A series of N-heterocyclic carbene (NHC)-stabilized phosphanylgallanes and highly temperature sensitive phosphanylalanes bearing aliphatic and aromatic substituents at the phosphorus atom were synthesized via salt metathesis reactions of IDipp-coordinated (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) triel halide precursors with alkali metal phosphanides. The gallium derivatives IDipp∙GaH2PHtBu (1) and IDipp∙GaH2P(tBu)2 (2) were isolated and structurally characterized, revealing monomeric species with distorted tetrahedral coordination at the group 13 element. The corresponding aluminum compounds IDipp·AlEt2PHtBu (3), IDipp·AlEt2PHPh (4), IDipp·AlEt2PPh2 (5), and IDipp·AlEt2PH2 (6) exhibit a strong dependence of their electronic and structural properties on the substitution pattern at the phosphorus atoms. X-ray diffraction studies show that Al─P bonds in the P atom substituted phosphanylalanes are elongated compared to IDipp·AlEt2PH2. Furthermore, computational analysis of the frontier molecular orbitals revealed the availability of the phosphorus lone pair. Comparison of donor abilities of the studied compounds with known substituted phosphanylboranes also indicates that these phosphanylalanes and -gallanes can serve as strong σ donating ligands. The reactivity of phosphanylgallanes toward SMe2·BH3 with formation of IDipp·GaH2PPh2·BH3 (9) and IDipp·GaH2PH2·BH3 (10), respectively, confirms the accessibility of the phosphorus lone pair and highlights their potential as Lewis base compounds.