The coordination chemistry of complexes containing carbon-phosphorus multiple bonds has recently evolved at a very rapid pace. Recently, two different types of coordinated isocyaphide (C≡PR) ligands have been prepared; complex 1 exhibits a semibridging isocyaphide ligand, while complex 2 contains a symmetrically-bridged isocyaphide ligand. These ligands are phosphorus analogs of isocyanide ligands (C≡NR) and were prepared in the coordination spheres of transition metal complexes, as free C≡PR ligands are unknown. In attempts to prepare further examples of coordinated isocyaphide ligands to compare their bonding properties with those of isocyanides, we have succeeded in preparing some very interesting, previously unknown C=P multiply bonded ligand complexes of nickel, palladium and platinum. Several examples of [eta]1-coordinated phosphavinyl complexes (3)* have been prepared which are phosphorus analogs of vinyl complexes. Compounds of the type 4* are the first examples of [eta]2-coordinated phosphavinyl phosphonium ligands, which act as three-electron donors, while compound 5 exhibits a phosphine-functionalized isocyaphide ligand that is bridging [mu]-[eta]2, [eta]2 between two nickel atoms acting as a six-electron donor. This type of phosphine functionalization is unknown in CNR, CO or CS chemistry, and shows a unique capability of isocyaphide ligands. Complex 6 exhibits an [eta]2-coordinated phosphonio-methylene(oxo)phosphorane ligand, while complex 7 contains an [eta]2-coordinated phosphonio-methylene(imino)metallaphosphorane ligand; these are the first examples of coordinated methylene(imino, oxo)phosphorane ligands;We have recently been successful in preparing semibridging isocyaphide complexes by reacting an [eta]1, [eta]2-bridged cyaphide (C≡P-) complex with alkyl halides. In the case of R = Me,* the first example of an [eta]1 ,[eta]2-bridged cationic isocyaphide complex was prepared and isolated as the triflate salt. These cationic isocyaphide complexes are attacked by halides with accompanying loss of phosphine to generate semi-bridging isocyaphide complexes. The [eta]1, [eta]2-bridged cyaphide complex* also reacted with transition metal complexes to afford the first examples of [eta]1 , [eta]1 , [eta]2-bridged trinuclear cyaphide complexes. These complexes containing new carbon phosphorus unsaturated ligands are valuable additions to the expanding field of low-valent phosphorus coordination chemistry. ftn*Please refer to the dissertation for diagrams.