Law of reciprocal proportion
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.