Where does the inverse seconds unit come from in the association constant?
First order association reaction:

$$\ce{A + B &lt;=&gt;[k_f][k_r] AB}$$

Rate of association (units: concentration per time) = $k_f\times[A]\times[B]$
 Since [A] and [B] are concentrations and LHS is conc. per time the units of $k_f$ has to be such that it gives conc per time when multiplied by conc2 ([A] ×[B])  Which is conc-1 × time-1

Rate of dissociation (conc. per time) = $k_r\times[AB]$
 Therefore units of $k_r$ would be time-1.

Dissociation constant is actually $\dfrac{k_r}{k_f}$; Its units therefore would be conc.