# Law of reciprocal proportion

Is it necessary that the valency of an element must have the same valency?

For eg. If 2 elements A and B combine with the same mass of C, is it necessary that they will have the same valency as they would have when combining AB?

The following are ladders of oxidation states for two elements. Simple examination shows there is no hard and fast rule for valence and stoichiometry overall. Valence need not be integral, either, for example, the Creutz-Taube ion.

Fe(-2), $\ce{Na2[Fe(CO)4]}$, Collman's reagent
Fe(0), $\ce{Fe(CO)5}$
Fe(0), $\ce{Fe3(CO)12}$
Fe(+2), $\ce{FeCl2}$
Fe(+3), $\ce{FeCl3}$
Fe(+4), $\ce{K4[FeO4]}$
Fe(+5), $\ce{K3[FeO4]}$
Fe(+6), $\ce{K2[FeO4]}$

V(-2), $\ce{[Na(diglyme)2][V(CO)6]}$
V(-1), $\ce{(Cp)V(CO)4}$
V(0), $\ce{V(CO)6}$

Chemistry seeks to establish rules. Then the fun starts - breaking the rules.

• V(CO)6 is a coordinated compound, you cannot use the oxidant number to describe such compounds. Apr 19 '14 at 0:35