We study the properties of intermediate four-atom complexes formed in bimolecular collisions, which may have the critical role for understanding of losses in experiments with ultracold alkali-metal molecules. We show that the variation in the nuclear spin–spin and quadrupole couplings may be strong enough to couple different rotational manifolds resulting in an increase in the density of states and lifetimes of the collision complexes. We estimate the nuclear spin–rotation, spin–spin, and quadrupole coupling constants for bialkali four-atom complexes using ab initio quantum-chemical methods and model the reaction kinetics using an extended rate equation approach. We also reveal that the interaction-induced variation of the electron spin–nuclear spin couplings may explain the recently observed long lifetime of alkali-metal three-atom complexes formed in atom-molecule collisions. Our results can be helpful for interpreting recent experimental data obtained with nonreactive systems which reported unexpectedly large loss rates and for designing future experiments utilizing polar molecules for the purpose of precision measurements and quantum technologies.

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• Atomic & molecular collisions
• Cold and ultracold molecules
• Ultracold collisions
• Molecules