I was thinking of the geometry of the transition metal complex in solid $\ce{K2[Cu(ox)2] · 4 H2O}$ solid around the $\ce{Cu(II)}$ ion.

At first I thought square planar, but is it possible or is there any chance that it could be octahedral with two water molecules since it is a hydrate?

Or do brackets always indicate what is directly complexed in this case?


Yes, square brackets are used to denote a coordination sphere, but they may sometimes be used incorrectly. Ideally you want to check the crystal structure, luckily there is one [1]:

The $\ce{Cu(II)}$ atom has square-planar geometry with two bidentate oxalate ligands. Each $\ce{K+}$ cation is coordinated by eight oxygen atoms from one bidentate and two unidentate oxalate groups and four water molecules.

enter image description here Fig. 1. (a) Crystal structures of $\ce{[K2Cu(ox)2 · 4 H2O]}$ (1) and (b) $\ce{[K2Cu(ox)2 · 2 H2O]}$ (2) with atom numbering scheme. The thermal ellipsoids are drawn at 50% probability.


  1. Fan, J.; Sun, W.-Y.; Okamura, T.; Yu, K.-B.; Ueyama, N. The X-Ray Crystal Structural Characterization of Dipotassium Bisoxalato Copper(II) Tetrahydrate, $\ce{[K2Cu(Ox)2·4 H2O]}$ $(\ce{Ox}$ = oxalate Dianion). Inorganica Chimica Acta 2001, 319 (1), 240–246. DOI: 10.1016/S0020-1693(01)00466-2.

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