The Structural Chemistry of Text‐Book Species: the Tartrato‐Cuprates in Fehling's Solution

Fehling's solution is an aqueous alkaline solution (pH ca. 13) of D ‐, L ‐, or rac ‐tartrato‐cuprates(II) and both excess tartrate and hydroxide. The isolation of crystalline solids succeeded for rac ‐tartrate‐containing samples. Crystal structure analyses revealed the formulas K 8 [Cu 10 ( rac ‐tartH ‐1 ) 4 ( rac ‐tartH ‐2 ) 4 (H 2 O) 4 ] · 18H 2 O ( 1 ), Li 4 [Cu 2 ( rac ‐tartH ‐2 ‐κ 2 O 1 , O 2 :κ 2 O 3 , O 4 ) 2 ] · 11.75H 2 O ( 2 ), and Na 6 [Cu( rac ‐tartH ‐2 ‐κ 2 O 2 , O 3 ) 2 ] · 14H 2 O ( 3 ). The cupric central atom is in a square‐planar tetracoordination in the bis(diolato) chelate in 3 , and in square‐pyramidal 4+1 coordination in the mixed carboxylato‐alkoxido chelates 1 and 2 . In terms of stoichiometry, 3 is closest to typical Fehling recipes.