Magnesium cobaltates ((Ar)nacnac)MgCo(COD)(2) (1-3) were synthesised by reacting ((Ar)nacnac)MgI(OEt2) with K[Co(eta(4)-COD)(2)] (COD = 1,5-cyclooctadiene) [(Ar)nacnac = CH(ArNCMe)(2); Ar = 2,4,6-Me-3-C6H2 (Mes), 2,6-Et-2-C6H3 (Dep), 2,6-iPr(2)-C(6)H(3)Mes (Dipp)]. Compounds 1-3 form contact ion-pairs in toluene, while solvent separated ion-pairs are formed in THF. The effect of ion-pairing on the reactivity is illustrated by reaction of 2 with tert-butylphosphaalkyne, which affords distinct 1,3-diphosphacyclobutadiene complexes. The heteroleptic sandwich complex [((Dep)nacnac)MgCo(P(2)C(2)tBu(2))](2) (4) is selectively formed in toluene, while the homoleptic bis(1,3-diphosphacyclobutadiene) complex [((Dep)nacnac)Mg(THF)(3)][Co (P(2)C(2)tBu(2))(2)] (5) is obtained in THF. Complex 4 is a precursor to further unusual phosphaorganometallic compounds. Substitution of the labile COD ligand in 4 by white phosphorus (P-4) enabled the synthesis of the phosphorus-rich sandwich compound [((Dep)nacnac)MgCoP4(P(2)C(2)tBu(2))](2) (6). The heterobimetallic complex (Cp*NiP(2)C(2)tBu(2))Co(COD) (7) was isolated after treatment of 4 with Cp*Ni(acac) (Cp* = C5Me5, acac = acetylacetonate).