Hexacoordinate phosphorus molecules involving nitrogen, oxygen, or sulfur ligands provide examples of Lewis acid-Lewis base hexacoordination.  For the two similar complexes shown below, the length of the C&ndash;P bond increases with decreasing length of the N&ndash;P bond; the strength of the C&ndash;P bond decreases with increasing strength of the N&ndash;P Lewis acid&ndash;Lewis base interaction.