 Solution

$M^1=\mathrm{Li}$ and $M^2=\mathrm{K}$. Both metals are bridged by the two diisopropylamide nitrogen atoms in a four-membered Li–N–K–N ring. The smaller lithium ion accommodates the two amides plus one THF oxygen, while the larger potassium ion has space for two THF donors and more readily adopts the higher coordination number. The principal Li–N and Li–O contacts should be shorter than their potassium counterparts because of the much smaller lithium ionic radius. A larger number of donors does not imply shorter individual K–donor bonds.