Relative nucleophilicity:
In a range of anions, nucleophilic strength parallels basicity if the nucleophilic center is similar atom. For instance the nucleophilic strengths of the following oxygen compounds (RO> HO>> RCO2) matches their order of basicity.
The same holds true for anions in which the nucleophilic center is an element in similar row of the periodic table (For example C, N, O, F). So, the order of nucleophilicity of the following anions (R3C > R2N > RO > F) is similar as their order of basicity. This tendency is connected to the electronegativities of these atoms. The more electronegative the atom (for example F), the more tightly it holds on to its electrons and the less obtainable these electrons are for creating new bonds (less nucleophilic).
The story turns into more complexes if we compare anions comprising nucleophilic centers from diverse parts of the periodic table. Now, relative nucleophilicity does not essentially match relative basicity. This is since the solvent employed in a reaction has a significant effect. In protic solvents like water or alcohol, the stronger nucleophiles are those that have a large nucleophilic center, i.e. an atom lower down the periodic table (example S is more nucleophilic than O but is less basic). This is since protic solvents can create hydrogen bonds to the anion. The smaller the anion, the stronger the solvation and the much more hard it is for the anion to react like a nucleophile.
The order of nucleophilicity of a number of common anions in protic solvents is like this:
SH> CN > I > OH > N3 > Br > CH3CO2 > Cl > F.
While an organic solvent is employed that is incapable of creating hydrogen bonds to the anion (for example DMF or DMSO) the order of nucleophilicity changes to more closely match that of basicity. For instance, the order of nucleophilicity of the halides in DMSO is F > Cl > Br > I.