[Dolphinsiu]

I cannot think out of the two following questions:

1. Why V(CO)6 reacts readily with sodium to form Na[V(CO)6]-?

2. Why an aqueous solution of [Co(NH3)5(H2O)]3+ is acidic? (is it due to H3O+?)

[Alpha-Omega]

#1

The EAN Rule states that in forming a complex, ligands are added until the total number of e- in the central metal atom/ion becomes the same as the number of e- in the next inert gas.

V(CO)6 is an example of a coordination complex that has very bulky ligands (CO)₆ which preclude the approach of additional ligands that would allow the metal to achieve the 18 electron configuration.

Many transition metal complexes do not follow this rule, and, furthermore, compounds which have fewer than 18 valence electrons tend to show enhanced reactivity. In fact, 18 electrons is often a recipe for non-reactivity in either a stoichiometric or catalytic sense. The fact that the rule is broken often and is mainly a guide to unreactive species does not detract from its usefulness - it remains an invaluable guide for the classification of compounds and as a predictor of structures and mechanism. For example, 18-electron compounds almost invariably react with donor ligands via pathways that are dissociative, whereas most catalytic processes rely on agents that react via associative steps. On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base like processes. (Ref:  Wikipedia)

Certain complexes

V(CO)₆ has 17 valence electrons

Hence the easy formation of the monoanion Na[V(CO)₆]^-

#2

Coordination of a water molecule to a metal center with a high positive charge greatly increases its acidity, compared to free water. For example, your cobalt complex acts as a weak acid in aqueous solution by donating a hydrogen ion to water:

[Co(NH₃)₅(H₂O)]³⁺ + H₂O  [Co(NH₃)₅(OH)]²⁺ + H₃O⁺

The K_a for uncoordinated water is only 10^-14, so the acidity of the coordinated water in the pentaamminecobalt complex is greatly increased.