A celebrated theorem of Kapranov states that the Atiyah class of the tangent
bundle of a complex manifold $X$ makes $T_X[-1]$ into a Lie algebra object in
$D^+(X)$, the bounded below derived category of coherent sheaves on $X$.
Furthermore Kapranov proved that, for a K\"ahler manifold $X$, the Dolbeault
resolution $\Omega^{\bullet-1}(T_X^{1,0})$ of $T_X[-1]$ is an $L_\infty$
algebra. In this paper, we prove that Kapranov's theorem holds in much wider
generality for vector bundles over Lie pairs. Given a Lie pair $(L,A)$, i.e. a
Lie algebroid $L$ together with a Lie subalgebroid $A$, we define the Atiyah
class $\alpha_E$ of an $A$-module $E$ (relative to $L$) as the obstruction to
the existence of an $A$-compatible $L$-connection on $E$. We prove that the
Atiyah classes $\alpha_{L/A}$ and $\alpha_E$ respectively make $L/A[-1]$ and
$E[-1]$ into a Lie algebra and a Lie algebra module in the bounded below
derived category $D^+(\mathcal{A})$, where $\mathcal{A}$ is the abelian
category of left $\mathcal{U}(A)$-modules and $\mathcal{U}(A)$ is the universal
enveloping algebra of $A$. Moreover, we produce a homotopy Leibniz algebra and
a homotopy Leibniz module stemming from the Atiyah classes of $L/A$ and $E$,
and inducing the aforesaid Lie structures in $D^+(\mathcal{A})$.Comment: 36 page