Metalation of dimesitylphosphane oxide, Mes2P(O)H (1), with alkali metal reagents (nBuLi, NaH, and A(hmds); A = K, Rb, and Cs) in THF yields the corresponding dimesitylphosphinites of lithium (2-thf), sodium (3-thf), potassium (4-thf), rubidium (5-thf), and caesium (6). Their molecular structures exhibit a broad and fascinating variety. Dinuclear compounds 2-thf, 3-thf, and 5-thf have central four-membered A2O2 rings, whereas the potassium congener crystallises as a tetranuclear complex with an inner A4O4 heterocubane cage. The tetranuclear caesium congener precipitates without thf coligands and exhibits a quite unique structure in its crystalline state. Due to their catalytic activity in hydrophosphorylation reactions, we focus on the solvent-structure relationship of the potassium derivatives. In hydrocarbons, [K4(O-PMes2)4]2 (4) is formed, and bidentate Lewis bases like dme and tmeda are unable to deaggregate this tetranuclear cage compound, but bases with a higher denticity (diglyme, triglyme, and pmdeta) split this cage compound into dinuclear complexes with central K2O2 rings. In addition, very bulky P-bound aryl groups like 2,4,6-triisopropylphenyl in dinuclear 8-thf hinder the formation of tetranuclear cage compounds, whereas 2-methylnaphthyl substituents are not bulky enough and the tetranuclear cage compound 7-thf is stabilised. For the 2,4,6-triisopropylphenyl substituent, the rubidium and caesium congeners 11 and 12 crystallise with two central A2O2 rings interconnected by π-interactions. A heteroleptic potassium complex 9-hmds, containing hmds as well as phosphinite anions, represents a snapshot on the way from the starting K(hmds) to the phosphinite-based heterocubane congener. Finally, heterobimetallic [{(thf)K}2Mg(O-PMes2)4] (10-thf) with tetrahedrally coordinated Mg centres has been isolated.