Mechanism for reaction of Tollens' reagent with aldehydes
Here are the two half reactions:

$$\begin{align}
\ce{[Ag(NH3)2]+ + e- &amp;-&gt; Ag^0 + 2NH3} \\
\ce{RCHO + 3OH- &amp;-&gt; RCO2- + 2H2O + 2e-}
\end{align}$$

which together yield the overall reaction

$$\ce{2[Ag(NH3)2]+ + RCHO + 3OH- -&gt; 2Ag^0 + RCO2- + 4NH3 + 2H2O}$$

Here is a diagram of the reaction mechanism. The carbonyl group is oxidized in the process and the $\ce{Ag^+}$ is reduced. The resultant oxidized aldehyde (now a radical cation) reacts with hydroxide to form a tetrahedral intermediate. A gem-diol like intermediate is formed via a hydrogen shift, which then continues on to the final carboxylate anion.