglobin-ethanol 

 complex (5-89). 3 ' 4 Other compounds of ferrihaemoglobin have a 

 magnetic moment not far from that given by the spin of a single 



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