The metathesis reaction of potassium (tris(tert-butyl)silyl)phosphanide with GaCl3 in a molar ratio of 1:1 leads to the formation of [Cl2GaP(H)(SiBu3)-Bu-t](2) (1) as a mixture of cis and trans isomers with very large (1)J(P,H) and (2)J(P,P) coupling constants. The molecular structure of 1 shows a Ga2P2 cycle with nearly planar coordinated phosphorus atoms under neglection of the hydrogen atoms and Ga-P distances of 239 pm. The reaction of GaCl3 with 3 equiv of potassium (tris(tert-butyl)silyl)phosphanide as well as the reaction of 1 with 2 equiv of KP(H)(SiBu3)-Bu-t yields [(Bu3SiP)-Bu-t(H)Ga(mu-PSi'Bu-3)](2) (2). The central moiety comprises a four-membered Ga2P2 cycle with one planar P atom and extremely short Ga-P bonds of approximately 226 pm, the other being in a pyramidal environment with an angle sum of 298.4degrees. The structure of 2 can be described as a GaPGa heteroallyl system which is bonded to a phosphanidyl substituent. This idea and its dependency on the steric demand of the trialkylsilyl groups are investigated by DFT calculations on different isomers of 2.