  A Riemann-Cartan manifold is a Riemannian manifold endowed with an affine
connection which is compatible with the metric tensor. This affine connection
is not necessarily torsion free. Under the assumption that the manifold is a
homogeneous space, the notion of homogeneous Riemann-Cartan space is introduced
in a natural way. For the case of the odd dimensional spheres
$\mathbb{S}^{2n+1}$ viewed as homogeneous spaces of the special unitary groups,
the classical Nomizu's Theorem on invariant connections has permitted to obtain
an algebraical description of all the connections which turn the spheres
$\mathbb{S}^{2n+1}$ into homogeneous Riemann-Cartan spaces. The expressions of
such connections as covariant derivatives are given by means of several
invariant tensors: the ones of the usual Sasakian structure of the sphere; an
invariant 3-differential form coming from a $3$-Sasakian structure on
$\mathbb{S}^7$; and the involved ones in the almost contact metric structure of
$\mathbb{S}^5$ provided by its natural embedding into the nearly K\"ahler
manifold $\mathbb{S}^6$. Furthermore, the invariant connections sharing
geodesics with the Levi-Civita one have also been completely described.
Finally, $\mathbb{S}^3$ and $\mathbb{S}^7$ are characterized as the unique
odd-dimensional spheres which admit nontrivial invariant connections satisfying
an Einstein-type equation.

Differential Geometry