Constructing an unprecedented {Mn} matryoshka doll with a [Mn(CO)] inorganic core and magnetocaloric effect.

An unprecedented inner [Mn(CO)] inorganic core and [Mn] metal-organic periphery compose a high-nuclearity homometallic single-valent {Mn} molecular aggregate with a [Mn] ⊂ [Mn] ⊂ [Mn] ⊂ [Mn] matryoshka doll-like skeleton that displays a significant magnetocaloric effect (MCE).