Electron spin resonance spectrum of bis(4-phenylamino-2-phenyliminopent-3- enato-NN?)copper(II); a complex having both a low Azz and low-energy d-d band

Bis(4-phenylamino-2-phenyliminopent-3-enato-NN?)copper(II) (I), having a low energy d-d band characteristic of near-tetrahedral stereochemistry, shows an e.s.r. spectrum (as a doped sample of the zinc analogue) with a low A?(Cu) (107 × 10-4 cm-1); the relevance of this spectrum to the stereochemistries proposed for 'blue' copper proteins is discussed.