Conformational Analysis of P,N‐Containing Eight‐Membered Heterocycles and Their Pt/Ni Complexes in Solution

In solution, 1,5‐diaza‐3,7‐diphosphacyclooctanes (P^R₂N^R′₂) are in equilibrium between the crown (CW; major) and chair–boat (CB; minor) forms. Aromatic substituent at the nitrogen atom shifts the equilibrium in favor of the CW form. [Pt(P^R₂N^R′₂)]Cl₂ complexes exist in solution in the CB (CB*) conformation irrespective of the substituent at the heteroatoms. Aromatic substituents on N notably decrease the barrier of CB–CB* conformational exchange. Biligand complexes ([M(P^R₂N^R′₂)₂]²⁺, M = Pt, Ni) are in CBCB₁ conformation with distortion of the metal coordination plane. The CBCB₁ form is in degenerate exchange with the CBCB₁* form. In general, distortion of the metal coordination plane and the intramolecular exchange barriers are higher in Ni complexes than in Pt ones. An Ar group at the nitrogen atom increases the heterocycle interconversion rate. Bulky P‐substituents increase the distortion of the metal coordination plane and slow down its “twisting” rate. Counterions with stronger coordinating ability increase the barriers of intramolecular conversion.