AbstractA novel mixed‐ligand erbium complex [(NacNac)ErL(THF)] (1) containing two types of sterically hindered ligands ‐ monoanionic N‐mesityl substituted β‐diketiminate (NacNac=(MesN−C(Me)CHC(Me)‐NMes)⁻, Mes=2,4,6‐Me₃C₆H₂) and dianionic modified diamide ligand with mesityls as the N‐aryl substituents (L=(MesN−C(H)(Me)‐C(=CH₂)‐NMes)²⁻) ‐ was isolated from the salt metathesis reaction of [(NacNac)Er(THF)₂] with potassium salt of doubly reduced diazabutadiene K₂(DAD‐Mes²⁻) (DAD‐Mes=MesN=C(Me)‐C(Me)=NMes) prepared in situ. A solid‐state structure of 1 was determined by single‐crystal X‐ray diffraction and revealed unusual asymmetrical diamide ligand with sp³‐ and sp²‐hybridized carbon backbone atoms and exometallacyclic double carbon‐carbon bond. In spite of the energy preference of symmetrical form of diamide ligand Lsym (Lsym=(MesN−C(Me)=C(Me)‐NMes)²⁻) calculated by DFT asymmetrical form L was found as minor product in the mixture of dipotassium salts obtained by reduction of DAD‐Mes.