Why Does the Tetrakis(trimethylphosphine)iridium(III) Hydridochloride Cation Adopt the Sterically and Electronically Unfavorable Cis Geometry?

Although the tetrakis(trimethylphosphine)iridium(III) hydridochloride cation, [HIrCl(PMe₃)₄]⁺, and similar complexes would intuitively be expected to adopt a trans geometry on electronic and sterical grounds, experimentally the cis geometry is found to prevail. Quantum chemical calculations suggest that the trans and cis structures are nearly isoenergetic, such that the cis:trans equilibrium is dominated by the higher entropy of the lower symmetry cis structure.