Are M-N bonds indeed inherently weaker when N is a tertiary rather than a primary or secondary nitrogen atom?

N-alkylation of amine ligands decreases the stability constants of their metal ion complexes, stabilizes low valent complexes, increases the acidity of the central cation and turns it into a harder acid. The major source of these effects is the decrease in the outer sphere solvation energy induced by the N-alkylation.