Metallacyclopentadienes, which possess two M–C(sp²) bonds and feature the structure of M(CR¹ CR²–CR³ CR⁴), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of organic molecules and useful precursors to main group element compounds, and have received considerable attention in organometallic chemistry, coordination chemistry and synthetic organic chemistry over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addition, their application in synthetic organic chemistry and polymer chemistry is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chemistry.

Multisubstituted guanidines have received much attention because of important applications in many fields, such as pharmaceutics, organometallic and coordination chemistry, and organic synthesis. Although classical methods are available for the preparation of guanidines, synthetic preparation approaches to guanidines are still in great demand. In this review, we summarize recent developments on synthetic methods via the C–N bond formation. Three aspects are included: transition-metal-catalyzed guanidine synthesis based on classical methods; catalytic guanylation reaction of amines with carbodiimides; and tandem catalytic guanylation/cyclization reactions.

Various metal complexes (e.g., lanthanides, early transition metals, and alkali metals) can serve as catalyst precursors for the catalytic addition of alkyne C–H, amine N–H, and phosphine P–H bonds to carbodiimides, to give a new family of propiolamidines, guanidines, and phophaguanidines, some of which were difficult to prepare previously. The catalytic reaction proceeds generally through nucleophilic addition of an M–ER (E = CR¹R², NR¹, PR¹) bond, which is formed by an acid–base reaction between a catalyst precursor and a RE–H bond, to a carbodiimide compound, followed by protonolysis of the resultant amidinate–(phospha)guanidinate species “{R′NC(ER)NR′}M” with RE–H.

The extensive synthetic application of carbodiimides in various types of N-heterocyclic systems from the 1980s to today is outlined.