Amidinate

Amidinate refers to a class of bidentate ligands derived from amidines by deprotonation of the central carbon. The resulting amidinate anion coordinates to metals through two adjacent nitrogens, typically forming a five-membered chelate ring. The common representation is [RNC(R')NR''], with R, R', and R'' as organic substituents that can be alkyl or aryl. The negative charge is delocalized over the two nitrogens and the central carbon, giving several resonance forms and contributing to strong binding to metal centers.

Preparation of amidinates usually involves deprotonation of N,N'-disubstituted amidines with strong bases such as alkali metal hydrides, organolithium reagents, or metal amide bases. The resulting salts, often lithium or sodium amidinates, can be isolated or used directly to form metal amidinate complexes. The substituents on the nitrogens (R', R'') and the aryl or alkyl groups on the carbon permit tunable steric and electronic properties, influencing reactivity and stability.

In coordination chemistry, amidinates are valued as strong σ-donor ligands with some π-donor character from delocalization. They stabilize a wide range of metal centers, including alkali, alkaline earth, and transition metals, and can form various architectures depending on substituents. The ligands commonly enforce chelation through two nitrogens, enabling robust M–N bonds and useful properties in catalysis and materials chemistry.

Applications of amidinate ligands span homogeneous catalysis, polymerization chemistry, and precursors to well-defined metal complexes. Their modular substituent framework allows systematic tuning of sterics and electronics to optimize catalytic activity, selectivity, and stability. See also amidine, imine, and related nucleophilic ligands in organometallic chemistry.