Charge density in Cu(glygly)(OH<SUB>2</SUB>)<SUB>2</SUB>·H<SUB>2</SUB>O at 10 K and the reproducibility of atomic orbital populations

An extensive accurate X-ray data set for diaqua(glycylglycinato)copper(ii) hydrate, Cu(glygly)(OH₂)₂·H₂O, at 10 K is reported. The molecular structure is very accurately defined. The Cu(glygly)(OH₂)₂ unit approximates to a square pyramid with the apical water molecule only loosely bound. The data have been analysed by multipole and valence orbital population models and the topology of the electron density has been examined for chemical bonding features. There are two independent molecules of the complex in the unit cell, and the agreement between their bonding features is used as a measure of the validity of obtaining quantitative valence-related parameters from X-ray data. The agreement is quite reasonable. Atomic charges evaluated by the Stockholder and monopole population methods do not conform with chemical expectations. Those from the sum of valence orbital populations and the atom-in-molecules approach are more reasonable but contain anomalies. The ‘hole’ in the 3d¹⁰ shell corresponding to the 3d⁹ configuration, predicted on the grounds of ligand field theory, is seen as a much lowered (1.16 e) population of 3d_z² relative to the other 3d orbitals (av. 1.65 e). Populations of valence hybrid orbitals conform with chemical expectations and, for the (sp²)_σ hybridised carbon atoms of C&z.dbd;O groups, relocation of electrons from a p_π orbital to those σ hybrids is seen. Bond critical points have been located between all the appropriate non-hydrogen atom pairs. The electron densities at these points show values typical for mixed ionic/covalent character in the Cu–O and Cu–N interactions and of covalent character in the C–C, O–C and N–C bonds. The Laplacian of the density at the bond critical points also behaves in a fashion which conforms with those bond types, although the scatter in the results is greater. There is little evidence of the copper atom having influenced the electron densities of the bonds between light atoms within the glycylglycinato moiety.